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1990-10-14
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DIGITAL RECORDING PRIMER
An introduction to PCM Digital recording and playback systems
by Rob Burr
By now, you must have heard a lot of buzz about digital recordings, digital
discs, and the like. What is it about this new way of making recordings that is
so new and different?
Let's examine the basic differences in the way each method records program
material. In conventional analog recording, sound is transformed into magnetic
level on tape in relation to loudness. Digital converts sound to binary
numbers, then records the numbers on tape.
In analog recording, the input signal causes a magnetic field to be recorded on
tape. The louder the sound, the greater the level of magnetism on tape.
Obviously, there is a limit to the amount of magnetism that tapes can handle.
There is also a given low limit of magnetism that is indistinguisable from the
level of "hiss" found on all analog recorders. The difference between these two
limits is the Signal-to-Noise Ratio. Typical broadcast studio recorders have a
ratio of over 70 decibels. (10 to the 7th power, or 10,000,000 to 1). This
sounds quite impressive until you realize that the human ear has a loud to soft
range of over 130 decibels.(10 to the 13th power, or 10,000,000,000,000 to 1)
Another problem in analog recording is know as "smear". This is the loss in
accuracy on transients (sudden level changes like drum attacks, etc.). This
problem increases with each playing of the tape. One of analog's good points is
its ability to deal with drop-outs (inconsistancies in magnetic coating on
tape). If tape is running at 15 inches per second, a small 1/50 inch drop-out
would produce a loss of sound for 1/750 second. This would be hard for the
average ear to detect.
In digital recording, the sound is sampled over 44 thousand times every second.
The trick occurs in a little circuit known as an Analog-to-digital converter.
This assigns a binary number that describes each sample, so the recorder sees a
fast steady stream of numbers. No sound is recorded on tape, only numbers. This
technique is known as Pulse Code Modulation (PCM). On playback the numbers are
converted back to sound, so no hiss is produced. The professional digital
recording system has a signal-to-noise ratio in excess of 90 db, making it over
100 times quieter than conventional analog systems. This increase in dynamic
range means that musical compositions containing low levels are not lost to
tape hiss. There is no degradation of the signal after each playing of a
digital tape, nor is there any smear. If a drop-out occurs, the computer must
reconstruct the missing numbers from previous and subsequent numbers in
micro-seconds. Another advantage of Digital recording is the absence of Wow and
Flutter because the rate at which the numbers are played back is controlled by
the "sync" or control track encoded in the tape.
Unless you happen to be the first one on the block to own a Compact Disc player
or digital cassette deck, the playback systems that we all have are analog
systems. Reel-to-reel and cassette decks, as well as phonograph records, are
very common tried and true mediums. As a matter of fact, the lowly LP, the same
basic phonograph system invented by Thomas Edison is still, by far, the most
popular format internationally. The finest discs in the world, Japanese
imported pressings, now boast a signal-to-noise ratio near 80 db. The standard
of quality of American record pressings is among the poorest of any large
record market. Many U.S. record buyers, therefore, buy imports to obtain
quality pressings of popular releases at a significantly higher price. The
problem of home taping has largely resulted from over-priced, poor quality
cassettes from record companies. Consumers prefer to tape their own versions on
cassette from albums or radio.
The Compact Disc is a 4.7 inch laser playback disk capable of containing up to
71 minutes of music per side in stereo. The disk is made up of micro-thin
tracks of binary numbers which are read by a laser beam and converted to sound.
In addition to program material, the Compact Disc can also contain other
imformation including the name and time of each of the songs. They can't wear
out because the laser never touches the surface of the record. Signal-to- noise
ratios exceeding 90 db are easily achieved with this system. Because of it's
disk form, the Compact Disc can be stamped out in great quantity for relatively
low cost. Recently, Compact Disc players have been selling for less than $500,
and the disks themselves vary from about $14 to $25 each.
Digital cassette players work essentially the same way as professional digital
recorders in the playback mode. Numbers are stored on tape which are converted
back to sound in the digital-to-analog converter. Because they must be recorded
in real time like video cassettes, as opposed to high speed duplication as with
standard audio cassettes, the cost per unit is greater than the disk format.
For this reason, digital cassettes will probably not acheive the popularity
that seems destined for the Compact Disc until a method is developed to
duplicate digital cassettes at high speed.