Behind the Scenes (Feb. 1978)

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I have been talking about the various aspects of digital recording since the 54th Audio Engineering Society Convention. I have described the prototype systems, given my evaluation of what digital tape playback demonstrations there have been and noted with the passing of subsequent conventions that we have been making slow but significant progress towards the realization of digital recording and the commercial availability of professional digital tape recorders.

At the 58th convention of the Audio Engineering Society, November 4 to 7, at the Waldorf in New York, it was apparent that it is finally safe to say that the era of digital audio has well and truly begun. I think this statement is justified in view of the exciting digital audio developments that were the highlight of the convention. How about a digital tape recorder capable of up to 32-channels, the result of a joint venture by the 3M Company and the British Broadcasting Corporation? How about a quadraphonic digital tape recorder from Dr. Tom Stockham's Soundstream Company? Perhaps you might be interested in a two-channel PCM recorder from Mitsubishi? How about an operational PCM cassette tape deck, also from Mitsubishi? To really put icing on the digital cake ... how about operational PCM-encoded discs with laser beam playback from Teac and Mitsubishi? To use that old cliche ... all this was mind-boggling, and then some! Now let's take a closer look at all these exciting new digital machines.

The 3M Company and the BBC held a joint press conference the day before the AES convention and described and demonstrated their digital audio mastering system. Essentially, the tape transport is a modified version of the 3M M-79 tape recorder. The M-79 in normal analog configuration can record 24 channels on two-inch wide tape. In the digital version, up to 32 channels can be recorded on one-inch wide tape. In addition to the main recorder, there is a smaller digital recorder, a "mixdown" unit, which can record four or two channels on quarter-inch tape. The recorders are 16-bit systems, with a sampling rate of 50,000 times per second at a tape speed of 45 inches per second. At this speed, 7200 feet of tape on a 12 1/2-inch reel gives 30 minutes of recording. The main mastering recorder can be operated remotely, offers the usual overdub, and in-sync track-to-track facilities common to analog recorders.

Both the 3M people and the BBC have been working independently on digital recording, but joined forces about two years ago. 3M contributed their expertise in computer technology, and the BBC in the design of extremely linear analog-to-digital and digital-to analog converters.


---TEAC PCM-Laser Record Player

For some time now, the BBC has linked together a number of cities in England with digital transmission of FM, thereby gaining their experience in AD-DA converters. The master and the mix-down recorders claim a frequency response of ±0.3 dB 30 Hz to 15 kHz and are-2 dB at 20 Hz and-3 dB at 20 kHz. Since frequency response is usually a little less than half the sampling rate (50K/sec), these specs seem reasonable. However, in light of this, I am a bit puzzled by statements made during the demonstration, that the system was "flat down to d.c." Harmonic and intermodulation distortion is rated at less than 0.03% 20 Hz to 20 kHz with input/output level of +15 dBM. As to signal-to-noise ratio, with a 16 bit system, with each bit handling about 6 dB, we have 96 dB, and that, friends, is really quiet! As with all digital recorders, wow and flutter, print-through, crosstalk, and modulation noise are unmeasurable.

The recorder uses a special 3M tape, and this combined with a sophisticated error-correction system (a lot of BBC input on this) essentially eliminates the problem of drop-outs.

How'd It Sound?

The demonstration tapes of some piano and big band music were very impressive (allowing for differing tastes in playback equipment) ... very clean, with a lot of punch, superb transient response, and a total absence of tape hiss. Needless to say, all the virtues of digital recording are attractive to the typical recording studio, but one feature of this new recorder is especially important, its ability to overdub with absolutely no incremental build-up of noise. One can only hope that the producers don't go overboard on this point and make recordings so over-dubbed that they are texturally too thick! The master and the mixdown digital recorders are to be sold as a system, with an anticipated price of around $150,000. 3M states that three such digital systems will be available in 1978, with delivery slated for July or August. 3M had a suite at the Waldorf and demonstrated their digital recording system throughout the convention.

Dr. Tom Stockham has been a pioneer in digital recording, as witness his interesting reconstructions of Caruso recordings, and his ongoing demonstrations of his Soundstream digital recorder for the past several years. At the Waldorf, he was demonstrating his new quadraphonic digital tape recorder (albeit with just the two front channels) with recordings of Arthur Fiedler and the Boston Pops playing Tchaikovsky's Capriccio Italien and Rimsky-Korsakov's Capriccio Espagnole. These were the result of simultaneous direct-to-disc and digital recording sessions for Crystal Clear Records, and I had the pleasure of being the recording engineer. I believe this was the first time a symphony orchestra was recorded digitally, and the recordings created a lot of interest and seemed to be well received. Dr. Stock ham makes the particular point that his machine is a full 16-bit straight binary conversions and recording system, with no compression, expansion or floating-point being used. The sampling rate is 48K per second, and frequency response is rated as 0 Hz to 17 kHz +.05/-1.0 dB (another version has response to 22 kHz). Signal-to-noise ratio is better than 90 dB (unweighted). Harmonic distortion is said to be 0.03 percent at any level below peak at all frequencies, while intermodulation distortion is 0.006 percent with high impedance loads. Tape speed is 30 ips, which gives 30-minutes recording with a 10 1/2-inch reel. It is interesting to note how much progress has been made in digital recording because Ampex is now supplying Type 460 tape, in this case, one inch wide which is specifically for digital recording, and is the tape used with the Soundstream recorder on the Boston sessions. The Soundstream recorder has an elaborate error correction system for the elimination of dropouts.

The quadraphonic model of the Soundstream is expected to sell between $60,000 and $70,000, with delivery scheduled within the first quarter of 1978. I know a modest-sized recording company (who chooses to remain anonymous) that has just about decided to "go digital" and buy a Soundstream recorder!

Japanese Entry

The Mitsubishi digital recorder that was shown at the convention is an update of the unit I described some months ago. It uses what is essentially a standard open-reel transport with quarter inch tape. The format is two channel stereo, and it is a 14-bit system with a 48k per second sampling rate.

Frequency response is claimed to be ±0.5 dB from d.c. to 20 kHz with a signal-to-noise ratio of better than 85 dB, and harmonic distortion of less than 0.1 percent at operational levels.

Tape speed is 15 ips, and with a special circuit, monitoring of the digital signal is possible. Most unusual is the claim that conventional tape-splice editing can be performed with this recorder, although electronic editing is also possible with an adaptor. An error-correcting circuit for dropouts is used in this unit, but I note that they state that dropouts are "reduced below a detectable threshold," rather than eliminated.

Mitsubishi was also showing a PCM cassette tape deck. Now don't flip, fellas! This is NOT the Philips compact cassette. Rather it is a take-off on the video-recorder theme, a la Betamax.

(Incidentally, no sign of Sony and their PCM unit, whatever the reason.) The unit is a helical-scanning rotary head recorder using the video-cassette and affording up to two hours recording.

For stereo, there are two PCM channels and one for bias. Frequency response is said to be ±0.5 dB from d.c. to 20 kHz, S/N ratio 80 dB, and distortion less than 0.03 percent. This is a 13-bit system, but uses logical compression.

Sampling rate is 47.5 K/second, and dropout compensation (note, not elimination) is interpolation from previous value. What this all means is that theoretically this system should not have the quality of the open-reel PCM. However, from the samples of pop music played for me, the sound was of very high quality, with exceptionally good transient response.


---3M Head

Laser Discs

One of the most intriguing developments in digital audio at the convention was the demonstration by Mitsubishi and Teac of pulse code modulation discs. A joint venture of these two companies and Tokyo Denka, the technology of these discs admittedly is a take-off on the Philips MCA videodisc. (Reportedly Philips is busy adapting their videodisc to audio use.) A photograph in the Teac room showed the PCM discs being made inside a glass case under "clean room" conditions. On a metallized disc, a gas laser is pulsed by the input from a digital tape and "burns" tiny pits in the disc, only one thousandth of the width of an LP record groove. These pits are pulses or "bits" of information on the encoded audio signal. The playback unit consists of a turntable spinning at 1800 rpm, under the control of a quartz-crystal oscillator servo; a focusing servo system, which keeps the beam of a helium/neon gas laser exactly the right size and focused perfectly on the pits on the disc; a tracking mirror servo which controls a mirror angle so that the laser beam is precisely in the middle of the pits, and finally a radial motion transport servo system that moves the entire optical assembly across the diameter of the disc. At present, the playing time for the disc is 30 minutes on one side. (Philips recently produced a laser disc that can be played on both sides.) However, with suitable modifications, it would be possible to store and playback all nine Beethoven symphonies! The frequency the PCM disc is rated at +0.1/-0.5 dB 10 Hz to 20 kHz, S/N ratio and dynamic range better than 98 dB, THD less than 0.1 percent, wow and flutter are quartz crystal accurate. With the laser beam playback, there is no record wear, no tracing distortion, no tracking error, no pinch effect, no acoustic feedback, and no surface noise. Really quite incredible! It is claimed that the discs can be duplicated in the same fashion as current LP records and for about the same cost. Some people at the convention said they never heard these discs working properly. Anytime I went in, everything was in order. The principle is perfectly sound ... it is only the accuracy and stability of the various servo systems that can cause problems. In any case, the sound from these laser discs was astounding.

Shades of the old days ... Teac was using a railroad train recording, and the dynamic range was truly startling, with the realism of the playback limited only by the quality and power of the speakers and amplifier. Music on the discs was super clean, open and transparent, and blissfully free of hiss.

Projected cost of the playback system is around $600.

3M Digital Mastering Unit

I find all these digital developments very exciting. I will go out on a limb and state that this is the way the audio industry will progress, and that those die-hards who think that digital audio is still five or more years down the road are deluding themselves. Of course, we will still have the old "chicken and the egg" bit ... we had it with binaural and stereo and many other developments. As always, it will be the dedicated audiophiles ... the nuts, if you will ... who will be first to embrace the digital technology and get the ball rolling. Ultimately, as it always has, the advancements will filter down to the lower echelons of audio and finally, to the mass market.

To the apprehensive manufacturers of today's equipment, casting a wary eye at these developments, it should be obvious that with the hundreds of millions of record playing units in the world, there will be a market for turntables and arms, phono cartridges, and conventional records for a very long time to come.

As to the 58th AES convention, it behaved like a good AES convention should. Which is to say that attendance was up, there were more exhibits than ever, with the 10th floor of the Waldorf positively loaded with active audio demonstrations (and a great place for inquisitive audiophiles to roam). As usual, the technical sessions were well attended, and there was no dearth of eye-brow raising papers. As to new products, there were some interesting items which I will go into the details of next month. Lastly, on a personal note ... I am very honored that the AES elected me a Fellow, and I received my award at the convention banquet.

(Source: Audio magazine, Feb. 1978, Bert Whyte)

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