EVERY LITTLE BIT HELPS
In the June issue, I described some of the testing and evaluation of Digital Compact Cassette (DCC) tapes, particularly in respect to the Precision Adaptive Sub-band Coding (PASC) system for data compression. At issue was whether the action of PASC is audible. Exhaustive tests at Decca Records in London seemed to indicate that PASC encoding had reached such a high degree of refinement and technical sophistication that it could not reliably be detected in NB comparisons of master tape versus PASC-encoded/decoded DCC tapes.
As a result of these tests, there was quite a bit of speculation that DCC (a recording format) is better than CD (a playback-only format), based on the wider dynamic range of 18-bit DCC tapes. Some of this speculation probably arose because the parameters for CD technology and replication are all cast in concrete and invariable, as dictated by the Sony/Philips Red Book.
This may foster the impression that CD sound quality cannot be improved be yond the optimum levels achieved by adherence to the Red Book standards.
Although we are certainly not going to see 24-bit digital recordings with a sampling rate of 125 kHz, as fantasized by some audiophiles, there have been improvements in CD sound quality. The earliest CD players had brickwall, 90-dB anti-aliasing filters which certainly degraded sound quality, causing high-frequency spuriae that grated on perceptive ears. The continually evolving D/A converter technology--multi-bit, MASH, one-bit, bitstream, and other variants--has expunged most of the offensive sounds. In fact, D/A converter technology has outpaced that of the typical A/D converters used in professional digital recorders. Studies have revealed that these recorders' A/D converters help create digital distortions and artifacts.
Recognizing this, manufacturers have been working to develop a new breed of A/D converter to address digital signal distortions. Most of these A/D converters are stereo units, accepting two signals from the mike preamps, and their circuitry provides 128-times oversampling and dither applications at various levels. Several new A/D converters, such as one from Sony and the Wadia 4000, afford 20-bit resolution.
Some converters will be offered as an eight-input module on a single chassis and then be used in multiples according to the number of tracks employed in the digital recorder.
In addition to new A/D converters, recording engineers are placing in creased emphasis on high-quality microphone preamps and improved on-location and in-house monitoring facilities. With all these new devices in their armamentarium, I asked three highly respected recording engineers to de scribe their current digital mastering techniques.
Tom Jung is justly famous for the superb, ultra-clean pop/jazz digital recordings he engineers for his dmp label. Tom has been making digital recordings since 1978 and is known as a tireless experimenter and innovator in his quest for sonic realism. Currently, he feeds various mikes (types and pat terns are chosen for particular recording applications) into high-quality mike preamps. He often feeds his main stereo mikes into FM Acoustics' Class-Amp M-1 mike preamp. The output of the Class-Amp M-1 and the outputs of the "sweetener" mike preamps are fed into four Wadia 4000 A/D converters. The digital output of the converters is fed into the eight input channels of a Yamaha DMR-8 mixer/digital recorder. This 20-bit linear digital recorder has stationary thin-film heads and allows 40-channel redundancy recording on metal tape. It provides 22 minutes of recording, but slave-unit serial recorders switch on at 21 1/2 minutes, taking over from the main unit. When the slave unit reaches 21 1/2 minutes, it automatically switches back to the main recorder, so Tom can continue to record as long as he wants! The Yamaha DMR-8 allows him to mix in the digital domain. Since the Yamaha met al-tape cassette is not configured for CD mastering, the recorder's digital AES/EBU and SPDIF outputs are fed into a professional R-DAT recorder, and this tape is used for CD mastering. Tom also makes extensive use of Peter D'Antonio's RPG Diffusor and Abffusor acoustical panel treatments in his session monitoring and, whenever possible, in recording venues. This cutting-edge-of-the-art digital recording technique is expensive, but Tom's pristine-clean recordings speak eloquently for his dedication to high-fidelity sound.
Tony Faulkner is one of Britain's pre-eminent recording engineers for classical music, and the countless recordings he has made for many labels are distinguished by the sheer musicality of his soundscapes. Tony is always interested in new recording technology as long as it appears to advance the art, but basically he takes a purist approach, which is to say that he firmly believes in minimal miking for most classical recordings. Over the years he has employed Blumlein and other stereo pair techniques. He generally likes the warmth and musicality of tube microphones and favors near-coincident pairs of Neumann M49c and M50c mikes as well as the AKG C24 stereo mike, all modified for low noise. Tony does much of his recording on an open-reel, 20-bit Mitsubishi PDX-8620 digital recorder. Also aware of the problems with conventional A/D converters, he has upgraded to Meridian's 607 and has already mastered five al bums with the Wadia 4000.
Tony is quite enthusiastic about his newest digital recording technique, which utilizes CD-R. He feeds his mike preamp into his A/D converter, then outputs the digital signal into a Meridian CD-R, which uses pre-grooved dye-polymer discs. The disc is edited on a Sonic Solutions digital workstation, then rerecorded on CD-R to create the digital master. Tony has found two CD mastering plants in England that accept the CD-R discs and use them directly to the CD glass master.
He asserts that CD-R is presently the most transparent digital recording for mat, with the lowest error rate, and that it provides digital masters of exceptional clarity and cleanness. Not the least of CD-R's attractions for classical recordings (which are usually done on location in various halls) are its very light weight and easy portability. These are major pluses, compared to the big Mitsubishi PDX-8620 open-reel recorder or the Sony 1630 U-matic. Tony reckons he has now mastered more than 20 albums using CD-R.
John Eargle, colleague and Audio contributor, is responsible for most of the remarkable digital recordings on Delos and is well known for his books on recording engineering and micro phones. Over the years he has developed a recording philosophy based on his technical expertise and his insights as a trained musician. John's recordings are a distillation of his understanding and application of microphones, acoustics, and the characteristics of the instruments in the symphony orchestra. The musical balances he achieves between the various orchestral choirs, and his placement of the orchestra in accordance with the acoustical characteristics of the re cording hall, are major contributors to the stunning sonic realism of his recordings.
John favors a modified near-coincident ORTF pair of cardioid mikes as his main pickup, flanked by omni mikes, with minimal use of stereo-pair sweetener mikes. He has been experimenting with A/D converters and currently is using the Wadia 4000. John feeds the output from his mike preamps to a specially modified Soundcraft 200B mixer. The mixer's two output channels feed into a Wadia 4000, and its digital output is recorded on a professional Fostex R-DAT recorder. This tape is later transferred to a Sony 1630, and subsequent editing is done on a Sony 3000 to make a CD mastering tape.
Other recording engineers have recognized the A/D converter problems and have been experimenting with new units. Jack Renner of Telarc has been using a proprietary A/D converter as well as the Wadia 4000. John New ton, who has made many recordings for Philips, EMI, and other labels, has been using Apogee Electronics A/D converters and the Wadia 4000. Of course, Sony Classical uses Sony's own 20-bit A/D converter, and Decca Records and other companies are using various new A/D converters, including 20-bit models. In all of this, you will note the trend to 20-bit A/D converters. A logical question is: Why use 20-bit converters, since the CD standard is a 16-bit system? The engineers say it gives them extra headroom and ensures that all 16 bits are "exercised" down to the least significant bit (LSB).
At my deadline for this column, Sony announced a new technique, called Super Bit Mapping (SBM), for transfer ring 20-bit masters to a 16-bit CD to yield sound quality beyond 16-bit resolution. SBM uses noise-shaping to shift quantization noise away from the frequencies the ear is most aware of.
Sony's white paper states:
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In order to shape the noise in this manner, the actual error at each point must be tracked. This requires that the signal be quantized with more bits than will actually be used. For example, while CD-quality audio is quantized at 16 bits, SBM requires a higher number, such as 20 bits. Each sample then consists of the most-significant 16 bits, with the quantization error consisting of the least-significant 4 bits. Once the quantization error (noise) is identified, it can be shaped by a noise-shaping filter and fed back into the original signal, thus producing the desired noise spectrum.
Note that the total noise power remains unchanged; only the spectrum is different.
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An added benefit of SBM is a significant reduction in transient harmonic distortion, and there is much more to this Sony technology that will be revealed in due course. In the meantime, 16-bit transfers from the new 20-bit A/D converters will likely be improved by SBM and give us further enhanced CD sound.
(Editor's Note: At a press conference at the recent Consumer Electronics Show in Chicago, I heard A/B demonstrations of CDs made the standard way and a second set of discs made with SBM. Two recordings were used, ones with which I fortunately was familiar: Dave Brubeck's "Take Five" and Bob Dylan's "Just Like a Woman." It took me a little while to tune into the difference, which at first sounded like it was merely some sort of equalization.
Some, but not all, of the intrusive old, analog tape noise was reduced, but more importantly the size of the musical illusion behind that "gray wall' of tape noise increased and became more three-dimensional. Further, the size of each instrument in Brubeck's group was reduced, and its place in that newly enlarged space was clarified. With the Dylan, his voice was much less harsh and sibilant, but yet more "voice-like" and realistic. This is a premium-quality sound from the CD medium, and I must confess I had thought CD's sonics were not to be improved upon. These SMB CDs are to be initially released on Sony Music's Legacy Series, and the best thing I can say about them is that I will search them out and pay the extra dollars to have them, for they are that much bet ter. -E.P.)
(adapted from Audio magazine, Aug. 1992; Bert Whyte)
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