|Home | Audio Magazine | Stereo Review magazine | Good Sound | Troubleshooting|
The Audio Engineering Society held its 55th convention as another "Weekend at the Waldorf" from October 29 to November 1st. Such is the growth of the dynamic audio industry that this was the biggest East Coast AES convention thus far, with a record first-day crowd of registrants swarming into the exhibit booths in the ballroom and the sound demonstration rooms on the 5th floor...and, of course, listening to the papers presented at the technical sessions. As I have pointed out before, no AES convention is dull, but some generate high excitement quotients through significant advances or breakthroughs in audio technology. The 55th AES convention may well be remembered as the beginning of the era of digital audio recording.
Digital recording has always been a "hot" topic among audio engineers. It has been endlessly discussed and treated as a "Holy Grail," a panacea for all the multitudinous ills of analog magnetic tape recording. For some years now, the debut of digital recording has always been "just around the corner." At this convention, two papers were presented and an actual demonstration of digital audio recording was given, which may indeed represent the first steps in the conversion of audio recording from analog to digital technology.
Mr. J. Stanley Kriz, of the Three Rivers Computer Corp., Pittsburgh, Pa., described his "Audio Analog-Digital Analog Conversion System," which is basic to the technology of moving audio signals into and out of the digital domain. Mr. Richard B. Warnock, of Soundstream, Inc., Salt Lake City, Utah, created quite a stir with his paper on "Longitudinal Digital Recording of Audio." Mr. Warnock's associate and president of Soundstream, Inc., is Dr. Thomas G. Stockham Jr., whom you may recall is the gentleman responsible for the fascinating digital reprocessing of old Caruso acoustical recordings that removed the honky megaphone sound from the recordings and revealed more of the true nature of Caruso's voice.
During the run of the convention, Dr. Stockham and Mr. Warnock held forth in a 9th floor suite and gave private demonstrations of their' digital recording system. Basically, the Soundstream digital recorder consists of a standard Honeywell computer transport operating at 30 ips (which gives 30 minutes recording from a 4600-foot reel of tape), plus various proprietary modules for analog–to-digital and digital-to-analog converters, sample and hold units, encoders and decoders for synch and data bits, memory buffers, input and output low pass filters, etc. The recorder demonstrated was a 15-bit system with a sampling rate of 37,500 per second, which gives sufficient resolution for a d.c. to 15 kHz audio bandwidth. One inch, high quality instrumentation tape is used, and this prototype machine has two channels for stereo recording, although as many as 40 channels could be accommodated on the one-inch tape. At this point, think I had better put this discussion on hold, as I'm sure distress flags are flying from readers for whom digital technology is terra incognito. What say you, Dr. Tom...can we con you into furnishing the readers of Audio with a basic primer on digital recording?
The Why and Wherefore
In the meanwhile, there are no doubt those people who ask a very basic question, why digital recording? What is the big deal? Understandably Mr. Warnock and Dr. Stockham have an axe to grind, so when they enumerate the problems of analog tape recording, this must be taken into consideration. Thus, while ultra-sophisticated machines like the Ampex ATR-100, have advanced the quality of analog recording by at least an order of magnitude, it is valid to discuss the inherent limitations of the medium.
In his paper Mr. Warnock notes the following problems with analog magnetic tape recording, inadequate dynamic range, i.e., low signal-to-noise ratio; inherent phase distortion; inherent harmonic distortion; insufficient transient response; modulation noise; cross talk; print through;,, multi-copy degradation; wow and flutter; inherent limitations in noise reduction systems; storage degradation with time, and limited low frequency response. Of the 12 problems listed, Mr. Warnock goes on to state that digital recording completely eliminates modulation noise; print through, wow and flutter, limitations to the low frequency response, and the need for noise reduction systems.
Phase distortion, harmonic distortion, and cross-talk are reduced to inaudibility under worst-case conditions. Dynamic range (S/N ratio) is significantly improved...and at a rated 90 dB below maximum signal level, that is putting it mildly! Significant improvement in the problems of transient response, storage degradation and multi-copy degradation is also claimed.
All this is very impressive, to say the least, and it is easy to understand why such a glamorous aura surrounds digital recording. A glimpse of how digital recording solves some of the analog recording problems is contained in this note on modulation excerpted from Mr. Warnock's paper:
"Modulation noise is an audible distortion in analog recording that increases and decreases with increasing and decreasing amplitude of the recorded waveform. It results from the varying head-to-tape spacing caused by dirt and dust particles and uneven oxide coating of the tape, among other things. This varying tape-to-head spacing causes a slight time-varying modulation of the originally recorded waveform. In digital recording, the audio content is in the encoded serial combination of ones and zeros, and although the tape to head spacing may change, that can affect only the amplitude of the ones and zeros and not their encoded audio content. Thus, modulation distortion is totally eliminated as a problem in the reproduced audio waveform, simply because the audio is encoded digitally." The Soundstream digital recorder has actually been used to make professional recordings. Jerry Bruck, an engineer highly regarded for his recordings of classical music, recorded Virgil Thompson's opera "The Mother of us All" on a 16-channel analog recorder and fed a 2-channel mixdown from his console to the digital recorder. For whatever reason, the recording venue was New Mexico in a hall with difficult acoustics, and it must be noted that many people do not find this particular piece the most accessible of music. At the demonstration I heard, Soundstream had a tough audience with John Eargle, John Woram, John Curl and editors Gene Pitts, Ed Canby, Barney Pisha, and yours truly on hand. On listening to excerpts from the opera, we were impressed the most with the totally silent background afforded by the 90 dB S/N ratio. Perhaps the particular kind of amplifier and speakers that were used in the demonstration colored our opinions, for while we thought the sound was very clean, with crisp transients and fine articulation on the voices, we felt that a top-quality analog recorder could have done as well. Another recording, this time of some people in a studio simply noodling on various instruments, was a more impressive demonstration of the digital machine's capabilities.
What probably influenced our judgments more than anything else was the fact that the top end rolled off above 7 or 8 kHz. This was explained as a function of the particular 15-kHz low-pass filters in the reproduce chain and of the sampling rate. Extension of the bandwidth to full 20 kHz can be accomplished by increasing the sampling rate and using appropriate filters. It all really comes down to the ability of being able to A/B between an analog and a digital recorder under controlled conditions with the same program material...and after all, the digital recorder was a prototype unit.
Like anything else, there are problems in digital recording that have not been fully resolved. For example, even with the very best instrumentation tape, dropouts can be devastating to the reconstruction process in playback. However, there are error correction procedures that can reduce audible errors due to dropouts to as few as one or two in five minutes of playback. Dr. Stockham and Mr. Warnock are to be congratulated for bringing digital recording out of the laboratory and for the advances they have embodied in their prototype recorder. Both men are confident that they will have full audio bandwidth digital recorders in production in the not too distant future. More power to them! Now that we have accorded digital recording the importance it deserves, lets get on with the rest of the 55th AES convention.
I have become nothing but redundant in reporting that every AES convention has a plethora of mixers and consoles. Why should this one be different? They were everywhere in the ballroom, intimidating in their multi slider, multi-switch, multi-knobbed, and multi-metered complexity, but for all that, the object of many covetous glances. This year a new element was on display, this being quite a number of specialized mixer/ turntable setups for the burgeoning disco market. Even old line hi-fi firms like Bozark are in the act, with their disco mixer garnering quite a number of sales. Shure Bros. surprised people with a low-cost spectrum analyzer and equalization system with easy-to-use LED read-outs.
Hard by the entrance to the ballroom, Ampex had their big display and was showing their 16and 24-channel recorders, as well as the super ATR100, which is now available with "ear and cue" facilities. (Patience men, my report on the ATR-100 is upcoming soon.) Totally unexpected from Ampex was their new ATR-700 tape recorder, which you could call "semipro" although it has features not usually found on that kind of unit. The machine takes up to 10 1/2-in. reels, has a three-motor, servo-lock capstan transport, sel-sync recording, equalization and bias adjustments on the front panel, a "dump edit" mode, and very fine specs on wow and flutter, distortion, and S/N ratio. The big news is its $1695 price, which means it should attract the interest of high-end audiophiles as well as industrial and governmental users.
Every year, the 5th floor demonstration rooms grow in number, and this year it looked like a mini hi-fi show.
There are, in fact, many hi-fi items on display. Pioneer had quite a line up, showing their new PLC-590 quartz phase-locked loop turntable, with direct reading meter to indicate precise turntable speed. Then there was the U-24 program selector, with a multitude of inputs for tape decks, phono inputs, power amp outputs, etc. The D-23 electronic crossover network permits up to a four-way multi-amplifier system. A new "slim-line" pre amp, the C-21, is simple in facilities but quite sophisticated in high performance electronics. Last, but not least, was the M-22 Class-A power amplifier. Rated at 30 watts per channel with a THD of 0.01, it has a separate power supply for each channel and generous heat-sinking. This kind of amplifier is a distinct departure for Pioneer. I used a prototype of this amp at home and found it a very clean, exceptionally smooth performer, although a bit lacking in bass power at the dynamic levels I favor. I'll bet this amp will be used in many bi- and tri-amplified speaker systems.
Dick Sequerra (of tuner fame) was demonstrating a mid-sized pyramidal shaped speaker system which was quite interesting. The room he was in was awful with standing waves, with the obviously extended bass of the speaker exciting them. Mid-range and high end were clean and very smooth, and stereo imaging seemed quite stable. Eli Passen of Gotham Audio was flipping people with the EMT 250 digital reverb system. With controls looking like the throttles on a 747, you can dial in the delay you desire, then the amount of reverb period, and you get a fabulous enhancement of acoustic perspective with no paucity of echo density. For the man who has everything, it's a mere 15Gs. Speaking of delay systems, Bob Berkovitz of Acoustic Research was once again demonstrating his great 16-channel system, this time using somewhat larger speakers than he had in Los Angeles. Oddly enough, I preferred the small units...perhaps because they were less obtrusive to the eye, which always helps in this kind of enhancement. Bob tells me the consumer version of the delay system draws ever nearer! The Technics room was loaded with tape machines...a new open-reel unit, the RS-150005, was quite remarkable. It features "isolated loop" tape :)rive, with a huge capstan driven by a quartz controlled, phase-locked, servo-controlled direct-drive motor, and affords a wow and flutter spec of an astonishing 0.018 percent W rms at 15 ips. Supply and take-up reels have separate d.c. motors. The unit accepts up to 10 1/2-in. reels. Tape motion is controlled by full IC logic. The RS-1500 has many other conveniences and features, one handy item being a real-time tape counter for the 15 ips speed. Price is $1500.00. Next unit in the Technics room was an "all-out" cassette recorder, the RS-990005. This machine has separate transport and electronics sections. The transport is a closed-loop, double capstan system with three direct-drive motors and a wow and flutter spec of 0.04 percent W rms. The transport has full IC logic, three heads for true monitoring, and even adjustable azimuth. On the electronics section, Dolby controls and bias and equalization calibration controls are all accessible on the front panel, and there are all sorts of conveniences too numerous to mention here. At $1500, this recorder is obviously meant to compete with the top decks in the field. Finally, Technics put their best foot forward with two Elcaset tape decks, the RS 75000S, a three-head unit with servo controlled d.c. motor, and the RS790US which features four heads (one for pilot tone), four direct-drive motors in closed-loop double-capstan configuration for a wow and flutter of 0.03 percent W rms. Would you believe a frequency response with Type 2 tape of 25-23,000 Hz ±3 dB or a S/N ratio with Type 2 tape without Dolby of 63 dB? Of course, it has Dolby NR, so you can add the usual 8-10 dB to the S/N. Here again, there are far too many features to list, but it is something to look forward to when Elcaset gets moving.
On the quadraphonic front, JVC was showing its new noise-gate CD 450 demodulator, but attracted most attention with their new two- and four-channel recording techniques.
Two artificial heads are used in the latter technique, but in spite of this, with special equalizing and time delay, signals are provided for loudspeaker listening, in the four-channel configuration. The two-channel set-up uses the same idea, but is for normal stereo listening. After the AES, as guests of JVC, we went quite deeply into these new techniques. We were also given a unique headphone/binaural mike affair and a dummy head, which does some remarkable things.
We'll bring you details of all this at an early date...it is utterly fascinating! Sansui kept its four-channel flags flying with the introduction of their 9001 receiver, with 60 watts/channel and the latest QS vario-matrix decoder/synthesizer. Also available as a separate unit is the QSD2 decoder/synthesizer.
As usual, there is just too much to cover at these conventions. If your goody has been passed over, cuss me when you next see me!
(Source: Audio magazine, Feb. 1977; Bert Whyte)
= = = =
Prev. | Next