A ROSE BY ANY (ANTI-) ALIAS
As I noted last month in my report on the 72nd AES Convention in Anaheim, digital audio was the star of the show. In addition to the digital equipment I covered last month, there were other interesting new digital items.
The dbx Model 7000 delta modulation digital audio processor I reported on in my January column caused quite a stir at its demonstration. A number of pop music recordings were played; in general, they had all the usual desirable digital attributes. The dynamic range of a recording of the Boston Symphony Orchestra was especially impressive. There were many favor able comments on the availability of a professional-quality 16-bit digital processor for $5,000, but I also heard reservations in respect to the editing capabilities of the unit.
Although there were a number of digital compact disc players around the show, the only demonstration I heard was in the TAD (professional speaker division of Pioneer) room.
They were using the neat-looking Pioneer CD player, and as I entered, I was greeted with a blast of raucous rock music. With one exception (JVC with some beautiful harpsichord music), at all the digital demos-with equipment capable of 90 dB dynamic range "rinky-dink" pop music was being played, with a dynamic range of 10 dB at best. C'mon, fellas, what's the point?
[Gee, Bert, even the worst of rock manages 20 dB or better, and the best has real, if not quite classical, dynamics. -I.B and E.P.]
In any case, I knew there were some classical CD discs at the TAD room because Bart Locanthi, their erudite consultant (and head of the AES digital committee) had told me so.
After some rummaging, Dvorak's New World (9th) Symphony performed by the Czech Philharmonic Orchestra was played on the CD unit and auditioned through TAD's big monitor system. I had some quibbles about mike placement, but in general it was very clean, dead quiet, and with such impressively wide dynamic range that one of the assembled "rock types" ex claimed, "Gee, that's almost too much!" In all fairness, however, I must say that I recently have been able to play some digital tapes of pop/rock persuasion that have actually made some tentative approaches to wide dynamic range. In the right creative hands, digital audio could revolutionize pop music recording.
A development that could have significant effect on digital recording and playback was the subject of a paper by John Meyer. John is the brilliant young engineer who designed the high-power speakers used for special effects in the film Apocalypse Now, and he currently manufactures high-power, phase-corrected speakers for monitoring and sound reinforcement.
John was also my colleague in my Crystal Clear direct-to-disc and digital recordings, being the designer of the special low-noise preamps for use with the Bruel & Kjaer 4133 instrumentation microphones I used. (At the convention, B & K introduced their own preamp for the 4133.) John's paper, "Time Correction of Anti-Aliasing Filters Used in Digital Audio Systems," details studies he has made of a number of digital audio processors which exhibit high-frequency phase--anomalies, due mainly to their anti-aliasing filters. John found that these processors exhibit a total delay which is the product of two components: A linear, pure delay resulting primarily from the data conversion process, and a nonlinear, frequency-dependent delay primarily due to the anti-aliasing filter. He points out that the phase shift starts around 5 kHz, and reaches 50° by 12 kHz, 100° by 15 kHz and 175° by 17 kHz! John freely admits that the audibility of high-frequency phase shift is a controversial subject. He notes, how ever, that there seems to be general agreement that "pure delay"--delay which is not frequency-dependent does not change the perceived character of a sound. On the other hand, he states that we seem to be sensitive to frequency-dependent time distortion.
John feels those who proclaim that digital recordings have a different high-frequency sound are reacting to this phase distortion.
----------The Meyer Sound MS-8201 stereo time correction filter.
---------The Studer A810 pro/broadcast tape recorder
On the basis of his studies, John has produced an analog all-pass stereo correction filter with complementary circuit group-delay characteristics, the Meyer Sound MS-8201. It is supplied as a 3 x 4 inch epoxy module for printed circuit mounting. Each channel is completely independent, with provision for separate power supplies for each channel. Power required is ±15 volts a.c. The unit has an active, balanced input, with a maximum output level of + 18 dBV into 10 kilohms. The MS-8201 can be used in digital recording and playback. John has found that most digital processors are 180° phase-inverted. Even without the MS-8201 filter, I can personally attest that if you reverse the leads on both of your speakers, you'll hear marked improvement in respect to imaging, depth, stage width and projection. The MS-8201 is adaptable to different digital processors and recorders.
Trim pots adjust the filter's delay to match the phase characteristics of the unit it's being used with, and it can be set up to correct the phase inversion of some digital units. How does the MS-8201 work? The corrected system be haves as a pure delay line, with constant delay to 18 kHz. In practical terms there is no phase shift up to 15 kHz and only 10° shift at 17 kHz. As to the audible effects on digital sound, an early report is very positive from Peter McGrath, who operates Sound Components in Coral Gables, Florida, and whose engineering skills are evident on his fine piano recordings. He states that with the MS-8201, recordings made on his Sony PCM-F1 have more precise and stable imaging, better depth and, most especially, smoother high-frequency response. In short, Peter is very enthusiastic about this de vice. John Meyer supplied an MS-8201 to Dave Fletcher, the head of Sumiko, importer of Supex and Grace phono cartridges. Dave packaged the MS-8201 in a neat cabinet with power sup ply and RCA input and output jacks.
Dave tells me the filter not only corrects digital processors, but works equally well on compact digital disc players, and even on digitally mastered analog phonograph records! Perhaps there is some slight alteration due to the phasing in the cutter head, but evidently the phase shift characteristics of the digital master are transferred to the lacquer and subsequently to the LP. Dave claims the filter makes digital/analog records tolerable to those people who are always complaining about "edgy digital string sound." The MS-8201 is available from Meyer Sound in San Leandro, California for $485. Dave has indicated that he may market the MS-8201 in a pack age similar to the unit he assembled.
Last month I mentioned that Studer had introduced a new analog tape recorder, the Model A810. One is tempted to say that perhaps this unit is Studer's "last hurrah" in analog tape recorders before digital becomes the "standard" recording technology. It would be hard to imagine an analog tape recorder with improvements be yond those Studer has incorporated in the A810. It is a recorder of great complexity with a staggering array of features and it literally would take a half-dozen columns to fully describe. The A810 is 19-inch mountable in rack, console or portable case. Any two speeds between 3 3/4 and 30 ips can be selected. Its servo-controlled capstan motor has capacitive speed-sensing and microprocessor quartz reference, while its electronically-controlled tape tension has servo-controlled spooling motors. The tape counter operates in real time for all tape speeds.
In the electronics section, record and playback amplifiers are phase correct ed. There is an elaborate microprocessor control keyboard for many functions, with such exotic amenities as an integrated test system which automatically monitors main functions. SMPTE intertrack time code recording is optional. There is an audio-frequency test generator with five frequencies. Re mote control is very complete, including remote calibration of the recorder! Digital technology gets into even this analog machine. Audio parameters are entered via keyboard. All adjustments are made via 8-bit A/D converters, with 256-step adjustment ranges. Such parameters as treble and bass correction, level and bias are read out on the tape-counter LCD. A nonvolatile electronic memory retains the calibration parameters even when the recorder has been switched off. I could go on and on, for this is just the tip of the iceberg. The basic A810 is expected to sell in the $6,000+ area.
At Gotham Audio (celebrating their 25th year), owner Steve Temmer introduced me to Horst Redlich, Technical Director of Teldec. As most audiophiles know, Teldec (jointly owned by Telefunken and Decca) has become very well-known for its audiophile-quality premium pressings. Herr Redlich and I discussed the fabulous new Teldec Direct Metal Mastering, a process first shown to me by Arthur Haddy, former Technical Director of Decca Records, in his London studios in 1976. Although it was applied only to videodisc technology at the time, Arthur indicated that someday audio discs would be cut in a similar manner.
Essentially, by means of a special Neumann lathe and cutter head, a copper-plated disc is used for disc mastering instead of the usual lacquer blank. The result is the creation of a direct-cut mother. The whole thing is a clever, complex and rewarding new technology. I have auditioned some DMM pressings, and they have no trace of surface noise, nor any ticks or pops or any other kind of noise! Like cavalry ever coming to the rescue, new technology has given the venerable phono graph record a new lease on life.
As usual, there were many other interesting items at the convention which must go unheralded, while others are significant enough to deserve separate, in-depth reports. In this category is Crown International's Tecron TEF System Ten. TEF is the acronym for "time, energy, frequency" measurement techniques, based on TDS time-delay spectrometry-invented by Audio Senior Editor and speaker tester, Richard C. Heyser. The Tecron TEF System Ten is a dedicated instrument for this technology. It does a great deal more than the assemblage of instruments heretofore needed for these measurements, and is available at a substantially lower cost.
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(Adapted from: Audio magazine, Mar. 1983; Bert Whyte )
Also see:
Dr. Thomas Stockham on the Future of Digital Recording (Feb. 1980)
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