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- Subject: FAQ: rec.audio.* Recording 2/99 (part 7 of 13)
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- Organization: Texas Instruments Corp.
- Summary: Answers to common questions about audio equipment, selecting,
- buying, set-up, tuning, use, repair, developments, and philosophy.
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- Archive-name: AudioFAQ/part7
- Last-modified: 2004/1/12
- Version: 2.16
-
- 14.0 Recording
- There are more different recording systems available today than
- ever before. Digital and analog are both available to the
- consumer. With the advent of consumer digital recorders, used
- pro analog recorders are becoming available for surprisingly low
- prices. Now may be the time for you to buy a microphone and
- recorder and make your first!
-
- 14.1 What is DAT? What is its status today?
- DAT (Digital Audio Tape) is currently the standard professional
- digital format for 2-track digital recording. DAT had a
- short-lived consumer presence, but never "made it". As digital
- recorders have no tolerance for clipping, using a DAT recorder
- takes a slightly different knack. The results can be worth it,
- however, as DAT format offers the same resolution and dynamic
- range as CDs. DATs record for up to 3 hours on a tape, and can
- run at three different sampling rates: 32 kHz, 44.1 kHz
- (for CD), and 48 kHz (the DAT standard). Longplay mode cuts
- frequency response to 14kHz but adds even more recording time.
-
- 14.2 What is DCC? What is its status today?
- DCC is Philips' attempt to modernize the regular cassette. DCC
- decks can play analog cassettes, and can record new Digital
- Compact Cassettes. They use stationary heads (DATs use rotary
- heads as do VCR's), and although they are digital, they use
- lossy compression to fit all the data on the cassette. Although
- DCC sound quality is far better than the 1960 standard cassette,
- the DCC does not have the sound quality present in DAT or CD.
- DCC may be a good choice for consumers who want to assemble mix
- tapes for cars or walkmans, but is not suitable for any
- professional applications.
-
- As of October 1996, DCC is quite affordable in price. Some
- DCC home recorders are under $200. However, blank DCC tapes
- are still hard to find and fairly expensive ($10 each for 90
- minute lengths). Also, DCC manufacturers are dropping DCC
- from their lines, indicating that it is either on the way
- out or never made it in.
-
- Although the ability to play analog cassettes is a strong
- advantage of DCC, many people have had trouble with oxide
- particles falling off analog cassettes and clogging the gap
- of the DCC head. This may be due to the extremely low
- quality of some analog cassette tapes and may be due to the
- very tiny gap of DCC heads.
-
- Caution: NEVER demagnetize DCC heads. This will permanently
- damage the heads.
-
- As of May 1997, Philips has announced plans to discontinue DCC.
-
- 14.3 What about writable compact discs? What is the status today?
- Recordable and rewritable CD recorders and discs are available,
- | and costs are dropping. As of Dec 2003, recorders have shown
- | up for <$30 and blank disks are advertised as low as $0.25
- each in bulk. Many people report destroying many disks before
- getting their machine working correctly, but once people learn
- the software and hardware steps, archival CDs can be made
- | inexpensively and routinely. There is definitely a difference
- | in discs and a difference in recorders. However, it is tough
- | to generalize on which are better or worse other than to say
- | that name brand discs are a safer bet than off-brand discs.
- For more on CD-R read this excellent document:
- http://www.fadden.com/cdrfaq/
-
- 14.4 What are Dolby B, C, and S, HX Pro, and DBX? Are they compatible?
- Dolby B, C, S, and DBX are techniques for increasing the
- signal/noise ratio of recordings. All work in similar ways:
- they compress the dynamic range of the sound during recording,
- then expand it back upon playback. As much as we would like
- it to be otherwise, you only get correct reproduction if you
- use Dolby B to play back a Dolby B tape. Same for Dolby C,
- Dolby S, and DBX. Dolby HX Pro is the exception.
-
- Dolby B works mostly with higher frequencies; it increases
- their levels during recording and decreases their levels, and
- the levels of high-frequency noise such as tape hiss, during
- playback.
-
- Dolby B tapes can be played back without Dolby B processing,
- but high frequencies are over-emphasized and the sound will
- be excessively bright. This can be compensated for to some
- extent by turning down the treble control. Audio novices
- often remark that commercially recorded tapes recorded using
- Dolby B sound dull when played back with Dolby B; this is
- because they are accustomed to the boosted high frequencies
- they hear when playing these tapes without Dolby.
-
- Dolby C achieves greater noise reduction (about 8-10 db) than
- Dolby B by working with a greater range of frequencies and
- altering relative levels more; this means that playing Dolby C
- tapes back with no Dolby processing or with Dolby B, leads to
- very bad frequency response and a sound that most people find
- unpleasent. Dolby C may also be more sensitive to variations
- among decks in exact frequency response, alignment, etc. Some
- people find that tapes recorded using Dolby C sound best only
- when played back on the deck on which they were recorded.
-
- Dolby S works with an even broader range of frequencies than
- Dolby C, and achieves slightly greater noise reduction. Its
- has three advantages over Dolby C: (1) many people find that
- tapes recorded and played back using Dolby S sound closer to
- the original than tapes done using Dolby C; (2) tapes recorded
- using Dolby S don't sound awful if played back on Dolby B decks,
- and (3) Dolby S seems to be less sensitive to variations among
- decks.
-
- DBX is similar to Dolby B, C, and S, but uses the same compression
- on all frequencies, high and low. However, DBX is mostly used
- in the professional market. Very little home DBX equipment is
- available, and some of that home equipment is no better than
- comparable Dolby B home systems. All DBX systems are compatible
- with all other DBX systems, but incompatible with Dolby. A DBX
- tape will sound terrible without DBX processing during playback.
-
- All compression/expansion systems suffer two problems. One is due
- to the fact that compressors can't compress a loud signal before
- they have heard a bit of it, so that little bit of loud signal
- will get through uncompressed. Likewise, quiet passages will not
- be expanded until after they are detected. These delays give rise
- to an audible problem often called "breathing".
-
- The other problem inherent in all compression/expansion systems
- is that if there are any frequency response errors in the tape
- recorder, they will be made worse by the compression/expansion.
- For example, if there is a 2dB dip in frequency response at 1kHz
- in the tape recorder, this will be accentuated to a 4dB dip if
- the compressor is using a 2:1 ratio. So compression/expansion
- trades noise for frequency response error. For that reason and
- the previously mentioned breathing, some people prefer to use
- their recorder without any noise reduction at all. They prefer
- a bit of noise to the other errors.
-
- Dolby HX Pro is not noise reduction and does not use
- compression or expansion. HX Pro is a technique developed by
- Dolby Labs to increase tape headroom by decreasing the bias
- when recording signals with a large high frequency component.
- This allows better transient response, particularly on less
- expensive tapes, and requires no processing when the tape is
- played back. Dolby HX tapes can be played back on any system
- with no decrease in quality.
-
- Dolby Corporation has developed other techniques and other
- acronyms for products related to surround sound. The phrase
- "contains Dolby" isn't as meaningful today as it used to be.
-
- 14.5 What is the best cassette deck under $400?
-
- 14.6 What is PASC? Can I hear the effects?
- PASC (Perceptual Audio Sub-band Coding) is a data-compression
- algorithm. It increases the length of recording that can be
- stored in a given number of data bits by eliminating sounds that
- the developers' research claims can not be perceived by human
- listeners. Its most important component is the omission of
- quiet sounds that occur at the same time and near the frequency
- of louder sounds. It provides up to a 4x increase in the length
- of recordings a given digital medium can hold; this is essential
- to allow full-length digital recordings on DCC (and on MD, which
- uses a different compression technique). It is not necessary
- to translate CD data to analog before compressing it using PASC,
- nor the reverse.
-
- It is very difficult to hear any degradation from PASC, but it
- is possible, depending on the source and listener. The effect
- is not a distinctive noise (like a hiss) nor a consistent
- diminution (like a notch in a speaker's response), but a broad,
- uncorrelated dropout in a changing collection of sounds that
- are masked by sounds that you can hear very easily.
-
- Since it is lossy, repeated PASC recording will cause
- progressive loss, and this signal damage may become easily
- noticeable. This is a side effect that recording companies
- hope will have the effect of discouraging piracy via DCC.
- DCC recorders do have digital inputs so can make one perfect
- copy of a master, but copy protection prevents digital
- duplication of a copy.
-
- For more information on audio compression, consult these
- articles (courtesy of Jonas Palm):
-
- R. Veldhuis, M. Breeuwer, R. van der Waal, "Subband Coding of
- Digital Audio Signals Without Loss of Quality," IEEE ICASSP,
- 1989, pp. 2009-2012.
-
- J. Johnston, "Perceptual Transform Coding of Wideband Stereo
- Signals," IEEE ICASSP, 1989, pp. 1993-1996.
-
- G. Davidson, L. Fielder, M. Antill, "High-Quality Audio Transform
- Coding at 128 kbits/s," IEEE ICASSP, 1990, pp. 1117-1120.
-
- J. Princen, A. Bradley, "Analysis/Synthesis Filter Bank Design
- Based on Time Domain Aliasing Cancellation," IEEE Trans ASSP,
- Oct. 1986, v. 34 n. 5, pp. 2161-2164.
-
- P. Duhamel, Y. Mahieux, J. Petit, "A Fast Algorithm for the
- Implementation of Filter Banks Based On 'Time Domain Aliasing
- Cancellation,'" IEEE ICASSP, 1991, pp. 2209-2212.
-
- J. Johnson, "Transform Coding of Audio Signals Using Perceptual
- Noise Criteria," Journ. Acoustical Society of America, Feb. 1988,
- pp. 314-323.
-
- 2nd Draft-Proposed Standard on Information Technology Coding of
- Moving Pictures and Associated Audio, document ISO/IEC
- JTC1/SC2/WG11 MPEG 90/001, Sept. 1990.
-
- G.Thiele, G. Stoll and M. Link "Low bit-rate coding of high-quality
- audio signals. An introduction to the MASCAM system." EBU Review
- No. 230
-
- 14.7 What is SCMS? Can I hear the effects?
- SCMS (Serial Copy Management System) is a copy-protection system
- intended to stop rampant piracy of commercial recordings to
- digital tape. SCMS allows the home taper to copy from a CD to a
- digital tape, but prevents anyone from digitally copying that
- new digital tape.
-
- You CANNOT hear SCMS.
-
- 14.8 How can I bypass SCMS?
- There are professional devices used by engineers to manipulate
- the digital bitstream, but they cost several hundred dollars and
- are not cost effective for consumers. If you need to make
- perfect digital copies of digital copies, buy a professional
- digital recorder. Pro models do not have SCMS, are more durable
- than consumer recorders, and may have better quality electronics
- than consumer models.
-
- 14.9 What's this about a tax on DAT?
- Every digital audio tape recorder and every blank digital tape
- sold in the USA is priced to include a "premium" or "tax". This
- tax is collected by the US Copyright Office and distributed to
- the recording artists and record companies that own the
- copyrights to commercial music. These fees are supposed to
- repay them for lost royalties.
-
- Many believe that this "tax" is illegal, because it represents
- an assumption that the buyer will use the recorder and tape to
- violate a copyright, and not to record their own works. A
- founding principle of the USA legal system is that everyone is
- assumed innocent until proven guilty.
-
- If you believe that this law is unjust, write your elected
- representatives.
-
- 14.10 Is it legal to copy an LP, CD, or pre-recorded tape?
- In the US today, it may be legal to copy LP's, CD's, etc. for
- your own private use (such as to copy a CD to play on your
- walkman). UK law specifically prohibits this, but it is almost
- never enforced. It is definitely not legal in the US, UK, or
- almost anywhere else, to copy these sources for commercial
- purposes, or to give the copies to others.
-
- It is as of yet unclear whether you own the rights to sell
- or give away a copy of a recording if you made the copy on media
- which was sold with an included digital audio tax.
-
- 14.11 How do I clean and demagnetize tape heads?
- First, a caution: DAT recorder tape heads are VERY fragile.
- Before cleaning the heads on a DAT recorder, get specific
- recommendations from a very knowledgeable source that is
- intimately familiar with DAT head cleaning. In the internet,
- a good source is the DAT-Heads-Digest FAQ. For more information
- on DAT-Heads-Digest, see section 20.2, below.
-
- To clean tape heads, use pure isopropyl alcohol and lint-free
- swabs. Wipe the metal parts of the transport with alcohol
- (DON'T wipe the rollers!) and allow them to dry. Throw the swab
- away after use. Be exceedingly careful when cleaning the heads
- on a DAT. DAT heads are notoriously easy to misalign by
- incorrect cleaning.
-
- Practical tape head demagnetizers are available for under $10.
- Try to find one with a plastic coated tip. If you can't find
- one which is plastic coated. you can slip a drinking straw or
- plastic tube over the tip for the same effect. This plastic
- will prevent the demagnetizer from scratching the head.
-
- Before plugging in the demagnetizer, remove all tapes from your
- working area and unplug the recorder. Hold the demagnetizer
- away from the recorder as you plug it in. Slowly bring the tip
- of the demagnetizer up to the tape head and slide it back and
- forth across each tape head for five one-second strokes. Then
- pull it away from the head slowly and go on to the next. After
- demagnetizing the heads, use the tip on each metal tape guide
- with a similar five strokes. Last, slowly pull the demagnetizer
- far away from the recorder and unplug it. Recording engineers
- use a demagnetizer before each recording session.
-
- 14.12 How do I adjust a tape recorder for best results?
- Adjusting a tape machine for best results usually requires
- special equipment and test tapes. Unless you know what you're
- doing, leave it for a pro. If you are serious about doing it,
- buy the service manual for your particular tape recorder. It
- will list a detailed procedure, as well as describe the correct
- test tape and tools.
-
- As for setting of record levels, it is best to experiment with
- different levels on different tape brands. Different
- formulation will reach saturation for different levels.
- Generally speaking, the transients on a Chrome tape should peak
- at about +6 dB above 0, though some formulations can take
- significantly hotter signals.
-
- 14.13 Where can I get new pinch rollers or drive belts?
- Projector-Recorder Belt Company
- Whitewater WI USA
- 800-558-9572
-
- 14.14 What is a good rubber (pinch) roller cleaner?
- Teac RC-1 available from
- J&R Music World
- 59-50 Queens-Midtown Expressway
- Maspeth NY 11378-9896 USA
- 800-221-8180 or 718-417-3737
- Tascam Rubber Cleaner RC-2 available from:
- Tape Warehouse
- Chamblee GA
- 1-404-458-1679
-
- 14.15 How can I program a recorder to tape a radio broadcast?
- Radio Shack and Panasonic make a clock/radio/cassette that can
- be set to record at a specific time. Radio Shack also sells 120
- minute cassettes, which can be used for 60 minutes per side.
- The recorders are not high quality, and the long tapes are
- fragile, but it works.
-
- You can buy "appliance timers" at hardware stores that will
- start and stop an appliance at a specific time. Radio Shack
- sells fancier versions of the same thing for more money. Gadget
- freaks love "X-10" control systems. These can be configured to
- do the same thing. All require a recorder that can be left in
- RECORD mode. Such recorders are identified by a "TIMER" switch
- on the front panel. Many cassette decks have a TIMER switch for
- use with timers.
-
- This can be set to start a recorder at a particular time. As
- the recorder will be started from a remote control rather than
- by the power line voltage, no timer switch is required. Radio
- Shack has a very similar product available for $99.95, may be
- less on sale.
-
- Carver made a remote with timer which could be programmed to
- start recording at a specific time, if you have a recorder
- with remote control capability.
-
- For the true nerd, there's the programmable remote sold as a
- Scientific Calculator, the HP-48. Audio remote control
- software for this fine adding machine exists. For more
- information, consult the HP-48 FAQ. The HP-48 FAQ contains
- pointers to a few remote control programs. The FAQ is archived
- at site rtfm.mit.edu in /pub/usenet-by-group/comp.sys.hp48
-
- You can also use a VCR for audio-only recording. Hook the audio
- in to the output of a radio, tuner, or receiver. You may also
- have to connect some video signal to the VCR so that the sync
- circuits work correctly.
-
- You can also use a computer's hard drive to record audio.
- Cybercorder 2000 shareware ($19.95 to register) schedules
- recordings on the computer sound card Line-In jack.
- http://skyhawktech.com
-
- 14.16 Will CrO2 or Metal tapes damage a deck made for normal tape?
- No. They will work fine. They are no more abrasive than common
- tape and may actually be less abrasive than very cheap tapes.
- Recorders which are designed for CrO2 or Metal tape have
- different bias settings and equalization settings to take best
- advantage of the greater headroom and to give flat response with
- these different types of tape. However, they use similar if not
- identical heads as less expensive tape recorders. Almost all
- tapes are in some way lubricated, and these lubricants minimize
- wear and squeaking.
-
- 14.17 Why do my old tapes squeak in my car cassette deck?
- One problem that will cause this is "binder ooze". The binder
- is the glue which holds the oxide particles to the backing.
- With time, this binder can ooze forward and actually get past
- the oxide particles, so that there is sticky stuff on the
- surface of the tape. When this sticky stuff goes past the
- heads, it can cause a slight stick, which will sound like a
- squeak. You won't feel it with your fingers, but it is there.
- If you have a prized tape with this problem, consider baking
- the tape in a home oven at a very low temperature, like 150F.
- This might cure the problem by drying out the binder.
-
- 14.18 Is VHS Hi-Fi sound perfect? Is Beta Hi-Fi sound perfect?
- The HiFi recording format is subject to two different problems:
- Head-switching noise and compression errors.
-
- To get perfect reproduction, the FM subcarrier waveform being
- played back by one audio head must perfectly match the waveform
- from the other head at the point of head switching if a glitch
- is to be avoided. If you record and then play the tape on the
- same VCR under exactly the same conditions, you have a
- reasonable chance of this working. But if the tape stretches
- just a bit, or you play it on another VCR whose heads are not in
- exactly the same position, or the tracking is off, the waveforms
- will no longer match exactly, and you will get a glitch in the
- recovered waveform every time the heads switch. This sounds
- like a 60 Hz buzz in the audio, which is often audible through
- headphones even if not through speakers.
-
- The same glitch will occur in the video waveform too, but since
- head switching always happens during vertical retrace, you won't
- see it.
-
- Some VCRs have azimuth correctors or Dynamic Track Following
- which minimize these problems (Philips V2000 and some VHS).
-
- The wonderful signal to noise ratio of VHS HiFi is achieved
- through the use of compression before recording and expansion
- after playback. The actual signal to noise ratio of the tape
- itself is about 35 dB and a 2.5:1 compressor is used to
- "squeeze" things to fit. Like all companders, this produces
- audible errors at certain places on certain signals, such as
- noise "tails" immediately after the end of particularly loud
- passages.
-
- Worse, compressors often have problems simply getting levels
- right. That is, if you record a series of tones, starting at
- -90 dB and working up in 1 dB increments to 0 dB, and then play
- them back, you will almost invariably have level errors. The
- trend from soft to loud will be there but the steps won't be
- accurate. Two or three of your tones might come out at
- essentially the same level, then the next one takes a big jump
- to catch up or even overshoot.
-
- For music, the result will be that the relative levels of some
- instruments, passages, etc. will not be accurate.
-
- This doesn't matter as much for movies, which tend to have
- steady volume level. Also, movie enjoyment is rarely hurt by
- these level errors. VHS and Beta HiFi is fine for reproduction
- of movie and tv soundtracks. They are also perfectly fine for
- non-critical audio applications. But VHS and Beta HiFi are not
- serious competitors to DAT, CD, open-reel analog tape, or even a
- high quality cassette deck.
-
- 14.19 How do HiFi VCRs compare to cassette recorders? DAT recorders?
- VHS HiFi and Beta HiFi are analog recording formats which use
- modulation techniques to record a video signal and a stereo
- audio signal on a videocassette. The audio capabilities
- typically surpass that of the "linear" audio tracks found on all
- video recorders, thus the "HiFi" designation. "HiFi" is
- essential for getting good sound quality on your video
- recordings and out of pre-recorded videos.
-
- HiFi is also touted as an excellent audio recorder for
- audio-only (no picture) applications. Progress in HiFi has
- modern VHS HiFi equipment on par with the best analog cassette
- recorders and close to that of the digital formats. VHS HiFi
- suffers generational loss and noise, but because of the high
- quality of the AFM (HiFi) track, these generational losses
- are minimal and not as severe as those of audio cassettes.
-
- Many people use VHS HiFi for recording radio broadcasts, since
- VCRs often have built-in timers and can record for up to 9
- hours. If you use a HiFi video recorder to record from an
- audio-only source, beware that some decks will not function
- properly without a video signal for synchronization. If you are
- interested in very good quality sound, use a deck with manual
- level control.
-
- 14.20 What is the difference between VHS HiFi and Beta HiFi?
- To record the video and HiFi sound signals onto the same tape
- area, VHS HiFi uses "depth multiplexing", while Beta HiFi uses
- "frequency multiplexing". That is, the FM signal for Beta HiFi
- occupies a different frequency band than do the Beta format's
- luminance and chroma signals, and is simply mixed with those
- signals and laid down on the tape by the video heads. In VHS
- the luminance and chroma signals were too close together in
- frequency for this to work. VHS HiFi uses a separate pair of
- heads on the spinning head drum to record the HiFi carrier.
- These heads' gaps are shaped so that the HiFi carrier is
- actually recorded at a different depth in the tape than the
- luminance and chroma signals.
-
- 14.21 Is there any good reason to buy a HiFi VCR for common TV shows?
- If you do not own a stereo TV, the purchase of a HiFi VCR will
- give you the capability to listen to stereo TV broadcasts to
- your system.
-
- 14.22 What is the best cassette tape?
- One simple answer to this question is that the best tape is the
- tape which was used to align your tape recorder. A second
- simple answer is that more expensive tapes are frequently
- better in terms of quality of the backing, durability of the
- oxide, accuracy of the shell and guides, and life.
-
- Background: When you make a tape recorder, you build electronic
- circuits which have specific, non-flat frequency response.
- These circuits correct for the non-flat response of the tape
- heads, the recording process, and the tape. These circuits can
- be adjusted after the recorder is made, but adjustment is
- tricky, and may or may not be successful with every tape made.
- The designer of the tape recorder picked one tape as their
- standard when they did the design, and built that recorder
- to work well with that particular tape. It may work better
- with a different tape, but it won't necessarily sound the
- best with what one person calls the best sounding tape.
-
- From a review of frequently given answers to this question,
- it is obvious that almost every brand of tape has its advocates.
- Many brands also have their detractors. Maxell and TDK tend to
- have a strong following, but that is in part because they own a
- large share of the US tape distribution market.
-
- 14.23 What is the best Reel-to-Reel tape?
- See 14.22. Just as cassette tape recorders are set up
- specifically for one type of tape, reel-to-reel tape recorders
- are equalized and biased so that they are best with one specific
- brand and model of tape. Just as more expensive cassette tapes
- will last longer and have less noise than cheaper ones, you can
- expect fewer dropouts, better quality control, and lower noise
- from more expensive reel-to-reel tapes.
-
- The major brands in reel-to-reel tape include Ampex, Scotch
- (3M), AGFA/BASF, and Maxell.
-
- 14.24 What is Type I, Type II, Type III, and Type IV cassette tape?
- These are IEC (International Electrotechnical Committee)
- standards. They provide broad standards for all tapes,
- and end the need to align a deck for an individual tape.
- Type 1 is for normal "iron oxide" tapes (Fe2O3), Type 2
- is for high-bias "chromium oxide" tapes (CrO2), Type 3
- (obsolete) is for FeCr (ferric chrome), and Type 4 is
- for Fe (Metal). Type 2 tapes tend to be more expensive
- than type 1, and type 4 tapes are the most expensive.
- This is because type 2 tapes tend to have less noise and
- flatter high frequency response than type 1, and type 4
- tapes tend to have even flatter highs and even less noise.
-
- Some Type 1 tapes are more expensive than other Type 2 tapes,
- and may be worth the extra price. More expensive tapes come
- in better shells, have better lubrication, fewer dropouts,
- smoother frequency response, and better uniformity from tape
- to tape. Even though the types imply a particular tape
- formulations, the type really refers to the tape performance.
- For example, some iron oxide tapes have an unusual oxide
- formulation with very small grains that conforms to the type
- 2 standard better than the type 1 standard. These tapes
- will be labeled type 2, but may not have any chrome in them.
-
- Most modern cassette recorders sense the tape type by the
- holes in the back of the housing and adjust bias and
- equalization to compensate for the differences. A few
- top cassette recorders (the Revox and several Nakamichis)
- automatically align to a particular tape by recording test
- tones and then setting their own equalization.
-
- In practice, each brand and model tape is slightly different.
- For the very best recordings, adjust your recorder for the
- tape you use most, or buy the tape which works best in your
- recorder. Manufacturers adjust each recorder for a specific
- tape at the factory. So the best tape might be the one
- referenced in the recorder owner's manual. In a recording
- studio, it is common to align the bias and equalization for
- the specific tape used, and stick with that tape.
-
- 14.25 Why do I have hum when I connect cable to my VCR (or TV), which is
- connected to my audio system?
- What you are experiencing is probably a "ground loop", caused
- by multiple connections from your equipment chassis ground to
-
- building ground. Since disconnecting the cable or building
- antenna from the VCR eliminates the hum, the cure is simple.
- The following info talks about "the cable" but works the same
- with a coax from a master antenna system.
-
- Go to Radio Shack and buy one each of:
-
- 15-1253, "300-ohm TV-VCR Matching Transformer"
- This looks like a little box with two screw terminals
- and a push-on male F (coax) connector.
-
- 15-1140, "75-ohm coax/300-ohm twin lead indoor/outdoor matching
- transformer"
- This is a longish box or tube, with a female F
- connector on one end and a bit of twin-lead coming from
- the other. The twin-lead ends in a pair of what are
- called "spade lugs" (shaped like U's).
-
- Note: each of these part numbers may have a "B" or other
- letter at the end. These indicate slightly different details
- of functionally equivalent parts. Don't worry about it.
-
- Connect the two spade lugs on the -1140 to the two screw
- terminals on the -1253. Make sure they don't touch each other;
- this shouldn't be difficult to get right. This gives you the
- "isolator", with a female coax connector on one end and a male
- coax connector on the other.
-
- Just insert the isolator "in line" in the incoming cable lead.
- ie treat it as you would a (very short) extension cord. You
- can do this right at the back of the VCR (or whatever the cable
- is hooked to).
-
- Only one of the two units called out here (15-1140) actually
- provides isolation. Two of the 15-1253 units back to back will
- NOT work. Two of the 15-1140 units back to back will work fine
- but will be less convenient.
-
- If you can't find these specific parts, and want to know if the
- substitutes you've found will work, test them with an ohmmeter,
- measuring from either the pin or shield of the coax side to
- either wire of the twin-lead side. If it's not an autoranging
- unit, set the meter to its highest resistance range. You want
- to see no connection (ie: infinite resistance, an open circuit)
- between them. As with the parts described above, only one of
- the coax/twinlead adapters needs to pass the test.
-
- This trick runs the signal through a PAIR of baluns. This is
- more than is absolutely required to solve this problem, and may
- weaken the signal slightly. This should not be a problem on
- most cable systems. But, some audio stores are beginning to
- carry a unit made expressly for this purpose. It contains a
- single 75 ohm to 75 ohm isolation transformer. This should
- introduce less signal loss. It will also be better shielded
- than the two baluns (see next paragraph). Under $10 would be an
- appropriate price.
-
- The back-to-back baluns may allow "ingress". That is, if you
- are near to a TV transmitter, the short length of twinlead may
- pick up broadcast TV signals and mix them with the cable,
- causing interference. If you can find a prepackaged 75 ohm
- isolation transformer as described in the preceding paragraph,
- it should be better in this regard.
-
- Mondial is selling a unit dubbed the "Magic-1"; this does the
- same job but with three capacitors instead of transformers. It
- is said to cause less than 1 dB of signal loss. On the other
- hand, it costs about $90.
-
- Yet another solution is to attack the problem at the line-level
- audio connection between the VCR and the rest of your stereo.
- Radio Shack's stereo ground isolators (270-054) are made for
- this purpose. These go in the line-level AUDIO connections
- between the VCR (or TV) and the rest of your sound system. If
- both the line in and line out jacks on the VCR are connected to
- the sound system, you'll need two of these isolators. They are
- audio frequency transformers and may add some distortion and
- frequency response error.
-
- | 14.26 Is Binaural better than stereo? What is Binaural?
- | Judge for yourself. There are samples of binaural recordings
- | available for free download at:
- | http://www.binaural.com
- | According to the Binaural FAQ (slightly edited to save space):
- | http://www.binaural.com/binfaq.html
- |
- | "Binaural...record(s) music and sounds with two tiny
- | omnidirectional mikes at the entrance to the ear canals on an
- | artificial head...This includes even the fleshy ridges of the
- | outer ears which modify the frequency balance of sounds
- | depending on the direction from which they originate...
- |
- | "...A stereophonic system...uses loudspeakers but requires an
- | infinite number of channels for perfect reproduction...
- | (Binaural) requires only two channels for perfect reproduction
- | but involves the use of a pair of head receivers [drivers] held
- | tightly to the ears for each listener. All listeners with such
- | a system can be given the illusion of sitting in the best seat
- | in the concert hall. Harvey Fletcher in the SMPTE Journal Vol.
- | 61, September 1953."
- |
- | "The binaural experience is striking, and requires no special
- | equipment besides stereo headphones and binaural recordings.
- | However, the 'perfect reproduction' mentioned by Fletcher is
- | not necessarily achieved by all listeners due to variations
- | in dummy heads, headphones and individual hearing. The
- | astonishing realism is heard by nearly all, even with the most
- | inexpensive headphones. But many have trouble localizing sounds
- | directly in front or in back, and for some the sounds seem to
- | occur inside their skull (just as with listening to stereo on
- | headphones) rather than outside. Better matching of HRTFs (Head
- | Related Transfer Functions) can correct some of these problems,
- | and with recent advances in digital signal processing there may
- | soon be a solution. It would involve a processor similar to the
- | Dolby Headphone circuit - which provides a virtual 5.1 surround
- | field on ordinary headphones, but allowing for the proper EQ
- | and phasing to map the binaural sounds seamlessly in a
- | 360-degree sphere around each listener."
-
- COPYRIGHT NOTICE
- The information contained here is collectively copyrighted by the
- authors. The right to reproduce this is hereby given, provided it is
- copied intact, with the text of sections 1 through 8, inclusive.
- However, the authors explicitly prohibit selling this document, any
- of its parts, or any document which contains parts of this document.
-
- --
- Bob Neidorff; Texas Instruments | Internet: neidorff@ti.com
- 50 Phillippe Cote St. | Voice : (US) 603-222-8541
- Manchester, NH 03101 USA
-
- Note: Texas Instruments has openings for Analog and Mixed
- Signal Design Engineers in Manchester, New Hampshire. If
- interested, please send resume in confidence to address above.
-