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- From: gabe@pgm.com (Gabe M. Wiener)
- Newsgroups: rec.audio.pro,rec.answers,news.answers
- Subject: rec.audio.pro FAQ (v 0.9)
- Followup-To: rec.audio.pro
- Date: 22 Dec 1996 10:35:06 -0500
- Organization: PGM Recordings / Quintessential Sound, Inc.
- Lines: 2252
- Sender: gabe@panix.com
- Approved: news-answers-request@MIT.EDU
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- Xref: senator-bedfellow.mit.edu rec.audio.pro:112194 rec.answers:26556 news.answers:90218
-
- Archive-name: AudioFAQ/pro-audio-faq
- Last-modified: 1996/06/25
- Version: 0.9
-
- Frequently Asked Questions (FAQ) file for rec.audio.pro
- Version 0.8
-
- Edited and compiled by Gabe Wiener (gabe@pgm.com)
-
- Many thanks to all who have contributed to the FAQ. Individual
- contributions are credited at the end of their respective sections.
- The core FAQ writers are listed below. The rubric in brackets will be
- used to indicate who has written a particular section.
-
- Scott Dorsey kludge@netcom.com [Scott]
- Christopher Hicks cmh@eng.cam.ac.uk [Chris]
- David Josephson david@josephson.com [David]
- Dick Pierce dpierce@world.std.com [Dick]
- Gabe Wiener gabe@pgm.com [Gabe]
-
- * Each author maintains and asserts full legal copyright on his
- * contribution to the FAQ. Compilation copyright (C) 1996 by
- * Gabe M. Wiener. All rights reserved. Permission is granted for
- * non-commercial electronic distribution of this document.
- * Distribution in any other form, or distribution as part of any
- * commercial product requires permission. Inquire to gabe@pgm.com.
-
- ---------
- TABLE OF CONTENTS FOR THE FAQ:
-
- Section I - Netiquette
-
- Q1.1 - What is this newsgroup for? What topics are appropriate here, and what
- topics are best saved for another newsgroup?
- Q1.2 - Do I have to be a "professional" to post here?
- Q1.3 - I need to ask the group for help with selecting a piece of equipment.
- What information should I provide in my message?
-
- Section II - The business of audio
-
- Q2.1 - How does one get started as a professional audio engineer?
- Q2.2 - Are audio schools worth the money? Which schools are best?
- Q2.3 - What are typical rates for various professional audio services?
-
- Section III - Audio Interconnections
-
- Q3.1 - How are professional transmission lines and levels different from
- consumer lines and levels? What is -10 and +4? What's a balanced
- or differential line?
- Q3.2 - What is meant by "impedance matching"? How is it done? Why is it
- necessary?
- Q3.3 - What is the difference between dBv, dBu, dBV, dBm, dB SPL, and plain
- old dB? Why not just use regular voltage and power measurements?
- Q3.4 - Which is it for XLRs? Pin 2 hot? Or pin 3 hot?
- Q3.5 - What is phantom power? What is T-power?
- Q3.6 - How do I interconnect balanced and unbalanced components?
- Q3.7 - What are ground loops and how do I avoid them?
- Q3.8 - What is the "Pin 1 problem" and how do I avoid it?
-
- Section IV - Analog tape recording
-
- Q4.1 - What does it mean to "align" a tape machine?
- Q4.2 - What is bias? What is overbias?
- Q4.3 - What is the difference between Dolby A, B, C, S, and SR? How do each
- of these systems work?
- Q4.4 - What is Dolby HX-Pro?
- Q4.5 - How does DBX compare to Dolby?
- Q4.6 - How much better are external microphone preamplifiers than those
- found in my portable recorder?
- Q4.7 - What is an MRL? Where do I get one?
-
- Section V - Digital recording and interconnection
-
- Q5.1 - What is sampling? What is a sampling rate?
- Q5.2 - What is oversampling?
- Q5.3 - What is the difference between a "1 bit" and a "multibit" converter?
- What is MASH? What is Delta/Sigma? Should I really care?
- Q5.4 - On an analog recorder, I was always taught to make sure the signal
- averages around 0 VU. But on my new DAT machine, 0 is all the way at
- the top of the scale. What's going on here?
- Q5.5 - Why doesn't MiniDisc or Digital Compact Cassette sound as good as DAT
- or CD? After all, they're both digital.
- Q5.6 - What is S/P-DIF? What is AES/EBU?
- Q5.7 - What is clock jitter?
- Q5.8 - How long can I run AES/EBU or S/P-DIF cables? What kind of cable
- should I use?
- Q5.9 - What is SCMS? How do I defeat it?
- Q5.10 - What is PCM-F1 format?
- Q5.11 - How do digital recorders handle selective synchronization?
- Q5.12 - How can a 44.1 kHz sampling rate be enough?
- Q5.13 - Doesn't the 44.1 kHz sampling rate make it impossible to
- reproduce square waves?
- Q5.14 - How can a 16-bit word-length be enough?
- Q5.15 - What's all this about 20- and 24-bit digital audio? Aren't
- CDs limited to 16 bits?
-
- Section VI - Digital editing and mastering
-
- Q6.1 - What is a digital audio workstation?
- Q6.2 - How is digital editing different from analog editing?
- Q6.3 - What is mastering?
- Q6.4 - What is normalizing?
- Q4.5 - I have a fully edited DAT that sounds just like I want it to sound on
- the CD. Is it okay to send it to the factory?
- Q6.6 - What is PCM-1630? What is PMCD?
- Q6.7 - When preparing a tape for CD, how hot should the levels be?
- Q6.8 - Where can I get CDs manufactured?
- Q6.9 - How are CD error rates measured, and what do they mean?
-
- Section VII - Market survey. What are my options if I want --
-
- Q7.1 - A portable DAT machine
- Q7.2 - A rack size DAT machine
- Q7.3 - An inexpensive stereo microphone
- Q7.4 - An inexpensive pair of microphones for stereo
- Q7.5 - A good microphone for recording vocals
- Q7.6 - A good microphone for recording [insert instrument here]
- Q7.7 - A a small mixer
- Q7.8 - A portable cassette machine
- Q7.9 - A computer sound card for my IBM PC or Mac
- Q7.10 - An eight-track digital recorder?
-
- Section VIII - Sound reinforcement
-
- Q8.1 - We have a fine church choir, but the congregation can't hear them.
- How do we mic the choir?
- Q8.2 - How do I 'ring out' a system?
- Q8.3 - How much power to I need for [insert venue here]?
- Q8.4 - How good is the Sabine feedback eliminator?
-
- Section IX - Sound restoration
-
- Q9.1 - How can I play old 78s?
- Q9.2 - How can I play Edison cylinders?
- Q9.3 - What are "Hill and Dale" recordings, and how do I play them back?
- Q9.4 - What exactly are NoNOISE and CEDAR? How are they used?
- Q9.5 - How do noise suppression systems like NoNOISE and CEDAR work?
- Q9.6 - What is forensic audio?
-
- Section X - Recording technique, Speakers, Acoustics, Sound
-
- Q10.1 - What are the various stereo microphone techniques?
- Q10.2 - How do I know which technique to use in a given circumstance?
- Q10.3 - How do I soundproof a room?
- Q10.4 - What is a near-field monitor?
- Q10.5 - What are the differences between "studio monitors" and home
- loudspeakers?
-
- Section XI - Industry information
-
- Q11.1 - Is there a directory of industry resources?
- Q11.2 - What are the industry periodicals?
- Q11.3 - What are the industry trade organizations?
- Q11.4 - Are there any conventions or trade shows that deal specifically
- with professional audio?
-
- Section XII - Miscellaneous
-
- Q12.1 - How do I modify Radio Shack PZMs?
- Q12.2 - Can I produce good demos at home?
- Q12.3 - How do I remove vocals from a song?
-
- Section XIII - Bibliography
-
- Q13.1 - Fundamentals of Audio Technology
- Q13.2 - Studio recording techniques
- Q13.3 - Live recording techniques
- Q13.4 - Digital audio theory and practice
- Q13.5 - Acoustics
- Q13.6 - Practical recording guides
-
- Section XIV
-
- Q14.1 - Who wrote the FAQ
- Q14.2 - How do you spell and pronounce the FAQ maintainer's surname?
-
- --------
- THE FAQ:
-
- Section I - Netiquette
-
- --
- Q1.1 - What is this newsgroup for? What topics are appropriate here, and what
- topics are best saved for another newsgroup?
-
- This newsgroup exists for the discussion of issues and topics related
- to professional audio engineering. We generally do not discuss issues
- relating to home audio reproduction, though they do occasionally come
- up. The rec.audio.* hierarchy of newsgroups is as follows:
-
- rec.audio.pro Issues pertaining to professional audio
- rec.audio.marketplace Buying and trading of consumer equipment
- rec.audio.tech Technical discussions about consumer audio
- rec.audio.opinion Everyone's $0.02 on consumer audio
- rec.audio.high-end High-end consumer audio discussions
- rec.audio.misc Everything else
-
- Please be sure to select the right newsgroup before posting.
-
- --
- Q1.2 - Do I have to be a "professional" to post here?
-
- No. Anyone is welcome to post on rec.audio.pro so long as the messages
- you post are endemic to the group in some way. If you are not an audio
- professional, we would ask that you read this FAQ in full before posting.
- You may find that some of your essential questions about our field are
- answered right here. But if not, feel free to ask us.
-
- --
- Q1.3 - I need to ask the group for help with selecting a piece of equipment.
- What information should I provide in my message?
-
- If you are going to post a request for advice on buying equipment,
- please provide the following information.
-
- Your application for the equipment
- What other equipment you will be using it with
- Your budget for the equipment
- Any specific requirements the equipment should have
-
- There is nothing worse than messages like "Can anyone recommend a DAT
- machine for me to buy???" Sure we can. But what do you want to _do_
- with it? We can recommend DAT machines for $400 or for $14,000.
-
- =====
- Section II - The business of audio
-
- --
- Q2.1 - How does one get started as a professional audio engineer?
-
- There are as many getting-started stories as there are audio
- engineers. The routes into the industry are highly dependent on what
- aspect of the industry one wishes to enter. For instance, many
- engineers who work in the classical-music field have at one time or
- another been classical performers. Others enter through their work in
- other musical genres, or through engineering programs at universities
- or technical schools. Without exception, everyone in the industry has
- learned at least a portion of their craft from watching those with
- more hands-on experience. Whether this comes from a formal internship
- or just from sustained observation and long-term question-asking, it
- is almost always universally true. [Gabe]
-
- --
- Q2.2 - Are audio schools worth the money? Which schools are best?
-
- An audio school will teach you the basics of the audio business,
- but just like any technical school, what they teach you may not be
- worth what you pay.
-
- There are several schools of thought:
-
- 1. Audio schools are great, you get trained on the gear that is used
- by top studios and costs millions of dollars, you get taught by pros
- in the field and you have job placement assistance after you graduate.
-
- 2. Going to an audio school is like wanting to learn aviation,
- and when you start flight school they teach you a 747. In the
- real world, you are probably not going to have 96 channel automated
- consoles on your first job. You are not going to mix your first
- live gig on a 48-channel 100,000 watt stadium PA rig. Better to
- start off on real world equipment and work your way up to the
- top-of-the-line stuff. Most recording studios are 24-track analog
- or less and most PA systems are 16 channel, 3,000 watts or less.
- Don't buy education for something you will never get to use after
- you leave the school.
-
- 3. Audio Schools are a waste of money. Instead of spending $18,000
- for a course and having nothing to show for it but a technical
- certificate (which everyone knows is no help at all getting a job),
- you would be better off spending the 18 grand on books and gear and
- learning by trial and error, or saving the 18 grand altogether and
- learning first from reading, and later from apprenticing.
- [jsaurman@cftnet.com (Jim Saurman)]
-
- Jim summarizes the opinions pretty well. Recognize that an altogether
- different option is to attend a full four-year college program. Many
- colleges and universities offer such programs. Examples include
- Peabody Conservatory, Cleveland Institute of Music, McGill University,
- New York University, University of Miami at Coral Gables, and the
- University of Massachusetts at Lowell. Without fail, graduates from
- these sorts of programs earn far more respect than graduates of any
- technical school. [Gabe]
-
- --
- Q2.3 - What are typical rates for various professional audio services?
-
- Depends on what you want to have done, and where.
-
- One can pay upwards of $300/hr for prime studio rental time in New York.
- In a small community however, one might find a project studio for $25/hr.
- Generally speaking, the rule is: the rarer the service, the more it will
- cost. In a community with dozens of small 8-track studios, you won't
- have to pay much. If you need emergency audio restoration, or mastering
- by a top-flight pop-music engineer, you can expect to drop many hundreds
- of dollars an hour. Like so many other things in this industry, there
- are no rules, and Smith's invisible hand guides the market. [Gabe]
-
- =====
- Section III - Audio Interconnections
-
- --
- Q3.1 - How are professional transmission lines and levels different from
- consumer lines and levels? What is -10 and +4? What's a balanced
- or differential line?
-
- Professional transmission lines differ from consumer lines in two
- ways. First, consumer lines tend to run about 14 dB lower in level
- than pro lines. Second, professional lines run in differential, or
- balanced, configuration.
-
- In a single-ended line, the signal travels down one conductor and
- returns along a shield. This is the simplest form of audio
- transmission, since it is essentially the same AC circuit you learned
- about in high-school physics. The problem here is that any noise
- or interference that creeps into the line will simply get added to
- the signal and you'll be stuck with it.
-
- In a differential line, there are three conductors. A shield, a
- normal "hot" lead, and a third lead called the "cold" or "inverting"
- lead, which carries a 180-degree inverted copy of the hot lead. Any
- interference that creeps into the cable thus affects both the hot and
- cold leads equally. At the receiving end, the hot and cold leads are
- summed using a differential amplifier, and any interference that has
- entered the circuit (called "common-mode information" since it is
- common to both the hot and cold leads), gets canceled out.
- Differential lines are thus better suited for long runs, or for
- situations where noise or interference may be a factor. [Gabe]
-
- --
- Q3.2 - What is meant by "impedance matching"? How is it done? Why is it
- necessary?
-
- We can talk about the characteristic impedance of an input, which is to
- say the ratio of voltage to current that it likes to see, or how much
- it loads down a source. (You can think of this as being an "AC resistance"
- and you would be mostly right, although it's actually the absolute
- magnitude of the vector drawn by the resistive and reactive load
- components. Dealing with line level signals, reactive components
- are going to be negligible, though).
-
- In general, in this modern world, most equipment has a low impedance
- output, going into relatively high impedance input. This wastes some
- amount of power, but because electricity is cheap and it's possible to
- build low-Z outputs easily today, this is not a big deal.
-
- With microphones, it _is_ a big deal, because the signal levels are
- very low, and the drive ability poor. As a result, we try and get the
- best efficiency possible from microphones to get the lowest noise
- floor. This is often done by using transformers to step up the voltage
- or step it down, to go into a higher or lower Z load. Transformers
- have some major disadvantages in that they can be significant sources
- of nonlinearity, but back in the days of tubes they were the only
- solution. Tubes have a very high-Z input, and building balanced inputs
- with tubes requires three devices instead of one. As a result, all
- mike preamps would have a 600 ohm balanced input, with a transformer,
- driving a preamp tube. Today, transistor circuits can be used for
- impedance matching, although they are often more costly and can be noisier
- in cases.
-
- As a result of the expense, consumer equipment was built with high-Z
- microphone inputs, and high-Z microphones. This resulted in more noise
- pickup problems, but was cheaper to make. Unfortunately this still
- held on into the modern day of the transistor, and a lot of high-Z
- consumer gear exists. Guitar pickups are generally high-Z devices,
- and require a direct box to reduce the impedance so that they can go into
- a standard 600 ohm mike preamp directly.
-
- Many years ago, the techniques that were used in audio came originally
- from telephone company practice. Phone systems operate with 150 or 600
- ohm balanced lines, and adoption of this practice into the audio industry
- caused those standards to be used. In the modern age where lines are
- relatively short and transformers considered problematic, the tendency
- has been to have low-Z outputs for all line level devices, driving
- high-Z inputs. While this is not the most efficient system, it is relatively
- foolproof, and appears on most consumer equipment. A substantial amount of
- professional gear, however, still uses internal balancing transformers or
- resistor networks to match to a perfect 600 ohm impedance. [Scott]
-
- [Ed. note: Modern equipment works on principles of voltage transfer
- rather than power transfer. Thus a standard audio circuit today is
- essentially a glorified voltage divider. You have a very low output
- impedance and a very high input impedance such that the most voltage
- is dropped across the load. This is not an impedance-matched circuit
- in the classic sense of the word. Rather, it is a "bridged" or
- "constant voltage" impedance match, and is the paradigm on which
- nearly all audio circuits operate nowadays. -Gabe]
-
- --
- Q3.3 - What is the difference between dBv, dBu, dBV, dBm, dB SPL, and plain
- old dB? Why not just use regular voltage and power measurements?
-
- Our ears respond logarithmically to increases in sound pressure level.
- In order to simplify the calculations of these levels, as well as the
- electrical equivalents of them in audio systems, the industry uses a
- logarithmic system to denote the values. Specifically, the decibel is
- used to denote logarithmic level above a given reference. For
- instance, when measuring sound pressure level, the basic reference
- against which we take measurements is the threshold of hearing for
- the average individual, 10^-12 W/m^2. The formula for dB SPL then
- becomes:
-
- 10 Log X / 10^-12 where X is the intensity in W/m^2
-
- The first people who were concerned about transmitting audio over
- wires were, of course, the telephone company. Thanks to Ma Bell we
- have a bunch of other decibel measurements. We can use the decibel to
- measure electrical power as well. In this case, the formula is
- referenced to 1 milliwatt in the denominator, and the unit is dBm. 1
- milliwatt was chosen as the canonical reference by Ma Bell. Since
- P=V^2 / R, we can also express not only power gain in dB but also
- voltage gain. In this case the equation changes a bit, since we have
- the ^2 exponent. When we take the logarithm, the exponent comes
- around into the coefficient, making our voltage formula 20 log.
- In the voltage scenario, the reference value becomes 0.775 V (the
- voltage drop across 600 ohms that results in 1 mW of power). The
- voltage measurement unit is dBv.
-
- The Europeans, not having any need to abide by Ma Bell's choice for a
- canonical value, chose 1V as their reference, and this is reflected
- as dBV instead of dBv. To avoid confusion, the Europeans write the
- American dBv as dBu. Confused yet? [Gabe]
-
- --
- Q3.4 - Which is it for XLRs? Pin 2 hot? Or pin 3 hot?
-
- Depends on whom you ask! Over the years, different manufacturers have
- adopted varying standards of pin 2 hot and pin 3 hot (and once in a
- while, pin *1* hot!). But nowadays most manufacturers have adopted
- pin 2 hot. Still, it is worth taking the extra minute or two to check
- the manual. The current AES standard is pin 2 hot. [Gabe]
-
- --
- Q3.5 - What is phantom power? What is T-power?
-
- Condenser microphones have internal electronics that need power
- to operate. Early condenser microphones were powered by
- batteries, or separate power supplies using multi-conductor
- cables. In the late 1960's, German microphone manufacturers
- developed 2 methods of sending power on the same wires that carry
- the signal from the microphone.
-
- The more common of these methods is called "phantom power" and is
- covered by DIN spec 45596. The positive terminal of a power
- supply is connected through resistors to both signal leads of a
- balanced microphone, and the negative terminal is connected to
- ground. 48 volts is the preferred value, with 6800 ohm resistors
- in each leg of the circuit, but lower voltages and lower resistor
- values are also used. The precise value of the resistors is not
- too critical, but the two resistors must be matched within 0.4%.
-
- Phantom power has the advantage that a dynamic or ribbon mic may
- be plugged in to a phantom powered microphone input and operate
- without damage, and a phantom powered mic can be plugged in to
- the same input and receive power. The only hazard is that in case
- of a shorted microphone cable, or certain old microphones having
- a grounded center tap output, current can flow through the
- microphone, damaging it. It's a good idea anyway to check cables
- regularly to see that there are no shorts between any of the
- pins, and the few ribbon or dynamic microphones with any circuit
- connection to ground can be identified and not used with phantom
- power.
-
- T-power (short for Tonaderspeisung, also called AB or parallel
- power, and covered by DIN spec 45595) was developed for portable
- applications, and is still common in film sound equipment.
- T-power is usually 12 volts, and the power is connected across
- the balanced pair through 180 ohm resistors. Only T-power mics
- may be connected to T-power inputs; dynamic or ribbon mics may be
- damaged and phantom powered mics will not operate properly. [David]
-
- --
- Q3.6 - How do I interconnect balanced and unbalanced components?
-
- First, let's define what the terms mean. The simplest audio
- circuit uses a single wire to carry the signal; the return path,
- which is needed for current to flow in the wire, is provided
- through a ground connection, usually through a shield around the
- wire. This system, called unbalanced transmission, is very
- susceptible to hum pickup and cannot be used for low level
- signals, like audio, for more than a few feet. Balanced
- transmission occurs when two separate and symmetrical wires are
- used to carry the signal. A balanced input is sensitive only to
- voltage that appears between the two input terminals; signals
- from one terminal to ground are canceled by the circuit.
-
- The simplest way to connect between balanced and unbalanced
- equipment is to use a transformer. The signals are magnetically
- coupled through the core of the transformer and either side may
- be balanced or unbalanced. Good transformers are expensive,
- however, and there are cheaper methods that can be used in some
- instances.
-
- An unbalanced output can be connected to a balanced input. For
- instance, from the unbalanced output of a CD player, connect the
- center pin to pin 2 of the balanced XLR input connector, and the
- ground to pins 1 and 3. To connect the balanced output of something
- to an unbalanced input requires different techniques depending on
- whether the output is active balanced (each side has a signal with
- respect to ground) or floating balanced (for instance, the secondary
- of a transformer with no center-tap connection). If it's an active
- balanced output, you can simply use half of it; connect pin 2 to the
- unbalanced input, and pin 1 to ground, leaving pin 3 floating. If this
- doesn't work (no or very weak signal) connect pin 3 of the output to
- pin 1 and ground and leave pin 2 connected to the unbalanced input
- center pin. Some active balanced outputs, particularly microphones,
- use the balanced circuit to cancel distortion, so this hookup may
- result in higher distortion than if a proper balanced-to-unbalanced
- converter such as a differential stage or a transformer were used.
- [David]
-
- --
- Q3.7 - What are ground loops and how do I avoid them?
-
- One of the most difficult troubleshooting tasks for the audio
- practitioner is finding the source of hum, buzz and other
- interfering signals in the audio signal. Often these are caused
- by "ground loops." This unfortunate and inaccurate term (it need
- not be in the "ground" path, and the "loop" is not what causes
- the problem) is poorly understood by most users of audio
- equipment. A better name for this phenomenon is "shared path
- coupling" because it happens when two signals share the same
- conductor path and couple to each other as a result.
-
- Another semantic problem that should be addressed early on is the
- idea that "ground" is one place where all currents go. It's not,
- there's nothing special about calling a signal "ground," current
- still flows through any path that's available to it.
-
- Referring to the discussion above regarding unbalanced signal
- paths, recall that there must be a complete circuit from the
- output of some device, through the input of another device and
- back to the "return" side of the output if any current is to
- flow. Current doesn't flow by itself, it must have a complete
- path. If there are multiple paths over which the current might
- flow, the current will be divided among them with most of the
- current flowing through the path having the least resistance. Any
- available path, regardless of the resistance in it, will carry
- some of the current, it's not a case of all the current following
- the path that has least resistance.
-
- For example, suppose we have two units connected together through
- a small piece of coaxial cable, and the units are also connected
- together at the wall outlet through their grounded power cords --
- the ground pins are connected to the chassis at each end. The
- audio signal goes along the center of the coaxial cable, and part
- of it might come back along the shield of the coax, but part will
- also go through the ground wire of one unit and back through the
- ground wire of the other unit. A problem arises when some other
- signal is also flowing through this same return path. The other
- signal might be another audio signal, video, data, or power. All
- of the currents in a wire add together, and the resistance of the
- wire causes a voltage to appear in proportion to the current
- flowing. All of these voltages add together, so there is a little
- bit of the video signal added to the audio, some of the power
- signal added to the video, some of the power signal added to the
- audio, etc. In rare instances, the "loop" of wire formed by the
- intended ground return path and the happenstance lower resistance
- return path formed by mounting hardware, power cords, etc. can
- form a magnetic pickup as well, so that magnetic fields radiated
- by transformers, CRT's, etc. can also induce a current in the
- "loop," which makes yet another source of noise voltage.
-
- This shared path coupling is a constant problem with unbalanced
- audio systems. Lots of different methods have been tried to get
- around the problem, many of them dangerous. Clipping off the
- ground leads of equipment so there is no common power line path
- between them simply makes any fault or leakage current follow
- some other path, back through the signal cable to some equipment
- that has a ground -- perhaps through the user's body, if all the
- ground pins have been removed. The only general solution to
- "ground loop" coupling with unbalanced equipment is to connect
- all the chassis together with a very low resistance path (copper
- strap or braid, for example), on the principle that since the
- resistance is so low, any leakage current will produce a
- correspondingly low signal voltage. It may also be effective to
- interrupt the ground path of shield conductors over signal wires;
- force the return path to go through the designated common strap
- while leaving the shield in place only for electrostatic
- screening.
-
- With balanced equipment, no current should be flowing in the
- shield conductors, and in fact performance should be identical
- with the shield left disconnected at one end (preferably the
- receiver end). Therefore balanced systems should be impervious to
- shared path coupling or "ground loop" problems but in fact they
- aren't, because most signals inside a given piece of equipment
- are unbalanced, and there are often return paths internal to the
- equipment that can be shared with return paths between other
- units of equipment connected to it. Especially with mixed
- digital, video and audio signals and high gain, high negative
- feedback amplifier circuitry, this can be a big problem -- small
- currents can create big effects -- and this brings us to the next
- question. [David]
-
- --
- Q3.8 - What is the "Pin 1 problem" and how do I avoid it?
-
- This is a special case of "ground loop" or shared path coupling.
- Recently this has been discussed in great detail and clarity by a
- group led by the consultant Neil Muncy of Toronto. Suppose you
- have a mixer, whose balanced output is connected to an
- amplifier's balanced input through a correctly wired cable. Both
- units are powered from the AC mains and one or both have some
- small amount of AC leakage current that travels to ground through
- all available ground paths -- including the shield of the cable
- that connects the two units. So far so good, no harm done because
- the circuit is balanced and any common mode voltage from current
- flowing through the shield will be canceled by the amplifier
- input. However... a small part of this leakage current also
- travels through the shield of the wire going from the back panel
- XLR connector to the PC board, through some "ground" traces on
- the PC board, and back out through the power line ground cable.
- No problem so far, except that some gain stage on that same PC
- board also uses that piece of ground trace in its negative
- feedback loop, and some part of that leakage signal will be added
- to the signal in that gain stage; it might be video, or data, or
- another audio signal, or (most commonly) power.
-
- The solution to this variant of shared path coupling is the same
- sort of approach that applies to other unbalanced signals: give
- the leakage current a very low resistance path to follow, and
- remove as many of the shared paths as possible. Within a unit of
- equipment, all the XLR connectors' pin 1 terminals should be
- connected to ground with very low resistance (big) wire or
- traces, and preferably all of the ground connections should be
- made at one point, the so-called "star ground" system. A brute
- force approach is to assume that the back panel is the star
- ground, and wire every connector's pin 1 solidly to the panel as
- directly as possible, and lift all the ground wires but one that
- go from the connectors to the circuitry. In this way, all the
- external leakage currents (the "fox" to use Neil Muncy's term)
- will be conducted through the back panel and out of the way,
- rather than running them through the ground traces on the PC
- board where they will mix with internal low level signals in high
- gain stages (the "hen house"). Individual wires can be run from
- points on the circuit board that need to be at "ground" potential
- to a common point on the back panel, which is designated a "zero
- signal reference point" (ZSRP). Equipment that has a reputation
- for being "quiet" and easy to use in many different applications
- is often found to be wired this way, while equipment that is
- "temperamental" if often found to be wired in such a way that
- leakage currents are easily coupled to internal signal lines.
-
- There's a simple test that can be done to check equipment
- susceptibility to this problem. Connect the output, preferably
- balanced and floating, of an ordinary audio oscillator to the pin
- 1 of any two XLR connectors on the equipment. Now operate the
- equipment through its various modes, gain settings, etc. You may
- be surprised to find the audio oscillator's signal appearing in
- many different places in the equipment. [David]
-
- =====
- Section IV - Analog tape recording
-
- --
- Q4.1 - What does it mean to "align" a tape machine?
-
- There are a number of standard adjustments on any analogue tape
- machine, which can roughly be broken up into mechanical and electronic
- adjustments. The mechanical adjustments include the head position
- (height, skew, and azimuth), and sometimes tape speed. Incorrect head
- height will result in poor S/N and leakage between channels, because
- the tracks on the head do not match up exactly with those on the tape.
- Incorrect tape skew will result in level differences between channels
- and uneven head wear, because there is more pressure on the top of the
- head than the bottom (or vice versa). Incorrect azimuth will result
- in loss of high frequency response and strange skewing of the stereo
- image. Tape speed error will result in tonal shifts, although on many
- machines with capstan speed controlled by crystal or line frequency,
- it is not adjustable.
-
- Electronic adjustments include level and bias adjustments for each
- channel. Some machines may have bias frequency adjustments, equalization
- adjustments for playback and record emphasis, pre-distortion adjustments,
- and a varied bevy of adjustments for noise reduction systems.
-
- Alignment is relatively simple, and the same general method applies
- from the smallest cassette deck to the largest multitrack machine.
- First, put a test tape on the machine. Use a real reference tape,
- from the manufacturer, from MRL, or a similarly legitimate lab. DO
- NOT EVER use a homebrew test tape that was recorded on a "known good"
- machine. You will regret it someday. Spend the money and get a real
- test tape (and not one of the flaky ones from RCA).
-
- 1. Speed adjustment (if necessary). Play back a 1 KHz reference tone
- and, using a frequency counter, adjust the tape speed for proper
- frequency output. There are strobe tapes available for this as well,
- but with cheap frequency counters available, this method is much easier.
-
- 2. Head height and skew adjustments. Better see your machine's manual
- on this one, because I have seen a variety of ways of doing this.
-
- 3. Azimuth adjustment. I find the easiest way to do this is to take
- the left and right outputs and connect them to the X and Y inputs
- of an oscilloscope, and play back a 1 KHz reference tone, while
- adjusting the azimuth until a perfectly-diagonal line appears.
- You can do this by ear if you are desperate, but I strongly recommend
- the lissajous method, which is faster and more accurate. On multitrack
- decks, use the two tracks as close as possible to the edge of the tape.
- Now you have the playback head azimuth set... put a 1 KHz source into
- the record input, with a blank tape on the machine, and adjust the
- azimuth of the record head for the proper diagonal line.
-
- 4. Playback eq adjustment (if necessary). This is a case of playing
- back various test tones at different frequencies, and adjusting the
- response curve of the deck to produce a flat output. You can also
- do this by playing back white noise and using a third-octave spectrum
- analyzer of great accuracy to adjust for flat response. Again, this
- is one to check your deck's manual for, because the actual
- adjustments vary from one machine to another, and you will want to
- use the test tape once again.
-
- 5. Record eq adjustment (if necessary). How this is done (and whether you
- want to do it after biasing the tape) depends a lot on your deck.
-
- 6. Bias adjustment. There are a lot of ways to do this. My favorite method
- is to use a white noise source, and adjust the bias until the source and
- tape output sound identical. Some people prefer to use a signal generator
- and set so that the levels of recorded tones at 1 KHz and 20 KHz are
- identical. I find I can get within .5 dB by ear, though your mileage
- may differ. [Ed. note: Many tapes have recommended overbias settings,
- and many decks will also provide a chart that correlates the amount of
- overbias against available tape formulations. -Gabe]
-
- 7. Record level adjustment. I use a distortion analyzer, and set the level
- so that at +3 dB, I get 3% distortion on the output. Some folks who are
- using very hot tape set the machines so that a certain magnetic flux is
- produced at the heads given a certain input, but I find setting for
- a given distortion point does well for me. If you don't have a distortion
- analyzer, use a 1 KHz tone source and set so that you have the onset of
- audible distortion at +3 dB, and you will be extremely close.
-
- [Ed. note: The traditional way to do this is to align the repro side
- of the machine using a calibration tape, and then to put the machine
- into record. Monitoring off the repro head, the operator then aligns
- the record electronics until the output is flat. -Gabe]
-
- At this point, you will be pretty much set. Whether you want to do this
- all on a regular basis is a good question. You should definitely go
- through the complete procedure if you ever change brands of tape. Checking
- the mechanical parameters on a regular basis is a good idea with some
- decks (like the Ampex 350), which tend to drift. Clean your heads
- every time you put a new reel on, and demagnetize regularly. [Scott]
-
- --
- Q4.2 - What is bias? What is overbias?
-
- With just the audio signal applied to a tape, the frequency response
- is very poor. High frequency response is much better than low
- frequency, and the low frequency distortion is very high. In 1906,
- the Poulson Telegraphone managed to record an intelligible voice on a
- magnetic medium, but it was not until the 1930s when this problem was
- solved by German engineers.
-
- To compensate for the tape characteristic, a very high frequency
- signal is applied to the tape in addition to the audio. This is
- typically in the 100 KHz range, far above the audio range. With the
- bias adjusted properly, the frequency response should be flat across
- the audible range. With too low bias, bass distortion will be the
- first audible sign, but with too much bias, the high frequency
- response will drop off.
-
- Incidentally, digital recording equipment takes advantage of the very
- nonlinearity that is a problem with analogue methods. It records a
- square wave on the tape, driving the tape into saturation at all
- times, and extracts the signal from the waveform edges. As a result,
- no bias is required. (For a good example of the various digital
- recording methods, check out NASA SP 5038, _Magnetic Tape Recording_.)
- [Scott]
-
- [Ed. note: For those looking for an understanding of why we need
- bias in the first place, here is one way to think about it. Tape
- consists of lots of small magnetic particles called domains. These
- domains are exposed to a magnetic field from the record head and
- oscillate in polarity as the AC signal voltage changes. Domains,
- being physical objects, have inertia. Every time the analog signal
- crosses from positive to negative and back again, the voltage passes
- the zero point for an instant. At this moment, the domain is at rest,
- and like any other physical object, there is a short period of inertia
- before it gets moving again. The result is the bizarre high-frequency
- performance characteristic that Scott described. The high frequency of
- a bias signal simply ensures that the domains are always kept in motion,
- negating the effect of inertia at audio frequencies. -Gabe]
-
- --
- Q4.3 - What is the difference between Dolby A, B, C, S, and SR? How do each
- of these systems work?
-
- The Dolby A, B, C, SR, and S noise reduction (NR) systems are non-linear
- level-dependent companders (compressors/expanders). They offer various
- amounts of noise reduction, as shown in the table below.
-
- Dolby HF NR LF NR Number Of Active Target
- System Effect Effect Frequency Bands Market Year
- ------ ------ ------ ---------------------------- --------- ----
- A 10 dB 10 dB 4 fixed Pro audio 1967
- B 10 dB -- 1 sliding (HF) Domestic 1970
- C 20 dB -- 1 sliding (HF) Domestic 1981
- SR 24 dB 10 dB 1 sliding (HF), 1 fixed (LF) Pro audio 1986
- S 24 dB 10 dB 1 sliding (HF), 1 fixed (LF) Domestic 1990
- ------ ------ ----- ---------------------------- --------- ----
-
- The band-splitting system used with Dolby A NR is a relatively costly
- technique, although it can deal with noise at all frequencies. The
- single sliding band techniques used in Dolby B and C systems are less
- costly, making them more suitable for consumer tape recording
- applications where the dominant noise contribution occurs at high
- frequencies.
-
- The typical on-record frequency response curves for the Dolby B NR
- system look something like those depicted below. The curves for Dolby C,
- SR, and S are similar, but the actual response levels and behaviour at
- high frequencies are modified to extract better performance form these
- more advanced systems.
-
- |
- 0dB -|----------------------------------------------------
- |
- |
- -10dB -|
- | /-------------------------------
- | /
- -20dB -|------------------/
- |
- |
- -30dB -| /-----------------------------
- | /
- | /
- -40dB -|----------------/
- |______________________________________________________
- | | |
- 20Hz 1kHz 20kHz
-
- The above picture attempts to show that the encoding process provides
- selective boost to high frequency signals (decoding is the exact
- reciprocal), and the curves correspond to the results achieved when no
- musical signal is applied. The amount of boost during the compansion
- depends on the signal level and its spectral content. For a tone at
- -40dB at 3 kHz, the boost applied to signals with frequencies above this
- would probably be the full 10dB allowed by the system. If the same tone
- were at a level of -20dB, then the boost would be less, maybe about 5dB.
- If the tone was at 0dB, then no boost would be supplied, as tape
- saturation would be increased (beyond it's normal amount).
-
- The single band of compansion utilized with Dolby B NR reaches
- sufficiently low in frequency to provide useful noise reduction when no
- signal is present. Its width changes dynamically in response to the
- spectral content of music signals. As an example, when used with a solo
- drum note the companding system will slide up in frequency so that the
- low frequency content of the drum will be passed through at its full
- level. On replay, the playback of the bass drum is allowed to pass
- through without modification to its level, while the expander lowers the
- volume at high frequencies above those of the bass drum, thus providing
- a reduction in tape hiss where there is no musical signal. If a guitar
- is now added to the music signal, the companding band slides further up
- in frequency allowing the bass drum and guitar signals through without
- any compansion, while still producing a worthwhile noise reduction
- effect at frequencies above those of the guitar.
-
- The Dolby B NR system is designed to start taking effect from 300Hz, and
- its action increases until it reaches a maximum of 10dB upwards of 4kHz.
- Dolby C improves on this by taking effect from 100Hz and providing about
- 15dB of NR at 400Hz, increasing to a maximum of 20dB in the critical
- hiss region from 2kHz to 10kHz. Dolby C also includes spectral skewing
- networks which introduce a rolloff above 10kHz prior to the compander
- when in encoding mode. This helps to reduce compander errors caused by
- unpredictable cassette response above 10kHz, and an inverse boost is
- added after the expander to compensate. Although this reduces the noise
- reduction effect above 10kHz, the ear's sensitivity to noise in that
- region is diminished, and the improved encode/decode tracking provides
- important improvements in overall system performance. An anti-saturation
- shelving network, beginning at about 2kHz, also acts on the high
- frequencies but it only affects the high-level signals that would cause
- tape saturation. A complementary network is provided in the decode chain
- to provide overall flat response.
-
- When the tape is played back, the inverse of the above process takes
- place. For an accurate decoding to occur, it is necessary that playback
- takes place with no offsets in levels between record and replay. i.e. If
- a 400 Hz tone is recorded at 0dB (or -20dB), then it must play back at
- 0dB (or -20dB). This will help ensure correct Dolby "tracking".
-
- Just think about it: if a -40dB tone at 8kHz was recorded with
- Dolby B on, then it would actually have a level of -30dB on tape.
- The same tone, if it were at a -20dB level, would have a level of
- about -15dB on tape. If the sensitivity of the tape was such
- that anything recorded at 0dB actually went on tape as -10dB,
- then you can see that the Dolby encoded tones would actually be
- at a lower level, and the system would have no way of determining
- this. It assumes 0dB in = 0dB out. Hence the signal would be
- decoded with the incorrect amount of de-boost.
-
- The Dolby SR and S NR systems provide slightly more NR than Dolby C at
- high frequencies, 24dB vs 20dB, but they also achieve a 10dB NR effect
- at low frequencies below 200Hz as well. This is obtained using a
- two-band approach, the low-frequencies being handled by a fixed-band
- processor, while the high frequencies are tackled by a sliding band
- processor. This reduces the potential for problems such as "noise
- pumping", caused by high-level low frequency transient signals (bass
- notes from drums, double basses, organs), raising the sound level in a
- cyclic fashion. Dolby SR and S also contain the spectral skewing and
- anti-saturation circuits for high-level high-frequency signals that are
- implemented with Dolby C. The performance of the sliding band is
- improved over that obtained with Dolby B and C NR systems by reducing
- the degree of sliding that occurs in the presence of high-frequency
- signals. This increases the noise reduction effect available at
- frequencies below those occurring in the music signal.
-
- An additional benefit of the Dolby S NR system for consumers is that the
- manufacturers of cassette decks who are licensed to use the system must
- adhere to a range of strict performance standards. These include an
- extended high frequency response, tighter overall response tolerances, a
- new standard ensuring head height accuracy, increased overload margin in
- the electronics, lower wow and flutter, and a head azimuth standard.
- These benefit users by enhancing the performance of cassette recorders
- as well as helping to ensure that tapes recorded on one deck will play
- back accurately on any other. [Witold Waldman - witold@aed.dsto.gov.au]
-
- --
- Q4.4 - What is Dolby HX-Pro?
-
- HX-Pro is a scheme to reduce the level of the bias signal when high
- frequency information is present in the recorded signal. Sufficient
- high frequency information will act to bias the tape itself, and by
- reducing the AC bias signal somewhat, additional signal can be applied
- without saturating the tape. This is a single-ended system; it
- requires no decoding on playback, because it merely permits more
- signal to be recorded on the tape. In theory it is an excellent idea,
- and some implementations have lived up to the promise of the method,
- although some other implementations have produced unpleasant
- artifacts. [Scott]
-
- --
- Q4.5 - How does DBX compare to Dolby?
-
- [Anyone? Anyone? -G]
-
- --
- Q4.6 - How much better are external microphone preamplifiers than those
- found in my portable recorder?
-
- Going by the rule that "external is better than internal," the
- external preamps are likely to sound better. Besides the issue of
- electrical shielding and interaction, it is simply the case that a
- designer who is spending *all* his time on a project designing only a
- preamp is likely to do a better job of it than a tape machine design
- team that has to worry how they're going to fit the preamp into the
- box and still have enough room for the rest of the tape machine. [Gabe]
-
- --
- Q4.7 - What is an MRL? Where do I get one?
-
- An MRL is a reference alignment tape from Magnetic Reference Laboratory.
- These tapes, available in every conceivable tape speed, tape width,
- equalization, and field strength, contain alignment tones useful in
- calibrating the electronics of analog tape machines.
-
- These tapes can be ordered from many pro audio dealers. If not, you
- can contact MRL directly at:
-
- Magnetic Reference Laboratory, Inc.
- 229 Polaris Avenue, Suite 4
- Mountain View, CA 94043
-
- Tel: (415) 965-8187
- Fax: (415) 965-8548
-
- =====
- Section V - Digital recording and interconnection
-
- --
- Q5.1 - What is sampling? What is a sampling rate?
-
- Sampling can be (roughly) defined as the capture of a continuously
- varying quantity at a precisely defined instant in time. Most usually,
- signals are sampled at a set of sample-points spaced regularly in
- time. Note that sampling in itself implies nothing about the
- representation of sample magnitude by a number. That process is called
- quantisation.
-
- The Nyquist theorem states that in order to faithfully capture all of
- the information in a signal of one-sided bandwidth B, it must be
- sampled at a rate greater than 2B. A direct corollary of this is that
- if we wish to sample at a rate of 2B then we must pre-filter the
- signal to a one-sided bandwidth of B, otherwise it will not be
- possible to accurately reconstruct the original signal from the
- samples. The frequency 2B that is the minimum sample rate to retain
- all of the signal information is called the Nyquist frequency.
-
- The spectrum of the sampled signal is the same as the spectrum of the
- continuous signal except that copies (known as aliases) of the
- original now appear centred on all integer multiples of the sample
- rate. As an example, if a signal of 20 kHz bandwidth is sampled at 50
- kHz then alias spectra appear from 30 - 70 kHz, 80 - 120 kHz, and so
- on. It is because the alias spectra must not overlap that a sample
- rate of greater than 2B is required. In digital audio we are
- concerned with the base-band - that is to say the signal components
- which extend from 0 to B. Therefore, to sample at the standard digital
- audio rate of 44.1 kHz requires the input signal to be band-limited to
- the range 0 Hz to 22.05 kHz. [Chris]
-
- --
- Q5.2 - What is oversampling?
-
- To take distortionless samples at 44.1kHz requires that the analogue
- signal be bandlimited to 22.05kHz. Since the audio band is reckoned to
- extend to 20kHz we require an analogue filter that cuts off very
- sharply between 20kHz and 22kHz to accomplish this. This is expensive,
- and suffers from all the ailments associated with analogue
- electronics.
-
- Oversampling is a technique whereby some of this filtering may be done
- (relatively cheaply and easily) in the digital domain. By sampling at
- a high rate (for example 4 times 44.1kHz, or 176.4kHz) the analogue
- filter can have a much lower slope since its transition band is now
- 20kHz to 88kHz (ie half of 176kHz). The samples are then passed
- through a digital filter with a sharp cutoff at 20kHz, after which
- three of every four are discarded, resulting in the sample stream at
- 44.1kHz that we require. [Chris]
-
- --
- Q5.3 - What is the difference between a "1 bit" and a "multibit" converter?
- What is MASH? What is Delta/Sigma? Should I really care?
-
- Audio data is stored on CD as 16-bit words. It is the job of the
- digital to analogue converter (DAC) to convert these numbers to a
- varying voltage. Many DAC chips do this by storing electric charge in
- capacitors (like water in buckets) and selectively emptying these
- buckets to the analogue ouput, thereby adding their contents. Others
- sum the outputs of current or voltage sources, but the operating
- principles are otherwise similar.
-
- A multi-bit converter has sixteen buckets corresponding to the sixteen
- bits of the input word, and sized 1, 2, 4, 8 ... 32768 charge units.
- Each word (ie sample) decoded from the disc is passed directly to the
- DAC, and those buckets corresponding to 1's in the input word are
- emptied to the output.
-
- To perform well the bucket sizes have to be accurate to within +/-
- half a charge unit; for the larger buckets this represents a tolerance
- tighter than 0.01%, which is difficult. Furthermore the image
- spectrum from 24kHz to 64kHz must be filtered out, requiring a
- complicated, expensive filter.
-
- Alternatively, by using some digital signal processing, the stream of
- 16-bit words at 44.1kHz can be transformed to a stream of shorter
- words at a higher rate. The two data streams represent the same signal
- in the audio band, but the new data stream has a lot of extra noise in
- it resulting from the wordlength reduction. This extra noise is made
- to appear mostly above 20kHz through the use of noise-shaping, and the
- oversampling ensures that the first image spectrum occurs at a much
- higher frequency than in the multi-bit case.
-
- This new data stream is now converted to an analogue voltage by a DAC
- of short word length; subsequently, most of the noise above 20kHz can
- be filtered out by a simple analogue filter without affecting the
- audio signal.
-
- Typical configurations use 1-bit words at 11.3MHz (256 times over-
- sampled), and 4-bit words at 2.8MHz (64 times oversampled). The
- former requires one bucket of arbitrary size (very simple); it is the
- basis of the Philips Bitstream range of converters. The latter
- requires four buckets of sizes 1, 2, 4 and 8 charge units, but the
- tolerance on these is relaxed to about 5%.
-
- MASH and other PWM systems are similar to Bitstream, but they vary the
- pulse width at the output of the digital signal processor. This can be
- likened to using a single bucket but with the provision to part fill
- it. For example, MASH allows the bucket to be filled to eleven
- different depths (this is where they get 3.5 bits from, as 2^(3.5) is
- approximately eleven).
-
- Lastly it is important to note that these are all simply different
- ways of performing the same function. It is easy to make a lousy CD
- player based around any of these technologies; it is rather more
- difficult to make an excellent one, regardless of the DAC technology
- employed. Each of the conversion methods has its advantages and
- disadvantages, and as ever it is the job of the engineer to balance a
- multitude of parameters to design a product that represents value for
- money to the consumer. [Chris]
-
- --
- Q5.4 - On an analog recorder, I was always taught to make sure the signal
- averages around 0 VU. But on my new DAT machine, 0 is all the way at
- the top of the scale. What's going on here?
-
- Analog recorders are operated such that the signal maintains a nominal
- level that strikes a good balance between signal-to-noise ratio and
- headroom. Further, since analog distorts very gently, you often can
- exceed your headroom in little bits and not really notice it.
-
- Digital is not nearly as forgiving. Since digital represents audio as
- numerical values, higher levels will eventually force you to run out
- of numbers. As a result, there is an absolute ceiling as to how hot
- you can record. If you record analog and have a nominal 12 dB of
- headroom, you'll probably be okay if you have one 15 dB transient that
- lasts for 1/10th of a second. The record amps _might_ overload, the
- tape _might_ saturate, but you'll probably be fine. In a digital
- system, those same 3 dB of overshoot would cause you to clip hard. It
- would not be subtle or forgiving. You would hear a definite snap as
- you ran out of room and chopped the top of your waveform off.
-
- The reality is that digital has NO HEADROOM, because there is no
- margin for overshoot. You simply must make sure that the entire
- dynamic range of the signal fits within the limits of the dynamic
- range of your recorder, without exception. The only meaningful
- absolute on a digital recorder, therefore, is the point at which you
- will go into overload. The result is the metering system we now have.
- 0 dB represents digital ceiling, or full-scale. The negative numbers
- on the scale represents your current level relative to the ceiling.
-
- Thus, to return to our example, if you have a transient with 15dB of
- overshoot past your nominal level, you must then place your nominal
- level at a maximum of -15 dB. 0 dB on the meters is the absolute limit
- of what you can record. [Gabe]
-
- --
- Q5.5 - Why doesn't MiniDisc or Digital Compact Cassette sound as good as DAT
- or CD? After all, they're both digital.
-
- Both MD and DCC use lossy compression algorithms (called ATRAC and
- PASC respectively); crudely, this means that the numbers coming out of
- the machine are not the same as those that went in. The algorithms use
- complex models of the way the ear works to discard the information
- that it thinks would not be heard anyway.
-
- For example, if a pin dropped simultaneously with a gunshot, it may be
- reasonable to suggest that it isn't worth bothering to record the
- sound of the pin! In fact it turns out that around 75 to 80 per cent
- of the data for typical music can be discarded with surprisingly
- little quality loss.
-
- However, nobody denies that there is a quality loss, particularly
- after a few generations of copying. This fact and others make both MD
- and DCC useful only as a consumer-delivery format. They have very
- little use in the studio as a recording or (heaven forbid!) mastering
- format. [Chris]
-
- --
- Q5.6 - What is S/P-DIF? What is AES/EBU?
-
- AES/EBU and S/P-DIF describe two similar protocols for communicating
- two-channel digital audio information over a serial link. They are
- slightly different in details, their basic format is almost identical,
- but there are enough differences that the two are, for all intents and
- purposes electrically incompatible. Both of these digital protocols
- are described fully in an international standard, IEC 958, available
- from the International Electrotechnical Commission.
-
- AES/EBU (which stands for the joint Audio Engineering Society/European
- Broadcasting Union standard) is the so-called "professional" protocol.
- It uses standard 3-pin XLR connectors and 110-ohm balanced
- differential cables for connection (no, standard microphone cables,
- not even good quality cables, won't work, even though it seems they
- might) and a 5 volt, differential signal.
-
- S/P-DIF (which stands for Sony/Philips Digital InterFace, a now
- obsolete standard superseded by IEC 958) is the so-called "consumer"
- format. It uses what appears to be standard RCA connectors and cables,
- but, in fact, require 75-ohm connectors and cables. Good quality video
- "patch" cables have proven adequate (no, standard "audio" patch cords,
- even excellent quality versions, have been shown not to work). The
- signals are 0.5 volts unbalanced.
-
- The actual datastream, are very similar. Each sample period a "frame"
- is transmitted. Each frame consists of two "subframes", one each for
- left and right channels, each subframe is 32 bits wide. In that
- subframe, 4 bits are used for synchronization, then up to 24 bits are
- usable for audio (the "consumer mode" format is limited to 16 bits).
- The remaining four bits are used for parity (the first level of error
- detection), validity, user status and channel status. 192 subframes
- are collected, and the 192 user bits and 192 channel status bits are
- collected into separate 24 8 bit status bytes for each channel.
-
- The channel status bytes are interesting, because they contain the
- important control information and the major differences between the
- two protocol formats. One bit tells whether the data stream is
- professional or consumer format. There are bits that specify
- (optionally) the sample rate, deemphasis standards, channel usage, and
- other information. The consumer format has several bits allocated to
- copy protection and control: the SCMS bits.
-
- Now, the notion that all of this is encoded in a standard may be
- reassuring, but a standard is nothing but a voluntary statement of
- common industry practice. There is a lot of incompatibility between
- equipment out there caused directly by subtle differences between
- interpretations and implementations. The result is that some
- equipment simply refuses to talk to each other. Even THAT
- possibility is stated in the standard! [Dick]
-
- --
- Q5.7 - What is clock jitter?
-
- Clock jitter is a colloquialism for what engineers would readily call
- time-domain distortion. Clock jitter does not actually change the
- physical content of the information being transmitted, only the time
- at which it is delivered. Depending on circumstance, this may or may
- not affect the ultimate decoded output.
-
- Let's look at this a little more closely. Digital audio is sent as a
- set of binary digits....1's and 0's. But that is only a logical
- construct. In order to transmit binary math electrically, we use
- square waves. Realize that although we have two mathematical states,
- we have to transmit such a construct using control voltages and
- comparators.
-
- All digital audio systems start with a crystal controlled oscillator
- producing a square wave signal that is used to synchronize the entire
- digital audio sampling and playback processes. Now, for a clock, we
- don't really care about the fact that the clock might be at state 1 or
- state 0 at any given moment. That doesn't give us any information.
- As a computer, I can't tell if my clock has just gotten to state 1, or
- if it's been sitting there for a microsecond. Thus it isn't the
- states we care about. Instead, we care about the state
- *changes*....when the clock shifts from one state to the other.
-
- Now, in a perfect square wave (no such thing exists), the change of
- state would be instantaneous. BOOM...it's done. But in reality, it
- doesn't work this way. Square waves contain high orders of harmonics.
- Fourier teaches us that all complex waveforms are made up of simpler
- waveforms. Thus, as we run through noisy electronics, long cables,
- inadvertent filtering circuits, we begin to lose some of our
- harmonics. When this happens, our square wave begins to lose form.
-
- The result of this is that our nice sharp corners become rounded. So
- our state changes are no longer precisely at the edge anymore, because
- there is no more edge. The pointy edge is now all fuzzy. It now
- depends on design of the electronic comparator circuit as to when the
- clock state will change, as the stage change has shifted. The clock
- is, essentially, jittering.
-
- People love to bark out "Bits is bits. A copy of a computer file
- works as well as the original." Yes, this is true. But these
- jittering bits can create audible distortion during the digital-to-
- analog conversion, and the industry is working hard to reduce the
- amount of jitter present in digital systems.
-
- Furthermore, emerging research is suggesting that certain types of
- jitter may produce digital copies with eccentricities that result in
- more jittery output on playback. The jury is still out on the
- specifics however. Stay tuned. [Gabe]
-
- --
- Q5.8 - What kind of cable AES/EBU or S/P-DIF cables should I use? How long
- can I run them?
-
- The best, quick answer is what cables you should NOT use!
-
- Even though AES/EBU cables look like orinary microphone cables, and S/P-DIF
- cables look like ordinary RCA interconnects, they are very different.
-
- Unlike microphone and audio-frequency interconnect cables, which are
- designed to handle signals in the normal audio bandwidth (let's say that
- goes as high as 50 kHz or more to be safe), the cables used for digital
- interconnects must handle a much wider bandwidth. At 44.1 kHz, the digital
- protocols are sending data at the rate of 2.8 million bits per second,
- resulting in a bandwidth (because of the biphase encoding method)
- of 5.6 MHz.
-
- This is no longer audio, but falls in the realm of bandwidths used by
- video. Now, considerations such as cable impedance and termination become
- very important, factors that have little or no effect below 50 kHz.
-
- The interface requirements call for the use of 110 ohm balanced cables for
- AES/EBU interconnects, and 75 ohm coaxial unbalanced interconnects for
- S/P-DIF interconnects. The used of the proper cable and the proper
- terminating connectors cannot be overemphasised. I can personally testify
- (having, in fact, looked at the interconnections between many different
- kinds of pro and consumer digital equipment) that ordinary microphone or
- RCA audio interconnects DO NOT WORK. It's not that the results sound
- subtly different, it's that much of the time, it the receiving equipment
- is simply unable to decode the resulting output, and simply shuts down.
-
- Fortunately, there is a ready solution for S/P-DIF cables. Any store that
- sells high quality 75 ohm RCA video interconnect (or "dubbing") connectors
- also sells high-quality S/P-DIF interconnects as well. They may not know
- it, but they do. This is because the signal and bandpass requirements for
- video and S/P-DIF cables are the same. National chains such as Radio Shack
- sell such cables, and the data seems to indicate that they are good digital
- interconnects.
-
- For AES/EBU, there are fewer, less common solutions. Companies such as
- Canare make excellent cables. Professional audio suppliers and distributors
- may be good sources for such cables. If you are handy with a soldering
- iron, then you can purchase 110 ohm balanced shielded cable and make your
- own (which I have done quite successfully). Cables such as Alpha Twinax,
- Carol Twin Coaxial, Belden 9207 twin axial, and the like, all work well for
- this application. Use high-quality XLR connectors (be warned that these
- cables are 0.330 inches in diameter and are a VERY tight fit in the
- neoprene strin reliefs of many connectors: warming them in hot water makes
- them pliable enough to work well).
-
- As to how long these cables can be, it's hard to say. However, a couple of
- general rules apply.
-
- S/P-DIF was NEVER intended to be a long-haul hardware interconnect. The
- relevant specifications talk of interconnect lengths less than 10 meters
- (33 feet). In fact, many pieces of equipment cannot tolerate cables even
- that long, due to the excessive capacitance and possibly induced common
- mode interference.
-
- AES/EBU is more tolerant of longer runs because it is balanced (thus more
- immune to interference) and it's run at a higher signal level (5 volts
- instead of 0.5 volts). The standards "allow signal transmission up to a few
- hundred meters in length."
-
- The reality is that much is highly dependent upon the actual conditions at
- hand. The requirements are that the received signal fit within certain
- requirements of rise time/period and voltage level, the so-called "eye
- diagram". In other words, regardless of what kind of cable you use, if it
- can't move the voltage at the receiver far enough soon enough, it simply
- isn't going to work.
-
- Another complicating factor is that both protocols allow a degree of
- multi-drop capability. This means a single transmitter can drive several
- receivers (the last of which must be terminated with the proper termination
- impedance). However, implementing multi-drop puts more stingent
- requirements on impedance matching. [Dick]
-
- --
- Q5.9 - What is SCMS? How do I defeat it?
-
- SCMS is the Serial Copy Management System, a form of copy protection
- that was mandated by Federal law (the Home Recording Rights Act).
- SCMS consists of a set of subcode flags that indicate to a digital
- recorder whether or not the source may be copied. Under the HRRA,
- consumers are permitted to make one digital generation, but no more.
- Thus when, for instance, the consumer copies a CD onto DAT, the SCMS
- flag is set on the copy, and no further generations can be made.
-
- SCMS is only mandated in consumer machines. Any recorder sold
- through professional channels, and which is intended for use in
- professional applications, does not have to implement it.
-
- There are several professional products, such as Digital Domain's
- FCN-1 format converter, which allow manipulation of the SCMS flags.
- These units exist so that professional engineers may adjust the
- subcode bits of the recordings they produce. [Gabe]
-
- --
- Q5.10 - What is PCM-F1 format?
-
- In the 1980s, before the DAT era, Sony produced a set of PCM adaptors
- that enabled one to record digital audio using a video cassette
- machine. These units had RCA audio connections for input and output,
- as well as video I/O that could be sent to, and received from, the
- VCR. At the time, these systems offered performance far in excess of
- conventional analog recorders available in the price category.
-
- Sony released many models, including the PCM-F1, PCM-501, PCM-601, and
- PCM-701. Perhaps the most interesting is the PCM-601, which has
- S/P-DIF digital I/O. These units are highly prized since they are the
- only units that can be used to make digital transfers of F1 tapes to
- modern hardware.
-
- There are some engineers who insist that, despite the clunkiness of the
- format by modern DAT standards, the F1 series was the best digital format
- ever developed. To this day, it is not surprising to see an F1 encoder
- on a classical recording session. [Gabe]
-
- --
- Q5.11 - How do digital recorders handle selective synchronization?
-
- Selective Synchronization, or "sel-sync" as it is often called, is the
- ability of a recorder to play and record simultaneously, allowing
- synchronous recording of new material onto specific tracks without
- erasing everything on tape. This technique is what makes overdubbing
- possible.
-
- On an analog recorder, audio tracks are discrete entities, and the
- sync head is really just a stack of individual heads, any one of which
- is capable of recording or playing back. Thus sel-sync is a
- relatively simple matter of putting some heads into record and others
- into repro.
-
- In the digital world, the problem is highly complex. First, A/D and
- D/A conversion involves an acquisition delay of several milliseconds.
- Second, and more importantly, digital tracks are not discrete. Rather,
- they are multiplexed together on a tape, along with subcode and other
- non-audio information. So how can you replace one track and leave the
- others untouched?
-
- The answer is a technique called "read before write" (RBW) or "read,
- modify, write" (RMW) which involves a second set of heads. The data
- is read from the tape and flushed into a buffer, where it can be
- modified, and ultimately written back to the tape. Thus when you
- "punch in" on a digital deck, you are physically re-writing all the
- tracks, not just the one you're overdubbing. You are not, however,
- changing the data on any track other than the one you want to
- replace. [Gabe]
-
- --
- Q5.12 - How can a 44.1 kHz sampling rate be enough to record all the
- harmonics of music? Doesn't that mean that we chop off all the harmonics
- above 20 khz? Doesn't this affect the music? After all, analog systems
- don't filter out all the information above 20 kHz, do they?
-
- This whole question is based on the premise that "analog systems don't
- filter out all the information above 20 kHz." Indeed there are mixers
- and power amplifiers and other electronic systems that are capable of
- stunningly wide bandwidth, often exceeding 100 kHz, the same cannot be
- said for the entire analog reproduction chain. The mechanical
- transducers, microphones, speaker and phono cartridges seldom have
- real response far exceeding 20 kHz. In fact, some of the most highly
- regarded large diaphragm condensor microphones often used in very high
- quality recordings seldom exceed 18 kHz bandwidth. Analog tape
- recorders rarely have bandwidths as wide as 25 kHz, and LP
- reproduction systems have similar limitations in reality.
-
- So while it may be possible to send very high frequency ultrasonic
- signals through parts of both analog and digital reproduction chains,
- there are, in both technologies, fundamental and insurmountable limits
- to the bandwidth that, in reality, lead to very similar actual
- reproducible bandwidths in each.
-
- Thus, one of the basic premises of the question is flawed. Analog
- systems DO filter out information above 20 kHz. Further, the frequency
- response and phase errors of even the very best well-maintained analog
- reproduction systems have response errors far exceeding those of even
- middle of the line digital equipment. Whether one person may find
- those errors tolerable or even likeable or not is a matter or personal
- preference that is beyond the scope of this or any other technical
- discussion.
-
- There are a variety of anecdotal tales that are advanced to "prove"
- that the ear can hear far beyond what is conventionally accepted as
- the 20 kHz upper limit (an upper limit that, for the most part,
- applies to young people only: modern high SPL music and noise levels
- has lead to a widespread deterioration in the hearing of the adult
- population at large, and especially amongst young males).
-
- For example, there is an apocryphal story about Rupert Neve that
- tells of a console channel that sounded particularly "bad". It was
- later discovered that it was oscillating at some ultrasonic frequency,
- like 48 kHz. Rupert Neve is rumored to have seized upon this as
- "proof" that the ear can hear well beyond 20 kHz. However, there exist
- an entire range of perfectly plausible mechanisms that require NO
- ultrasonic acuity to detect such a problem. For example, the existence
- of ANY nonlinearity in the system would result in the production of
- intermodulation tones that would fall well within the 20 kHz audio
- band and certainly would make it sound awful. Even the problem that
- was causing the oscillation itself could lead to massive artifacts at
- much lower frequencies that would completely account for the alleged
- sound of the mixer in the complete absence of a 48 kHz "whistle."
-
- Whether 20 kHz is an adequate bandwidth is a debatable subject.
- However, several important facts have to be remembered. First, BOTH
- analog AND digital reproduction systems suffer from roughly the same
- bandwidth limiting. Second, digital systems using properly implemented
- oversampling techniques have far less severe phase and frequency
- response errors within the audible band. No analog storage and
- reproduction system can match the phase and response linearity of a
- digital system, both at low and high frequencies. Once those
- demonstrable facts are acknowledged, then the discussion about
- supra-20 kHz aural detectability can continue, knowing that, if it is
- demonstrated to be significant, both systems are provably deficient.
- [Dick]
-
- --
- Q5.13 - Yeah, well what about square waves? I've seen square wave
- tests of digital systems that show a lot of ringing. Isn't that bad?
-
- Square waves are a mathematically precisely defined signal. One of the
- ways to describe a perfect square wave is as the sum an infinite series
- of sine waves in a precise phase, harmonic and amplitude relationship.
- The relation is:
-
- 1 1 1 1
- F(t) = sin(wt) + -sin(3wt) + -sin(5wt) + -sin(7wt) + -sin(9wt) ...
- 3 5 7 9
-
- where t is time, w is "radian frequency", or 2 pi times frequency.
-
- Remember, we require an infinite number of terms to describe a perfect
- square wave. If we limit the number of terms to, say, 10 terms, (such as
- the case with a 1 kHz square wave perfectly band limited to 20 kHz),
- there simply aren't enough terms to describe a perfect square wave.
- What will result is a square wave with the highest harmonic imposed on
- top as "ringing." In fact, this appearance indicates that the phase
- and frequency response is perfect out to 20 kHz, and the bandwidth
- limiting is limiting the number of terms in the series.
-
- Well, what would a perfect analog system do with square waves? As it
- turns out, if you take a high quality 15 IPS tape recorder, bias and
- adjust it for the flattest possible frequency response over the widest
- possible bandwidth, the result looks remarkably like that of a good
- digital system for exactly the same reasons.
-
- On the other hand, adjust the analog tape recorder for a square wave
- response that has no ringing, but the fastest possible rise time. Now
- listen to it: it sounds remarkably dull and muffled compared to the
- input. Why? Because in order to achieve that square wave response, it's
- necessary to severely roll off the high end response in order to
- suppress the high-frequency components needed to achieve fastest rise
- time. [Dick]
-
- --
- Q5.14 - How can a 16-bit word length be enough to record all the detail
- in music? Doesn't that mean that the sound below -96 dB gets lost in the
- noise? Since it is commonly understood that humans can perceive audio
- that IS below the noise floor, aren't we losing something in digital
- that we don't lose in analog?
-
- You're correct in saying that human hearing is capable of perceiving
- audio that is well below the noise floor (we won't say what kind of
- noise floor just yet). The reason it can do this is through a process
- the ear and brain employ called averaging.
-
- If we look at a single sample in a digital system or an instantaneous
- shapshot in an analog system, the resulting value that we measure will
- consist of some part signal and some part ambiguity. Regardless of the
- real value of the signal, the presence of noise in the analog system
- or quantization in the digital system sets a limit on the accuracy to
- which we can unambiguously know what the original signal value was. So
- on an individual sample or instantaneous snapshot, there is no way
- that either ear or measurement instrument can detect signals that are
- buried below either the noise or the quantization level (when properly
- dithered).
-
- However, if we look at (or listen to) much more than a single sample,
- through the process of averaging, both instruments and the ear are
- capable of detecting real signals below the noise floor. Let's look at
- the simple case of a constant voltage that is 1/10th the value of the
- noise floor. At the instantaneous or sample point, the noise value
- overwhelms the signal completely. But, as we collect more consecutive
- snapshots or samples, an interesting thing begins to happen. The noise
- (or dither) is random and its long term average is, in fact, 0. But the
- signal has a definite value, 1/10. Average the signal long enough, and the
- average value due to the noise approaches 0, but the average value of
- the signal remains constant at 1/10.
-
- A somewhat analogous process happens with high frequency tones. In
- this case the averaging effect is that of a narrow-band filter. The
- spectrum of the noise (or simple dither) is broadband, but the
- spectrum of the tone is very narrow band. Place a filter centered on
- the tone and while we make the filter narrower and narrower, the
- contribution of the noise gets less and less, but the contribution of
- the signal remains the same.
-
- Both the ear and measurement instruments are capable of averaging
- and filtering, and together are capable of pulling real signals from
- deep down within the noise, as long as the signals have one of two
- properties: either a period that is long compared to the inherent
- sampling period of the signal in a digital system or long compared to
- the reciprocal of the bandwidth in an analog system, or a periodic
- signal that remains periodic for a comparably long time.
-
- Special measurement instrument were developed decades ago that were
- capable of easily detecting real signals that were 60 dB below the
- broadband noise floor. And these devices are equally capable of
- detecting signals under similar conditions in properly dithered
- digital systems as well.
-
- How much the ear is capable of detecting is dependent upon many
- conditions, such as the frequency and relative strength of the tone,
- as well as individual factors such as aging, hearing damage and the
- like.
-
- But the same rules apply to both analog systems with noise and digital
- systems with decorrelated quantization noise. [Dick]
-
- Q5.15 - Q5.14 - What's all this about 20- and 24-bit digital audio? Aren't
- CDs limited to 16 bits?
-
- Yes, CDs are limited to 16 bits, but we can use >16-bit systems to produce
- 16-bit CDs with higher quality than we could otherwise.
-
- We are able to record audio with effective 20-bit resolution nowadays.
- The finest A/D converter systems have THD+N values around -118 dB with
- linearity extending far below even that. When it comes time to reduce
- our word-length to 16 bits, we can use any one of a variety of noise
- shaping curves, the job of which is to mix with our 24-bit audio, shift
- the dither spectrum of the noise into areas where our ears are less
- sensitive, thus enabling the noise component to comprise audio information
- at the spectral areas where our ears are most sensitive. See Lipschitz's
- seminal papers for fuller detail on this subject.
-
- Furthermore, we often perform DSP calculations on our audio, and to that
- end it is worthwhile to carry out the arithmetic with as much precision
- as we can in order to avoid rounding errors. Most digital mixers carry
- their math out to 24-bit precision at the I/O, with significantly longer
- word lengths internally. As a result, two 16-bit signals mixed together
- can produce a valid 24-bit output word. For that matter, a 16-bit signal
- subjected to a level change can produce a 24-bit output if desired (except,
- of course, for a level change that is a multiple of 6 dB, as that's just
- a shift left or right).
-
- The number of noise shaping curves available today is staggering. Sony
- SBM, Weiss, Meridian 618, Sonic TBM, Apogee UV-22, Prism SNS, Lexicon
- PONS, Waves, and, of course, the classic Lipschitz curve are just a few
- of the multitudinous options that now exist. [Gabe]
-
- =====
- Section VI - Digital editing and mastering
-
- --
- Q6.1 - What is a digital audio workstation?
-
- A digital audio workstation (DAW) is one of our newest audio buzzwords,
- and applies to nearly any computer system that is meant to handle or
- process digital audio in some way. For the most part however, the
- term refers to computer-based nonlinear editing systems. These systems
- can comprise a $500 board that gets thrown into a PC, or can refer to
- a $150,000 dedicated digital mastering desk. [Gabe]
-
- --
- Q6.2 - How is digital editing different from analog editing?
-
- In the days of analog editing, one edited with a razor blade and a
- diagonal splicing block. Making a cut meant scrubbing the tape over
- the head, marking it with a grease pencil, cutting, and then taping
- the whole thing back together. Analog editing (particularly on music)
- was as much art as it was craft, and good music editors were worth
- their weight in gold.
-
- In many circles, analog editing has gone the way of the Edsel,
- replaced by digital workstation editing. For complex tasks, DAW-based
- editing offers remarkable speed, the ability to tweak an edit after
- you make it, a plethora of crossfade parameters that can be optimized
- for the edit being made, and most importantly, the ability to undo
- mistakes with a keystroke. Nearly all commercial releases are being
- edited digitally nowadays. Since satisfactory editing systems can
- be had for around $1,000, even home recordists are catching onto the
- advantages. More elaborate systems can cost tens of thousands of
- dollars.
-
- There are certain areas where analog editing still predominates,
- however. Radio is sometimes cited as an example, though this has begun
- to change thanks to products like the Orban DSE 7000. The needs of
- radio production are often quite different from those of music editors,
- and a number of products (the Orban being a fine example) have sprung
- up to fill the niche. Nonetheless, in spite of the rapid growth of
- DAWs in the radio market, razor blades are still found in daily use
- in radio stations. [Gabe]
-
- --
- Q6.3 - What is mastering?
-
- Mastering is a multifaceted term that is often misunderstood. Back in
- the days of vinyl records, mastering involved the actual cutting of
- the master that would be used for pressing. This often involved a
- variety of sonic adjustments so that the mixed tape would ultimately
- be properly rendered on vinyl.
-
- The age of the CD has changed the meaning of the term quite a bit.
- There are now two elements often called mastering. The first is the
- eminently straightforward process of preparing a master for pressing.
- As most mixdowns now occur on DAT, this often involves the relatively
- simple tasks of generating the PQ subcode necessary for CD replication.
- PQ subcode is the data stream that contains information such as the
- number of tracks on a disc, the location of the start points of each
- track, the clock display information, and the like. This information
- is created during mastering and prepared as a PQ data burst which the
- pressing plant uses to make the glass pressing master.
-
- Mastering's more common meaning, however, is the art of making a
- recording sound "commercial." Is is the last chance one has to get
- the recording sounding the way it ought to. Tasks often done in
- mastering include: adjustment of time between pieces, quality of
- fade-in/out, relation of levels between tracks (such that the listener
- doesn't have to go swinging the volume control all over the place),
- program EQ to achieve a desired consistency, compression to make one's
- disc sound LOUDER than others on the market, the list goes on.
-
- A good mastering engineer can often take a poorly-produced recording
- and make it suitable for the market. A bad one can make a good
- recording sound terrible. Some recordings are so well produced,
- mixed, and edited that all they need is to be given PQ subcode and
- sent right out. Other recordings are made by people on ego trips, who
- think they know everything about recording, and who make recordings
- that are, technically speaking, wretched trash.
-
- Good mastering professionals are acquainted with many styles of music,
- and know what it is that their clients hope to achieve. They then use
- their tools either lightly or severely to accomplish all the multiple
- steps involved in preparing a disc for pressing. [Gabe]
-
- --
- Q6.4 - What is normalizing?
-
- Normalizing means bringing a digital audio signal up in level such
- that the highest peak in the recording is at full scale. As we saw in
- Q5.4, 0 dB represents the highest level that our digital system can
- produce. If our highest level is, for instance, -6 dB, then the
- absolute signal level produced by the player will be 6 dB lower than
- it could have been. Normalizing just maximizes the output so that the
- signal appears louder.
-
- Contrary to many frequently-held opinions, normalizing does NOT
- improve the dynamic range of the recording in any way, since as you
- bring up the signal, you also bring up the noise. The signal-to-noise
- ratio is a function of the original recording level. If you have a
- peak at -6 dB, that's 6 dB of dynamic range you didn't use, and when
- you normalize it to 0 dB, your noise floor will rise an equivalent
- amount.
-
- Normalizing may help optimize the gain structure on playback, however.
- Since the resultant signal will be hotter, you'll hear less noise from
- your playback system.
-
- But the most common reason for normalizing is to make one's recordings
- sound, LOUDER, BRIGHTER, and have more PUNCH, since we all know that
- louder recordings are better, right? :-) [Gabe]
-
- --
- Q6.5 - I have a fully edited DAT that sounds just like I want it to sound on
- the CD. Is it okay to send it to the factory?
-
- This is a highly case-specific question. Some people truly have the
- experience to produce DATs on mixdown or editing that are ready to go
- in every conceivable way. Often these people can send their tapes out
- for pressing without the added expense of a mastering house.
-
- However, if you do not have this sort of expertise, and if your only
- reason for wanting to send it out immediately is because you know that
- it is technically possible to press from what you have, then you would
- be advised to let a mastering engineer listen to and work on your
- material. A good mastering engineer will often turn up problems and
- debatable issues that you didn't even know were there. Also, any
- decent mastering house will provide you with a master on a format
- significantly more robust than DAT.
-
- DAT is a fine reference tape format, but it simply is not the sort of
- thing you want to be sending to a pressing plant. [Gabe]
-
- --
- Q6.6 - What is PCM-1630? What is PMCD?
-
- PCM-1630 is a modulation format designed to be recorded to 3/4"
- videotape. It was, for many years, the only way one could deliver a
- digital program and the ancillary PQ information to the factory for
- pressing. The PCM-1630 format is still widely used for CD production.
-
- But PCM-1630 is now certainly an obsolete system, as there are many
- new formats that are superior to it in every way. One of the most
- popular formats for pressing now is PMCD (Pre-Master Compact Disc).
- This format, developed by Sonic Solutions, allows for CD pressing
- masters to be written out to CD-Rs that can be sent to the factory
- directly. These CD-Rs contain a PQ burst written into the leadout
- of the discs.
-
- Some plants have gone a step further and now accept regular CD-Rs,
- Exabyte tapes, or even DATs for pressing. The danger here is that
- some users may think that they can prepare their own masters without
- the slightest understanding of what the technical specifications are.
- For instance, users preparing their own CD-Rs must do so in one
- complete pass. It is not permitted, for instance, to stop the CD-R
- deck between songs, as this creates unreadable frames that will cause
- the disc to be rejected at the plant. [Gabe]
-
- --
- Q6.7 - When preparing a tape for CD, how hot should the levels be?
-
- Ideally you should record a digital master such that your highest
- level is at 0 dB, if only to maximize the dynamic range of your
- recordings. Many people like their CDs to be loud, and thus they
- will normalize anyway even if they don't hit 0 dB during recording.
-
- Some classical recordings are deliberately recorded with peaks
- that are significantly lower than 0 dB. This is done in order to
- prevent quiet instruments such as lutes and harpsichords from
- being played too loud. If you record a quiet instrument such as
- a harpsichord out to 0 dB, the listener would have to put the
- volume control all the way at the bottom in order to get a
- realistic level, and the inclination would be to play it at a
- "normal" listening level. By dropping the mastering level, the
- listener is more likely to set the playback level appropriately.
-
- --
- Q6.8 - Where can I get CDs manufactured?
-
- [To come]
-
- --
- Q6.9 - How are CD error rates measured, and what do they mean?
-
- [Forthcoming. -Gabe]
-
- =====
- Section VII - Market survey. What are my options if I want --
-
- --
- Q7.1 - A portable DAT machine
-
- --
- Q7.2 - A rack size DAT machine
-
- --
- Q7.3 - An inexpensive stereo microphone
-
- --
- Q7.4 - An inexpensive pair of microphones for stereo
-
- --
- Q7.5 - A good microphone for recording vocals
-
- --
- Q7.6 - A good microphone for recording [insert instrument here]
-
- --
- Q7.7 - A a small mixer
-
- --
- Q7.8 - A portable cassette machine
-
- --
- Q7.9 - A computer sound card for my IBM PC or Mac
-
- --
- Q7.10 - An eight-track digital recorder?
-
- =====
- Section VIII - Sound reinforcement
-
- --
- Q8.1 - We have a fine church choir, but the congregation can't hear them.
- How do we mic the choir?
-
- --
- Q8.2 - How do I 'ring out' a system?
-
- --
- Q8.3 - How much power to I need for [insert venue here]?
-
- --
- Q8.4 - How good is the Sabine feedback eliminator?
-
- =====
- Section IX - Sound restoration
-
- --
- Q9.1 - How can I play old 78s?
-
- First rule of thumb: DO NOT PLAY THEM WITH AN LP STYLUS!
-
- The grooves on 78s are gigantic compared to the microgroove LPs.
- As a result, specialized styli are needed for proper playback of 78s.
- Also, the RIAA equalization curve normally used for LPs has no
- relation to the frequencies that were equalized on 78 recordings.
-
- The easiest stylus to obtain is the Shure V15 with the 78 pickup. This
- is a good, though not great, 78 stylus and will do an okay job at playing
- most discs.
-
- The serious 78 collector will want to obtain not only a collection of
- 78 styli for the various discs in their collection (groove sizes varied,
- and most serious collectors own a handful of styli), but also a preamp
- with variable equalization curves. One supplier of all this apparatus
- is Audio-78 Archival Supplies at (415) 457-7878. [Gabe]
-
- --
- Q9.2 - How can I play Edison cylinders?
-
- Edison cylinders are best played back with Audio 78's adaptor. You
- remove the horn and reproducer element from your cylinder machine,
- install their electric reproducer, and connect it up to a phono
- preamp, sans RIAA. [Gabe]
-
- --
- Q9.3 - What are "Hill and Dale" recordings, and how do I play them back?
-
- Hill & Dale recordings are discs where the grooves move vertically
- instead of horizontally. Edison, for instance, cut his discs this
- way. In order to play Edison discs, one needs a special glass ball
- stylus that is 3.7-4.0 mil wide. This is available from Audio 78,
- among other places.
-
- Also, since the information is vertical instead of horizontal, one
- must rewire a stereo phono cartridge to reject the normal horizontal
- information and reproduce only the normally-discarded vertical
- information. This is easily accomplished by wiring a stereo cartridge
- in mono, summing the channels, with one channel out of phase. In
- other words, connect the cartridge as follows:
-
- ______________
- | ____ to preamp
- + L - + R - |
- | | | |
- | |_________| |
- |__________________
-
- One caveat: if your cartridge has one lug shorted to ground, make
- sure that this lug is connected to the ground on your preamplifier.
- It doesn't actually matter which channel you invert.
-
- Some preamps like the FM Acoustics 222 or the OWL 1 have a switch
- that will do this for you without rewiring. [Gabe]
-
- --
- Q9.4 - What exactly are NoNOISE and CEDAR? How are they used?
-
- NoNOISE and CEDAR are systems for noise removal. Both of them approach
- the same sorts of noise, but use different algorithms and have different
- user interfaces, often with differing effectiveness.
-
- Noise can be broken down into several categories:
-
- IMPULSIVE NOISE: Pops, clicks, thumps, snaps.
- CRACKLE: The low-level "bacon frying" effect heard on
- LPs and 78s.
- HISS: Tape hiss, surface noise, amplifier hiss, broadband noise
- BUZZ: 60 Hz hum, any other steady-state noise that is relatively
- narrow-band.
-
- NoNOISE and CEDAR are two (expensive) techniques for removing many of
- these ailments. It is rare that it is possible to remove all of the
- problem, nor is it ever possible to remove it with no degradation of
- the program material. [Gabe]
-
- --
- Q9.5 - How do noise suppression systems like NoNOISE and CEDAR work?
-
- Digital techniques have been applied to many facets of sound processing
- and recording and have, on the whole, been found to give results far
- superior to their analogue counterparts. Nowhere is this more true than
- in the field of audio restoration, where excellent processes have been
- developed for removal of impulsive noise (thumps, clicks and ticks) and
- attenuation of continuous broadband noise (such as tape hiss). Example
- techniques for these two are outlined below.
-
- Impulsive noise
-
- In this category we include many types of disturbance, from the click
- generated by a scratch on a 78rpm disc, to the tiny tick created by a
- single corrupt bit in a digital data stream. Also included is crackly
- surface noise from 78's (that sounds like a frying pan), though this
- requires somewhat different treatment; however the outline presented
- below is fairly similar for both processes. Typically, audible clicks
- are of a few microseconds to a few milliseconds in duration, and their
- density can be up to a few thousand clicks per second on poor-quality
- material.
-
- First the audio is split into short blocks of maybe 10ms duration. A
- model is fitted to each block; this model can be thought of as a
- description of the signal in simple mathematical terms. The model is
- chosen such that musical data is a good fit, but the impulsive noise
- is a poor fit.
-
- For example, a simple model could be a sum of sinewaves, whose number,
- frequencies, amplitudes and phases are the model parameters. The para-
- meters are calculated such that when these sinewaves are added together
- they match the musical parts of the signal accurately, but match the
- impulsive noise badly.
-
- Now the model can be thought of as a prediction of the music. In
- undamaged sections the prediction is close (since music is known to
- consist of a sum of sinewaves, at least approximately); during clicks
- and pops etc. the prediction is poor, because the model has been
- designed to match the music, and not the noise.
-
- Now we can achieve impulsive noise removal by replacing the data that
- fits the model badly (ie the clicks) with data predicted by the model,
- which is known to be a close approximation to the music.
-
- Broadband Noise
-
- Broadband noise is usually better tackled in the frequency domain. What
- this entails is taking a block of data (as in the impulsive noise case)
- but then calculating its spectrum. From the spectrum an estimate can
- be made of which frequencies contain mostly signal, and which contain
- mostly noise.
-
- To help in making this discrimination we first take a "fingerprint" of
- the noise from an unrecorded section, such as the lead-in groove of a
- record, or a silence between movements of a symphony. This spectrum of
- this fingerprint is then compared with the spectrum of each block of
- musical data in order to decide what is noise and what is music.
-
- The denoising process itself can be thought of as an automagically-
- controlled, cut-only graphic equaliser. For each block, the algorithm
- adjusts the attenuation of each frequency band so as to let the music
- through, but not the noise. If the SNR in a particular band is high
- (ie lots of signal, little noise) then the gain is left close to
- unity. If the SNR is poor in a given band, then that band is heavily
- attenuated. [Chris]
-
- --
- Q9.6 - What is forensic audio?
-
- Forensic audio is audio services for legal applications. Forensics
- breaks down into four main categories.
-
- TAPE ENHANCEMENT: Digital and analog processing to restore
- verbal clarity and make tapes easier to understand in a courtroom
- situation.
-
- AUTHENTICITY: Electronic and physical microscopic examination
- of a tape to prove that it has not been tampered with, altered,
- or otherwise changed from its original state. Another common
- authenticity challenge is to determine whether a given was tape
- was indeed made on a given machine.
-
- VOICE IDENTIFICATION: Voice ID, or voiceprinting, is the science
- that attempts to determine what was said, and by whom. A variety
- of analog and digital analysis processes are used to analyze the
- frequency and amplitude characteristics of a human voice and
- compare it against known samples.
-
- [More to come on this] [Gabe]
-
- =====
- Section X - Recording technique, Speakers, Acoustics, Sound
-
- --
- Q10.1 - What are the various stereo microphone techniques?
-
- [I'm working on it! -Gabe]
-
- --
- Q10.2 - How do I know which technique to use in a given circumstance?
-
- [This one too! -Gabe]
-
- --
- Q10.3 - How do I soundproof a room?
-
- Despite what you may have seen in the movies or elsewhere, egg crates
- on the wall don't work!
-
- First, understand what's meant by "soundproofing". Here we mean the
- means and methods to prevent sound from the outside getting in, or
- sound from the inside getting out. The acoustics within the room are
- another matter altogether.
-
- There are three very important requirements for soundproofing: mass,
- absorption, and isolation. Actually, there are also three others:
- mass, absorption, and isolation. And to finish the job, you should
- also use: mass, absorption, and isolation.
-
- Sound is the mechanical vibration propagating through a material. The
- level of the sound is directly related to the size of those
- vibrations. The more massive an object is, the harder it is to move
- and the smaller the amplitude of the vibration set up in it under the
- influence of an external sound. That's why well-isolated rooms are
- very massive rooms. A solid concrete wall will transmit much less
- sound then a standard wood-framed, gypsum board wall. And a thicker
- concrete wall transmits less than a thinner one: not so much because
- of the distance, but mostly because it's heavier.
-
- Secondly, sound won't be transmitted between two objects unless it's
- mechanically coupled. Air is not the best coupling mechanism. But
- solid objects usually are. That's why well isolated rooms are often
- set on springs and rubber isolators. It's also why you may see
- rooms-within rooms: The inner room is isolated from the outer, and
- there may be a layer of absorptive material in the space between the
- two. That's also why you'll also see two sets of doors into a
- recording studio: so the sound does not couple directly through the
- door (and those doors are also very heavy!).
-
- If you are trying to isolate the sound in one room from an adjoining
- room, one way is to build a second wall, not attached to the first.
- This can go a long way to increasing the mechanical isolation. Try
- using two sheets of drywall instead of one on each wall, and use 5/8"
- drywall instead of 3/8", it's heavier.
-
- But remember: make it heavy, and isolate it. Absorptive materials like
- foam wedges or Sonex and such can only control the acoustics in the
- room: they will do nothing to prevent sound from getting in or out to
- begin with. [Dick]
-
- --
- Q10.4 - What is a near-field monitor?
-
- A near field monitor is one that is design to be listened to in the
- near field. Simple, eh?
-
- The "near field" of a loudspeaker is area where the direct,
- unreflected sound from the speaker dominates significantly over the
- indirect and reflected sound, sound bouncing off walls, floors,
- ceilings, the console. Monitoring in the near field can be useful
- because the influence of the room on the sound is minimized.
-
- Near field monitors have to be physically rather small, because you
- essentially need a small relative sound source to listen to (imagine
- sitting two feet away from an 18" woofer and a large multi- cellular
- horn!). The physics of loudspeakers puts severe constraints on the
- efficiency, power capabilities and low frequency response of small
- boxes, so these small, near-field monitors can be inefficient and not
- have the lowest octave of bass and not play ungodly loud. [Dick]
-
- --
- Q10.5 - What are the differences between "studio monitors" and home
- loudspeakers?
-
- It depends upon who you ask. There are speakers called "monitor"
- speakers that are found almost exclusively in homes and never in
- studios.
-
- The purpose of a monitor speaker is to monitor the recording and
- editing process. If you buy the concept that they are but one tool in
- the process (and probably the most frequently used single tool at
- that), and if you buy the concept that your tools should be flawless,
- than the requirements for a monitor speaker are easy to state (but
- hard to achieve): they should be the most neutral, revealing and
- unbiased possible. They are the final link between your work and your
- ears, and if they hide something, you'll never hear it. If they color
- something, you might be tempted to uncolor it incorrectly the other
- way.
-
- There is another camp that suggests that monitor speakers should
- represent the lowest common denominator in the target audience. The
- editing and mix process should be done so that the results sound good
- over the worst car speaker or boom box around. While such an idea has
- validity as a means of verifying that the mix will sound good over
- such speakers, using them exclusively for the process invites (as has
- been thoroughly demonstrated in many examples of absolutely terrible
- sounding albums) the possibility of making gross mistakes that simply
- can't be heard in the mixing process. [Dick]
-
- --
- Q10.6 - My near field monitors are affecting the colors on my video
- monitor. What can I do to shield the speakers?
-
- Despite a lot of folk lore and some very impressive sounding wisdom
- here on the net and in showrooms, there is effectively nothing that
- you can do to the speakers or the monitor, short of moving them away
- from one another, that will solve this problem.
-
- The problem comes from the magnetic field created by and surrounding
- the magnets in the loudspeaker. It's possible to design a magnet that
- has very little external field, but it can be an expensive proposition
- for a manufacturer. If the magnets do have large external fields, the
- only technique that works is by solving the problem at the source: the
- magnet. Special cancelling magnets are used, sometimes in conjunction
- with a "cup" shield directly around the magnet.
-
- You'll hear suggestions from people about placing a sheet of iron or
- steel between the speakers and the monitor. That might change the
- field, but it will not eliminate it. As often as not, it will make it
- worse.
-
- You'll also here from people about shielding the speaker by lining the
- enclosure with lead or copper. This method is absolutely guaranteed
- to fail: lead, copper, aluminum, tin, zinc and other such materials
- have NO magnetic properties at all, they will simply make the speaker
- heavier and won't solve the problem at all. There is but one material
- that has a shot at working: something called mu-metal, a heavy, very
- expensive, material designed for magnetic shield that requires
- extremely sophisticated and difficult fabrication and annealing
- techniques. Its cost is far greater than buying a new set of speakers
- that does not have the problem, and it may not even work if the
- majority of the offending field is radiated from the front of the
- speaker, which you obviously can't shield.
-
- Try moving the speakers relative to your monitor. Often, moving them
- an inch or two is enough to cure the problem or at least make it
- acceptable. Sometimes, placing the speakers on their sides with the
- woofers (the major offenders in most cases) farthest away from the
- monitor works. [Dick]
-
- -----
- Section XI - Industry information
-
- Q11.1 - Is there a directory of industry resources?
- Q11.2 - What are the industry periodicals?
- Q11.3 - What are the industry trade organizations?
- Q11.4 - Are there any conventions or trade shows that deal specifically
- with professional audio?
-
- -----
- Section XII - Miscellaneous
-
- Q12.1 - How do I modify Radio Shack PZMs?
-
- [Chris?]
-
- Q12.2 - Can I produce good demos at home?
-
- [Who wants to do this?]
-
- [That's a request for a writer, NOT a professional opinion!]
-
- Q12.3 - How do I remove vocals from a song?
-
- You probably want a device called the Thompson Vocal Eliminator made
- by LT Sound in Atlanta, Georgia. The device will cancel out any
- vocal that is mono and panned dead-center. The unit works by
- filtering out the low frequencies, phase cancelling the rest of the
- signal, and then mixing the filtered bass back in. The result is a
- signal with the center, common information cancelled out. Sometimes
- it works well, other times it sounds awful. [Gabe]
-
- -----
- Section XIII - Bibliography
-
- Q13.1 - Fundamentals of Audio Technology
- Q13.2 - Studio recording techniques
- Q13.3 - Live recording techniques
-
- Q13.4 - Digital audio theory and practice
-
- * Ken C. Pohlmann, "Principles of Digital Audio," SAMS/Prentice Hall, 1993
- [Excellent introduction and explanation of all aspects of digital audio
- principals and practices]
-
- * John Watkinson, "The Art of Digital Audio," Focal Press, 1989
- [ditto]
-
- * Francis Rumsey and John Watkinson, "The Digital Interface Handbook,"
- Focal Press, 1993
- [deals with interfacing standards and protocols for both audio and video]
-
- * IEC Standard Publication 958, "Digital Audio Interface," International
- Electro-Technical Commission, 1989, 1993
- [THE standard!]
-
- * Claude E. Shannon and Warren Weaver, "The Mathematical Theory of
- Communication," U. Chicago Press, 1963. [The seminal work on digital
- sampling...includes the original 1948 BSTJ sampling paper]
-
- Q13.5 - Acoustics
-
- * Leo Beranek, "Acoustics," New York, American Institue of Physics, 1986
- [The bible, but heavily mathematical, very thick and obtuse, a good book
- despite the Lincoln Hall disaster.]
-
- * F. Alton Everest, "The Master Handbook of Acoustics," Tab Books 1989
- [A good entry level text on acoustics, studio and listening room design,
- not heavily mathematical].
-
- * Arthur H. Benade, "Fundamentals of Musical Acoustics," New York, Dover
- Publications, 1990
- [A thorough book, more on the acoustics of PRODUCING music rather than
- REPRODUCING it.]
-
- * Any good physics text book is useful for keeping the bunk at bay.
-
- Q13.6 - Practical recording guides
-
- -----
- Section XIV - Miscellaneous
-
- Q14.1 - Who wrote the FAQ?
-
- [Mini-bios of the FAQ maintainers...one day]
-
- Q14.2 - How do you spell and pronounce the FAQ maintainer's surname?
-
- Those who paid attention during fourth grade English learned the rule
- "I before E except after C." There is no C in my name, and therefore
- the I comes before the E. My last name is spelled "Wiener" not "Weiner."
-
- And it rhymes with cleaner, and starts with a W, not a V. [Gabe]
-
- =====
-
-
- --
- Gabe Wiener Dir., PGM Early Music Recordings |"I am terrified at the thought
- A Div. of Quintessential Sound, Inc., New York | that so much hideous and bad
- Recording-Mastering-Restoration (212) 586-4200 | music may be put on records
- gabe@pgm.com http://www.pgm.com | forever."--Sir Arthur Sullivan
-