Behind The Scenes (Jan. 1972)

by Bert Whyte

I BEGAN to write about four-channel stereo in 1969. Now, writing this first column for 1972, it comes as a mild shock to realize that this major advance in sound reproduction is still in its gestation period. However, there are many signs and portents which lead me to feel that sometime during this year four-channel stereo will finally come to term and become a viable entity in consumer audio.

I think it is important to understand that when four-channel stereo becomes a commercial reality, it will appear in a number of configurations. The term "four-channel stereo" is very loosely used these days. Consider these permutations for example: whether the format is discrete open reel tape, discrete Q-8 cartridge or matrix disc, if the music is of the pop "surround sound" type with equal amplitude fore and aft, it is, strictly speaking, four-channel monophonic sound. Discrete open reel tape and discrete Q-8 cartridge can give us true four-channel stereo with the rear channels reproducing ambient information. The matrix disc cannot of course give us true discrete four-channel stereo. However, the rear channels can carry somewhat compromised ambient information, which from a psycho-acoustic viewpoint may present a fairly effective simulation of stereo. I personally feel that the tape formats and the disc will co-exist. The suggestion put forth by some people, that tape utilize the matrix system, is patently ridiculous. Why take the ideal format for four-channel stereo and subvert it with a matrix, just to save on tape? Sure, discrete open reel tape is the purists' medium. Those who can afford and appreciate the advantages of this format, should not be penalized with a compromised product.

Speaking of matrix systems, I'm sure you all have read the advertisements about various decoders. You have also read about the Dyna system. You have heard terms like "derived four-channel stereo" and "ambient extraction" bandied about. The gist of these ads is that you can feed your ordinary garden-variety two-channel stereo discs into a decoder (with amplifiers and speakers for the rear channels of course) or in the case of the Dyna system, one or two extra speakers, and obtain "four-channel stereo effects." Well, with all due respect to all involved, this is not even a simulation of four-channel stereo. In the first place, the recovery of "sum and difference" information is strictly a matter of happenstance. In other words, you encounter this information on a random basis; it is not a constant factor in all recordings. In the second place, if the record does have some left minus right information, it is reproduced in the rear speakers without the time delay characteristic of a true four-channel stereo recording. In other words, in a typical four-channel stereo recording, depending on hall acoustics, the "rear" microphones pick up the multiple reflections from the walls, ceiling, and floor some number of milliseconds after the onset of the initial wavefronts of the instruments of the orchestra. Ideally, the delay should be no more than 33 to 35 mS, which is the limit of sound fusion in the brain. More than that, and you encounter echo and "slap-back." Now don't get me wrong! The word "enhancement" covers a lot of ground, and I know quite a few people who are very pleased with this "derived four-channel stereo." It is just not the "McCoy," not the "real thing." For those who enjoy this kind of sound, especially the Dyna advocates, here is a tip for you: In the early years of the stereo disc, from 1958 through 1963, London, Angel, and Deutsche Grammophon were using the European "MS" (middle/side) stereo recording technique. This utilized a cardioid and a figure-eight microphone placed together, with the cardioid facing forward and the figure-eight at right angles to the cardioid with the "loops" of the "eight" to the left and right. The mikes were hung at an appropriate height and distance from the orchestra on the centerline of the hall. (A bit later special stereo mikes with the two patterns in a single housing were evolved for this task.) This "MS" mike technique produced a lot of left minus right information, and with your Dyna hookup you can extract this information on virtually every record made during that era. With our American "spaced array" mike technique (which is now used by almost every recording company here and in Europe), finding the difference information is mostly a matter of luck.

So dig out all of those older stereo recordings made by the companies I mentioned if you want practically guaranteed results! One can readily understand that people who have a lot of money invested in a large stereo record collection would opt for any technique that offered a simulation of four-channel stereo. Most of the techniques available rely heavily on psychoacoustic phenomena, such as the well-known Haas precedence effect. The thing one must accept is the evidence of one's ears, in spite of the fact that intellectually you know you are being fooled.

If the techniques are sophisticated enough, such as the Eargle process tapes I described some months ago, the four-channel stereo simulation is virtually perfect. As you may remember, Mr. Eargle used a combination of time delay, plus reverberation and selective equalization in his process. His was strictly a studio technique, but one of the most important aspects of the process, the time delay, can be applicable to four-channel stereo simulations in the home. It goes without saying that whether real or simulated; four-channel stereo requires four channels of amplification and four speakers.

The most readily available and the least expensive form of time delay available to the audiophile are the well-known spring reverberation units.

There are a number of units on the market, differing mainly in the length of the spring and in damping methods.

Some units use multiple springs. In any case, while this type of delay can be surprisingly effective, these units are all easily over-driven, and even with moderate input they produce a characteristic "boinnggg" sound which is distinctly artificial and unpleasant.

There is one spring unit, a joint development of Harman-Kardon and the Para-Orban company, which not only affords delays as long as 7 ms (quite a bit for these devices) but has a very low residual "boinnggg" noise. At the moment, plans to market the unit are sort of up in the air.

One of the most simple, but most effective means of time delay was devised by Madsen of the Danish Bang and Olufsen company. On a standard tape deck he mounted a special playback head with a double gap, with separate playback pre-amps for each gap. (Bad nomenclature here because what we are really dealing with is a double head.) The distance between the first and second gaps determines the delay, in this case, 10 ms. In practice, the signal from the first gap goes to the front amplifiers and speakers, the signal from the second gap, is the same information delayed 10 ms and sent to the rear amplifiers and speakers. This technique restricts one to the use of tape ... but is that bad? Unfortunately, as far as I know there are no plans to market the device at the present time.

There are two electro-acoustical devices for the generation of time delays. One is a Japanese product (whose brand name I've sworn not to reveal at this time) which is sold everywhere in the world ... except the United States! Basically a quadraphonic preamplifier, the unit has within it, a honeycomb type of plastic extrusion which is actually a continuous air passage of some 58 feet in length. At the beginning of the tube a tiny mylar-dome transducer is sealed in place. A third of the way down the tube, a tiny microphone is inserted in the tube and sealed. Another third of the way down the tube is a similar mike and at the end of the last third of the tube, still another mike. These mikes are connected to standard mike pre-amplifiers, with control pots. The positions of the mikes in the tube give delays of 15, 30 and 45 ms, the amplitude of each being separately adjustable. Used with restraint this unit is capable of some interesting simulations.

The drawbacks are a certain "honkiness"--a nasal sound to the delay, a considerable attenuation of high frequencies, and rapid increase in distortion if driven too hard. I think with some modifications to correct these deficiencies, this unit has considerable potential.

A much more sophisticated electro-acoustical delay tube has been devised by the very clever Professor Duane Cooper of the University of Illinois.

A full description of the device was presented by the inventor in the April and May 1971 issues of AUDIO. Essentially a looped copper tube is used, with appropriate driver and pickup elements and an equalizer circuit.

The delay obtained with this unit is on the order of 14 to 16 ms, and is said to be wide band, with low distortion and a good signal-to-noise ratio. Mr. Bill Putnum of United Recording in Los Angeles has concluded an arrangement with Prof. Cooper to manufacture the delay tube as a commercially available product, and I have been promised an early production prototype with which to experiment. The anticipated price of the unit is around $600.00. For the ultimate in delay devices and for those with Texas oil baron pocketbooks, we have the Delta T, the invention of two MIT professors. This is a totally solid-state device which accepts analog signals, converts them to digital and stores them in a memory core, and then reconverts to analog with delays available up to 320 ms in 5 ms increments. The Delta T is distributed by the well-known professional audio firm, Gotham Audio Corp., of New York. Mr. Eli Passen of Gotham was kind enough to furnish me with a Delta T unit, and I have spent many hours experimenting with this fascinating instrument. Most high quality pre-amps have sufficient output to drive the Delta T. In use, the spare power amplifier output found on most pre -amps is connected to the input of the Delta T. The Delta T can be supplied with as many as five output modules, each of which is capable of the full 320 ms range of delay. The delay is simply set with either or both of two rotary switches, one for coarse adjustment in 40 ms steps, the other a fine adjustment in steps of 5 ms. In four -channel stereo simulation, the coarse switch is rarely used. On each module is a rocker switch which enables you to switch between the direct information of the front channels (delay off) and the delayed sound in the rear channels (delay on). I should have mentioned that the stereo output of the pre -amp is fed into a Y connector and thence into the single input of the Delta T. This of course is monophonic, but you feed this into two of the output modules, which in turn feed into the rear channel amplifiers and speakers.

This mono input and double (not stereo) output is common to most of the delay devices I have described. Since you are dealing with non -coherent sound that is almost totally non-localizable, this mono rear channel sound is not a problem. The dynamic range of the Delta T is over 60 dB, more than enough to handle almost any kind of program material. I found that in my living room, delays between 10 and 30 ms were the most effective. In fact, with the Delta T it was easy to detect the 33 ms limit of sound fusion in the brain, by simply switching in an exaggerated delay of 50 or more ms, raising the level of the rear channels and playing music with plenty of repetitive transients. The resounding "slapback" under these conditions was an eloquent witness for the validity of the fusion thesis. It was quite astonishing to select a nice two -channel stereo recording, run it through the Delta T with about 20-25 ms delay, and have the whole room open up with a spaciousness and heightened sense of "participation" and reality that was hard to believe. Switch off the delay and the effect is dramatic . .. the whole sonic perspective seems intolerably cramped and circumscribed. Believe me, it is one helluva effect, but at $4000.00, this can hardly be described as home type equipment. The Delta T is of course a tool for the modern re- cording studio. For example, take a group of six string players, mike them through the Delta T with an appropriate amount of delay, and all of a sudden the six violins sound like 12 violins. In the wild special effects that rock producers want these days, a highly exaggerated delay is deliberately used with certain instruments. The Delta T also has obvious uses in the public address field.

In closing I should also mention that the Delta T would be a highly precise and controllable delay device to replace the 80 inches per second tape delay used in the Eargle process four-channel tapes.

There would seem to be little doubt that a good delay unit can add an impressive quotient of realism in the home listening situation. Now if only someone can come up with a reasonably priced unit... . Early in November, the New York hi-fi press corps was present at a joint conference held by RCA Records, JVC, and Panasonic, which was billed as a "progress report on the four -channel discrete disc." It was known that RCA was investigating the JVC discrete disc, but the entry of Panasonic into the act was a surprise. We had the usual speeches from management and engineering, containing the usual ambiguities. And we had a demonstration of the discrete disc with a new Hugo Montenegro recording, which according to Hugo in person, wí3s especially arranged and recorded for the four - channel medium. We also had a clever example of how discrete the JVC disc is by the expediency of four people speaking at the same time ... in German from one loudspeaker, Italian from another, Japanese from the third loud- speaker, and in English from the fourth unit! Now in the words of a famous politician . . . I want to make myself perfectly clear. The demonstration that I heard was very impressive. The sound was very clean, the discreteness of the stereo beyond question ... in all aspects a rousing success! Alas, there are certain problems with this concept, which may not be resolved for some time to come.

The JVC disc has been described in these pages before. You know that it works on a sort of modified multiplex principle, that the carrier requires a phono cartridge with response to 45-50 kHz, that a demodulator is required.

Now here are some of the statements that were made at the conference. "The life of a discrete disc in laboratory testing when played on stereo home instrument -type equipment already is equivalent to that of stereo records played on the same equipment." "Partial mixed-system playback compatibility has been achieved. Discrete discs, played first in stereo equipment and then on four -channel equipment, have a playback life which has greatly increased within the past four months." "The playing time for a discrete disc now equals the playing time of stereo records at the time of stereo's introduction without sacrifice of the basic signal- to-noise ratio of record systems." I have underlined the kicker in that last sentence ... which means that playing time is roughly 20 to 22 minutes. Indeed, the demo record given to me has perhaps 2 3/4 inches of recording on a side. As to the first statement about discrete disc life played back on four-channel equipment, although there are no statistics available, with present record playback technology, I'd be inclined to accept this as fact value.

The mixed system playback is quite another matter. The crux of things is quite simply whether the high frequency carrier will be wiped off the discrete disc in a comparatively short time. We were told the life of the disc in this circumstance had been increased by a factor of two. But they never gave us the base figure! In answer to a question from the floor regarding the life expectancy of the carrier, it was stated that the JVC discrete disc would not be released to the public until the record could be played on the normal stereo equipment in the hands of the consumer, without degradation. When asked how soon this would be forthcoming, the answer was rather vague.

However, it must be noted that JVC and Panasonic must evidently believe that introduction of the discrete disc is not far off, because between them they had six exhibit rooms crammed with handsome, well -designed working prototypes of all sorts of four-channel equipment. There were straight de- modulators, demodulators with a stereo amplifier for the rear channels, preamps with demodulator, integrated amps with demodulator, four-channel stereo receivers with the demodulator, even four-channel compact systems, and of course the vital phono cartridges.

One by Panasonic is a semiconductor piezo-electric type. The JVC cartridge is magnetic.

As noted, the sound was very good indeed, the glittering array of equipment most impressive ... the sad part of the whole thing, apart from the problems reviewed, is that the JVC discrete disc cannot be broadcast. At least not in the foreseeable future. The Donen system would make it possible, but the FCC would have to do a radical about-face to embrace this idea. No matter what the people at the conference pooh-poohed about this, the inability to broadcast the JVC disc is a severe limitation. Perhaps the JVC discrete disc can stand on its own as a sort of audiophile special. It certainly has the best sound and it would be 'kind of a shame to lose one of the few chances we'll have to get a discrete four-channel stereo disc.

(Audio magazine, Jan. 1972; Bert Whyte)

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