October 1969 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
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It is hard to imagine anyone
who has not heard of the Dolby noise reduction process, even if he/she has no idea what
it is. Dr. Ray
Dolby developed his process in 1965, although it was not patented until 1969 - the
year this article appeared in Radio-Electronics magazine. At the time, "Dolbyized"
audio systems were not available in the consumer marketplace because the price was prohibitively
high - $1,495* for a basic A301 system. Only about 25 units per month were being
produced, primarily for recording studios and reproduction factories. Dolby's magic
that can reduce noise by 15 dB works on the
companding
(portmanteau of compression and expansion) principle, thereby eliminating or greatly
suppressing the discernable "hiss." Dolby B is still the most common version in use
after nearly half a century.
* $1,495 in 1969 is the equivalent of $10,620 in 2018 money per the BLS
Inflation Calculator.
Other Dolby articles from vintage electronics magazines include: "The Dolby System - How it Works"
(October 1969 Radio-Electronics), "All About Dolby" (June 1971
Radio-Electronics), "The
Dolby Technique for Reducing Noise" (August 1972 Popular Electronics),
"The
Dolby Noise-Reduction System" (May 1969 Electronics World).
The Dolby System - How it Works

Rack-mounted A301 uses 4 plug-in PC cards for bands in identical stereo
channels.
Taking the hiss from hi-fi tape recording.
by Walter G. Salm
Rapidly gaining acceptance by major recording companies, the Dolby system cuts tape
hiss 10-15 dB and reduces print - through 10 dB. Although you need a 'black
box' to hear a Dolbyized tape, prerecorded tapes are sounding better than ever.
There's an ever-present annoyance with tape recordings. It's called tape noise. It
takes various forms, with hiss being the most readily apparent to the listener. The other
noises are tape recorder rumble and scrape, and that ever-present gremlin, print-through.
These noises occur at the tape recorder as the recording is being made-or in the case
of print-through, after the tape is made. The noise isn't present in the signal coming
to the tape equipment from the mixing console; up to that point, the signal's nice and
clean.
Various methods have been tried to eliminate noise, hiss in particular - exotic filtering
systems, compression or expansion amplifiers, and a variety of related devices. Manufacturers
frequently announce a "totally new" noise-reduction system. Most of these systems work
up to a point, but many inject "swishing" noises into the recording because of the compressor
amplifier's relatively slow attack time. It can't react fast enough to the rapidly changing
audio dynamics of the input.
Low-Passage Noise

A tape deck using one Dolby frequency band to cut tape hiss is manufactured
by KLH.
Where are these tape noises most noticeable? You don't need a cram course in audio
to know - it's always in the low-level music passages. In the loud sections of a recording,
there are plenty of desirable decibels to mask any intruding noise, but hit that pianissimo
section and suddenly the hiss comes storming through, along with print, scrape and rumble.
If the intrusive noise could be drastically reduced in these low passages, so the reasoning
goes, then the overall noise level would be reduced.
Working on this premise, Dr. Ray Dolby developed his A301 noise-reduction system,
and started manufacturing it in England a couple of years ago. This system is inserted
in the signal path between the console and the tape equipment during the taping session.
During playback, the tape's signal must pass through the A301 again on its way to the
amplifier. A Dolbyized tape cannot be played back without the benefit of reprocessing
by the A301.
The A301 itself is actually a dual unit - two systems for two-channel stereo. The
same stereo unit that's used for recording can be used for playback, simply by changing
the connectors and flipping switches. The basic A301 is tagged at $1495, so it's not
a toy that the casual home recording enthusiast can buy for his amusement. Tape-to-tape
dubs do not require further Dolbyized (as long as tape-to-tape equalization remains the
same).
The overall effect is to reduce hiss and other noise factors by 10 to 15 dB!
A listening test demonstrates its effectiveness graphically. A-B comparison with and
without the A301 in the system makes one thing immediately apparent - with the Dolby,
there's absolutely no discernible hiss, not one little bit of it.
As a starting point, the Dolby system divides the audio spectrum into four frequency
bands: 0-80 Hz, 80-3000 Hz, 3-9 kHz and 9 kHz to the upper limit
of the audio spectrum. Crossover transitions between these bands are exceptionally smooth,
with no measurable discontinuities or attenuation.

Fig. 1 - System's feed-back loops provide necessary expansion and
compression to override background noise.
In the process of making a recording, low-level signals are boosted before reaching
the tape recorder and compressed by an identical amount in playback. The expansion/compression
moves the low-level signal up at least 10 dB. This 100 dB edge over intrusive
noise is maintained during expansion. The net result is that noise level is at least
10 dB lower than without the Dolby process.
In his description to the Audio Engineering Society in 1967, Dolby characterized the
basic elements of his black box as shown in Fig. 1. G1 and G2 are
identical signal multipliers controlled by the signal's amplitude, frequency and dynamic
properties. During recording, network G1 passes low-level signals to the adder,
which adds a component (signal expansion) to the signal on its way to the recorder. During
playback, G2 p passes low-level components to the subtractor, which partially
cancels these noise elements in the signal. G2 also partially cancels low-level
(desirable) signal components, thus offsetting (compressing) the signal that had been
partially expanded through G1's action.
Mathematically, Dolby explains that if the input to the recording processor is x (some
function of time), the signal in the channel is y, and the output signal from the reproducing
processor is z, the condition is described as:
y = [1 + G1 (x)] x
(1)
and
z = y - zG2(z) or
z = { 1/[1 + G2(z)] }y
(2)
Combining equations (1) and (2):
z = { [1 + G1(x)] / [1 + G2(z)] }x.
(3)
Solving these equations, G1 = G2 and z = x. These solutions
show that the output signal will equal the input signal if G1 and G2
are the same, provided G(z) does not become -1 (no oscillation) and the functions in
(1) and (2) are continuous and single-valued - in other words, no tracking ambiguity.
It looks fine on paper, and the proof of the math is in the listening: it works. In
fact, it works so well that, in A-B tests, the listener can't help but wonder if someone
hasn't jacked up the hiss level in the unprocessed tape - the difference is that marked
and apparent. But no, there's been no monkeying with the tape - just the normal amount
of dubbing and processing.
Recording Companies "Dolbyize"

Identical A301 halves can serve for stereo or a record-playback monitor.

A301 boosts low-level audio before recording, compressing it in playback.
First to recognize the value of this new system have been the major recording companies.
Since every recording-tape and disc - must first go through several generations of tape
dubs, eliminating the noise that creeps into these transfers and mixes makes the final,
non-Dolbyized product that much cleaner.
True, the prerecorded tape purchased by the consumer has its own hiss and other noises,
but all the preceding noise in the production chain has been eliminated. Thus, instead
of a noise level as high as 10 to 14 dB, the finished product may have a noise level
of only 3 dB - a more-than-tolerable level. Tapes commercially produced this way
sound so good, in fact, that duplicators such as Ampex have started touting their remarkably
noise-free quality. Although discs have had very low noise levels for some years, those
pressed from Dolbyized master tapes sound notably quieter and cleaner.
But it's no simple matter for a recording company to Dolbyize. First, there's the
very limited production of A301's. Ray Dolby has a baker's dozen technicians in his London
shop lovingly hand-crafting his A301's with a Rolls-Royce kind of zeal and perfectionism.
Total production capacity now is about 25 units per month, and a large recording company
may require at least that many units to handle its manifold operations.
Dolby won't be hurried. He's a perfectionist, and he sees this perfectionism as part
of his stock in trade. A brilliant designer, he was largely responsible for the development
of the electronic circuitry for the original Ampex video tape recorders while he was
still a teenage undergraduate at Stanford University. He received a PhD in physics from
Cambridge in 1961.
Dolby at Popular Prices
The price picture isn't all that gloomy. There's one way the home recording enthusiast
can get his feet wet with the Dolby system. It's called the KLH recorder. This is KLH's
first entry in the tape field, and it's a lulu. In addition to a host of professional-caliber
specs, the deck has a built-in Dolby system - and all for $600. [A recently introduced
KLH Dolby tape deck sells for less than half this price.]
Actually, it's not a full-blown Dolby. The machine just uses the third frequency band
since this is where most hiss occurs, The KLH Dolby produces tapes recorded at 3-3/4 ips with no higher noise level than tapes recorded at 15 ips without Dolby treatment.
The KLH Dolby does a dandy job of killing hiss, and could be a very worthwhile investment
for the serious recording hobbyist who wants to maintain the best possible quality in
his home recordings. But what happens when he wants to dub and maintain noise free tapes?
Buy two KLH recorders? This might be what the Cambridge whiz-kids had in mind, especially
since they have an exclusive license to use the Dolby circuit for manufacturing in the
US.
The System's Noncompatibility
The Dolby system isn't a panacea. It can't clean up already recorded tapes. Its function
is to process the sound going onto the tape, and then to reprocess it as it leaves the
tape. It just won't clean up noise already indelibly etched in those oxide particles.
Another problem: in today's ever-expanding tape channel capabilities, the A301's price
and relative unavailability makes it impossible to use with the 8- and 16-channel audio
mastering that is becoming more and more popular. This might involve having as many as
16 units in a single recording studio control room for making just one master tape. Fortunately,
such multitrack originals are mostly used for "Acid" rock and other popular sounds which
don't make very critical demands so far as recorded noise levels are concerned. It's
in the realm of the serious recording - which is usually taped in old-fashioned, two-channel
stereo - that the Dolby system can really come into its own.
Another problematical restriction: Dolbyized tapes simply can't be played back on
an ordinary tape system. The A301 must be in there to expand in the right places. Without
it, the dynamics and recorded sound in general will come out sounding pretty weird. But
in this era of electronic music and all of its strangeness to our more conventional ears,
this may well open the door to a new avenue of experimentation with electronically generated
musical effects. The engineer (or musician?) may play with a console that controls the
amount of Dolby expansion to be imposed on a Dolbyized tape master.
And what about that A301? It's just a black box with some connectors on it. No knobs
to twiddle; no meters to watch: no circuit failures to keep the service techs out of
mischief. It's downright maddening! No knobs; really, Dr. Dolby!
Posted November 28, 2018
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