Bert Whyte
NOW THAT AUDIO'S 25th anniversary has been duly noted and celebrated and we have finished our saunter "down memory lane," it's time to get back to the present and perhaps sneak a few looks at the future. As I write this, it is on the eve of my departure for the Los Angeles AES convention, which in addition to the usual interest of the audio papers and exhibits, will have the added stimulus of much that is new in quadraphonics. My report on these activities will appear in due time. By the time you read this I will be at the sixth annual audio seminar at Brigham Young University in Provo, Utah. This year the four-day seminar will be devoted entirely to four-channel stereo. As an exciting added attraction this year, at the conclusion of the seminar a chartered plane will fly a group of participants to Los Angeles for actual working quadraphonic sessions at Bill Putnam's United Recording studios. At the seminar I will have the pleasure of presenting the opening lecture, to be followed by lectures and demonstrations by such quad-sound luminaries as John Eargle of Altec; Jim Cunningham of Sound Market Recording; Dr. Duane H. Cooper of the University of Illinois; Bill Putnam of United Recording and his Chief Engineer, Jerry Feree; John Neal of Glen Glenn Sound, and Bill Robinson of William L. Robinson & Assoc. Among the subjects to be covered will be quadraphonic live recording, quadraphonic dub-down, matrix systems, quadraphonic disc mastering, control room engineering, and quadraphonic recording hardware. As you can see, that brash young upstart, four-channel stereo, is now thoroughly entrenched in the audio scene. I expect some really exciting quadraphonic developments at this seminar and a full report will be forthcoming.
Last year in the pages of AUDIO, Dr. Duane Cooper of the University of Illinois presented a two-part article on the construction of an acoustical delay line. The article aroused considerable interest, but although Dr. Cooper gave excellently detailed instructions on the fabrication of his "as. still a rather formidable … many people … unit.
The device was certainly beyond my resources of time and skill, and I remember thinking somewhat wistfully how I'd like to have one for its obvious uses in quadraphonic synthesis.
Among those who were impressed and fascinated by Dr. Cooper's delay unit was Bill Putnam of United Recording in Hollywood. Bill contacted Dr. Cooper and after a thorough evaluation of the delay unit, he was convinced of the basic soundness of the design.
Further discussions with Dr. Cooper resulted in a decision to undertake some developmental work on the device at Bill's United Recording Electronics Industries, with a view towards eventual commercial production. After months of experimentation a practical unit evolved and Bill decided to run off a few prototypes to farm out to various interested parties for their reactions.
Newly christened the "Cooper Time Cube," Bill was kind enough to send me one, which I have been using and testing for weeks now, and I am completely enthralled by the device. The "Time Cube" is now a standard UREI production unit and consists of a grey finished plywood housing, 24x24x9 in., which contains two coiled acoustical delay lines with their respective transducers, plus a 31/2x19 in. rack mounting electronics chassis containing a power supply, four UA 1109 card-type amplifiers, equalizing networks, VU meter, and operating controls. Also furnished are two standard 25 ft. microphone extension cables with XLR-3 type connectors for interconnecting the chassis to the housing.
In Dr. Cooper's original design, the acoustical delay lines were made of copper. In the production unit the delay lines are made of rigid-walled polyethylene tubing. This has the advantage of light weight and extremely smooth bore as well as freedom from ringing.
When the delay coils are placed in the housing, the housing is then filled with styrofoam pellets in sufficient quantity to produce a very high density pack around the coils. This serves to protect and isolate the coils, as well as furnish acoustic damping.
The Cooper Time Cube is described as a dual-channel audio delay-line system, with channel A having a delay of 16 milliseconds, and channel B 14 mS. The two channels may be cascaded for a single delay of 30 mS. The reason for the slightly differing delay times is to avoid comb-filter effects, and it is felt that in using the unit for quadraphonic synthesis, the differing delays help to produce a better ambient sound field. At this point, a word about how this acoustical delay system works would be in order, and this is very well described in the literature that accompanies the Time Cube.
"Delay is accomplished by utilizing the relatively slow velocity of sound wave transmission through air, as compared to the speed of electro-magnetic waves or electric current through a conductor; speed of sound in air is 1130 feet per second ... speed of light or electricity is 186,000 miles per second.
Therefore, the delay of sound through air, with respect to electronic signals through conductors can be approximated at 0.884 milliseconds per foot.
In the Cooper Time Cube an input signal to channel A is amplified and pre-equalized, then used to energize a dynamic transducer at one end of a coiled, rigid polyethylene tube containing one atmosphere air pressure. At the other end of the coiled tube is a special coupling assembly in which another transducer reconverts the acoustical signal to an electronic signal equivalent to the input, but delayed by the I6-mS acoustical length of the sound column. This signal is then post equalized and re-amplified to line level.
An input to channel B is similarly processed through the second acoustical transmission line having a delay of 14 mS." Acoustic delay lines are, of course, nothing really new. But up to now their drawbacks have always been restricted usable bandwidth and poor transfer characteristics due to acoustic reflections in the transmission line. Dr. Cooper's ingenious design solved these problems and subsequent improvements by Bill Putnam in acoustical equalizing techniques produced an even flatter frequency response and wider bandwidth. The Time Cube is unique as compared to earlier acoustical delay lines, in that distortion is very low, THD being on the order of less than 0.5% and with a signal-to-noise ratio greater than 70 dB. The applications for a high quality acoustical delay line are almost the same as for the Delta T digital delay unit discussed here several months ago, the main difference being that the Delta T has delays up to 320 mS which gives it an edge over the Time Cube in sound reinforcement work. The Time Cube can be used to "synthesize" quadraphonic sound from two-channel stereo material, to spatially enhance stereo or quadraphonic recordings, in loudness enhancement, and by delaying the "send" to reverberation chambers, or plates, to improve the illusion of a large room by approximating the longer dimension of the first order reflected sound (early sound) preceding the onset of reverberation.
My principal interest in the Time Cube is in the quadraphonic synthesis of regular stereo material whether it be on tape or disc. Now you can't call the Time Cube a stereo device, but the fact that it has dual channels is a great aid in creating the spatial ambience of the non-coherent "room sound." With the Time Cube we once again have the Haas precedence effect working for us.
Haas, Madsen, and others have shown that the spatial enhancement created by the delay, can be effected by delays as short as 2 mS, and that when the delay reaches approximately 35 mS (some scientists believe it can go as high as 50 mS), sound "fusion" ceases and the delayed sound may be heard as a discernible echo of the original sound source. Thus the 16 and 14 mS delays were chosen to keep them well within the limit of sound fusion.
The Time Cube is very easy to set up for quadraphonic simulation. The tape output of your front pre-amplifier is fed into the front panel input jacks on the Time Cube electronics chassis.
The output of the electronics is fed into a high level input on the rear channel pre-amplifier. The Time Cube's chassis has a mike cable leading to the delay line housing and feeding input signals. The output signal of the delay line feeds into the chassis via another mike cable. The VU meter on the chassis is of the switchable variety and there are two control pots. Each source, whether it be disc or any form of tape, must have available a level signal for initial set-up. This can be had in the form of various test discs and tapes.
The Time Cube is adjusted to the particular source by having the signal of the disc or tapes brought up to "0" VU on the meter by use of the input pots.
Once adjusted, nothing further is necessary until you change the nature of your source. After everything is set up, you just "dial" into your rear channels the level of ambience you desire. Needless to say, this quad synthesizing works best for classical music, but interesting effects can be created with pop material. If your source is of really good quality, the results can be quite spectacular. Most dramatic of all is to play 15 ips Dolby "A" stereo tapes of classical works made in a good hall.
The rear ambience you create with the delay give an astonishing "roundness" and a great sense of "liveness" and increased presence to the overall sound, and the absence of noise in either the front or rear heightens the feeling of "on-the-spot" realism.
The Time Cube is a professional product for the recording studio and considering the fact that it has two channels of delay, the $850 price tag is reasonable. I noted that the card amplifiers in the unit and other circuits used discrete components. There are some obvious places where IC's could be used and the price of the unit reduced considerably. In a recent chat with Bill Putnam, he gave a few hints that ultimately there might be an audiophile model. I certainly hope so, for once you have had one of the Time Cubes in your home and have become used to that wonderfully increased realism of your normal two-channel stereo material, you are not very keen on going back to so-called "normal" listening.
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In Memoriam
Dr. Frederick V. Hunt
Dr. Frederick V. Hunt, formerly Gordon McKay Professor of Applied Physics and Rumford Professor of Physics at Harvard University, died of a heart attack early on April 21 while attending the meeting of the Acoustical Society of America in Buffalo.
Ted Hunt combined the theoretical knowledge of the physicist with the practicality of the engineer and the wisdom of an educator. Only the evening before his death, at a committee meeting chaired by one of his former students, he was urging the advantages of a general educational background in … compared with early of its branches… Acoustical Society of America in 1951-2, received its Pioneers in Underwater Acoustics Medal in 1965 and its Gold Medal in 1969. In 1954 he received the Emile Berliner Award of the Audio Engineering Society "for an outstanding achievement in the field of audio engineering," and in 1965 the John H. Potts Memorial Award "for outstanding achievement in tracing distortion encountered in disc reproduction." He served as President of the AES in 1969-70. In 1947 he received the Presidential Medal of Merit for his work in sonar and torpedoes as Director of the wartime Harvard Underwater Sound Laboratory, and in 1970 the Navy awarded him their Distinguished Public Service Medal at the time of his retirement from Harvard. He then moved to California where he continued his work, as a research associate at the Scripps Institute of Oceanography.
In the world of audio Professor Hunt was particularly known for his work on pick-ups and loudspeakers. With J. A. Pierce, also of Harvard, he described a pick-up "with unbelievable response, flat within plus or minus 3dB from 30 to 18,000 cycles" and a 5 gram needle pressure, in the March, 1938 issue of Electronics, and the physical principles underlying it in the Journal of the Acoustical Society in July, 1938. His October, 1962 paper in the Journal of the Audio Engineering Society on "The Rational Design of Phonograph Pickups" is regarded as a classic, and he was at the time of his death planning a new edition of his book "Electroacoustics" (now out of print but available in facsimile from University Microfilms). He will be sorely missed by his many friends and colleagues in all branches of acoustics.
-G.L.W.
(Audio magazine, Jul. 1972; Bert Whyte)
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