Sony DTC-1000ES R-DAT recorder & Sony PCM-1630 A/D converter Page 2

Once the digital data is recovered off-tape, deprocessing can begin. For replay, decoding is 4x oversampled, ultra-flat passband, high-order digitally filtered and D/A converted, using Philips's dual-channel 16-bit TDA-1541 chip at present. The final output filter is a simple three-pole Bessel type, and the replay chain is linear phase. (The record section is not linear phase owing to the uncompensated input brickwall filter.) Relatively few op-amp chips are used in the filter-output section.

One small difference concerns sampling rate. The 1630 is set to CD standard 44.1kHz (or 44.056kHz). The R-DAT normally works at 48kHz, but it can decode at 44.1kHz, and will also decode and recode DBS (Direct Broadcast by Satellite) digital inputs at 32kHz. Current DAT recorders have a copy-inhibit system which prevents them recording in digital mode from the digital output of a CD player if the appropriate flag is set in the CD's PQ subcode.

Sound Quality
Any representation of the sound quality of a recording system is more complicated than might at first appear. Out in the real world, with live music, the sound quality available as a direct feed from a good microphone transcends that generally replicated and available to the consumer. So good is this quality that, even with the losses imposed by relatively primitive recorders, much still remains. Arguably, the recording technique and the performance itself are far more important than the recorder used for capture and storage.

In this context, it must be said that well-designed digital recorders can do a very competent job. Fine results have been obtained and can be demonstrated from many decks, including Sony's PCM-F1, PCM-701es, DTC-1000es, and PCM-1630. In fact, there is little to choose between them on replay merit. All were free from spurious noise, wow, and flutter; none suffer from high-frequency compression, or irregularities of frequency response or channel balance. Still more important to the operator, no specific tape selection or alignment is necessary, as the replay quality is essentially tape-independent.

In objective terms, the overall sound quality of one of these digital PCM units exceeded that of the channels of a typical modern multi-track mixer; most studio engineers regard digital recording as virtually perfect for this reason. It is worth noting that the $2000 domestic PCM unit essentially equals the sonic performance of a professional machine costing in the region of $23,000; in this respect, competent digital audio is a great leveler. One might then question the sound quality of a studio multi-track console.

However, we can also adopt an alternative standpoint, and place the so-called "perfect" or "nearly perfect" digital recorder in a high-quality audio chain. It can then be regarded as performing as a unity-gain amplifier with programmable time delay! What does it then sound like in a top-class domestic system? Well, if viewed as a preamp line stage and interposed between one's existing preamp or a good CD player, then the inevitable loss in quality may be put in understandable terms. To define the context, assume that, for the present, I scale subjective audio quality for electronics in the range 0–15, with the top score as present state of the art, "0" a large table radio, and "3" a good budget rack system. Competent-sounding audio lies in the 7–11 range.

Top CD sound scores 12.5 at present: against this score, the PCM machines' record/replay cycle achieved ratings between 6.5 and 7.3 when inserted into the reference system, a surprisingly small spread.

Given this modest rating, the sonic differences between PCMs were not particularly significant, the main differences concerning an easier, sweeter sound, with an improvement in stereo perspectives noted with the R-DAT and PCM-1630 when compared with the earlier non-oversampled PCM-701es. Overall, there was a consistent loss in dynamics and drama, a softening of bass definition, and a reduction in transparency, stereo depth, and treble detail. These effects, however, were not identifiably "digital" in nature: if anything, they related to the usual losses encountered with some of the more complex electronics of the "high-tech" preamplifiers from major Japanese companies.

A grade "10" CD player, recorded and replayed through any of the PCM units, ends up at grade 6.5 to 7—clearly, no audiophile will record CD via the analog domain on to R-DAT except for background-music purposes. In theory, direct digital recording is possible, since the R-DAT will operate at a 44.1kHz (CD format) sampling rate, as well as 48 and 32kHz; given a good, CD-quality equivalent replay section in the recorder, it would be able to record and replay with fidelity equal to CD. This, however, has been blocked by the "copy prohibit" instruction encoded on CD and obeyed by domestic R-DAT recorders.

Copycode
The additional proposal pioneered by CBS, concerning a spoiler code to be imposed on all records, consists of an intermittent 300Hz-wide notch, 60dB or deeper, at 3840Hz (see JGH's discussion elsewhere in this issue). CBS claims this to be inaudible, but many industry experts judge it to introduce a degradation in fidelity. Future R-DAT recorders may be forced by law to incorporate circuitry to recognize the presence of this notch and refuse to record. Consumers would thus be prevented from exploiting the true potential of a domestic digital recorder as a rightful component in a digitally based home audio system. At the very best, the CBS Copycode proposal must be carefully assessed. If it is recognized, as I think it should be, as a threat to fidelity, it should be stopped.

Comment
The domestic R-DAT format is a successful piece of design and engineering: as typified by this Sony machine, it works very well, with no identifiable code errors, low mechanical noise, convenient tape/time access, and—in context—good fidelity. The lab performance was most impressive, and closely approached the professional equivalent. The latter, Sony's PCM-1630, shows just how far commercial digital recording technology has come in terms of dynamic range, low-level linearity, and general accuracy. For semiprofessional recording, however, an R-DAT machine would do nearly as well, matching the performance of the full-sized system for simpler productions. If switched to 44.1kHz (some internal modifications are needed, modifications that would be subject to a $50,000 fine under the bill currently before Congress), it could be used for the first stage of CD mastering.

Considered as a hi-fi component, Sony's DTC-1000es R-DAT works admirably. Nevertheless, two major questions arise. First, why doesn't the recorder at least match the perceived quality of a good CD player? There is an interesting idea of precedence and logic here. Second, if the copyright constraints prevent effective domestic use of an R-DAT machine except for occasional live recording, thus restricting replay to as-yet-poorly planned prerecorded tapes, what could the machine be used for?

Taking the first point, we have to remember that assessing the sound of a PCM as a complete line stage is illogical, since normally (hopefully!) we would encode once and the replay/decode would be via a CD player with a digital input (the handbook for the '1000 illustrates use with Sony's DAS-703es). Thus this test is unrealistically arduous. Nevertheless, some of the shortfall, in my view, is not the fault of the digital part of the process but is tied up with the rather lengthy cascades of audio chips used between the input and the outputs of a PCM system. Certainly the simpler consumer models—the PCM-701es, DTC-1000es, sounded "sharper" and more dynamic than the more complex 1630. Improvement in this area is a function of good audio systems engineering, and I suspect that there is considerable scope here.

As regards the second question, I believe that R-DAT is a clever design lacking in mass-market consumer relevance, and is emasculated in terms of its digital interfacing. Perhaps the Japanese Electronics establishment believed that anything with "Digital Audio" on it would sell. At present, R-DAT decks are big and expensive, while tape prices are higher than those of compact cassette. (3M has announced that its 120-minute tape will cost $12.) Prerecorded material hardly exists, and in any case is only compatible with itself! DAT certainly has a future, but at present it must be considered a long-term one, with possibly five years before worthwhile sales, and 10 before dominance of the quality cassette market. As for CD sales, I think that they will be largely unaffected. Surely R-DAT represents the recipe for minimal sales of a new replay technology.

COMPANY INFO
Sony Consumer Products
Sony Drive
Park Ridge, NJ 07656
Product not yet released in the USA (1987)
ARTICLE CONTENTS

COMMENTS
Axiom05's picture

Maybe this is a dumb question but why do we see no pre-ringing from the anti-aliasing filter?

John Atkinson's picture
Axiom05 wrote:
why do we see no pre-ringing from the anti-aliasing filter?

It's a minimum-phase filter, presumably operating in the analog domain.

John Atkinson
Technical Editor, Stereophile

bdiament's picture

We sure have come a long way from the 1630 and DAT, thank goodness!
While Apogee's retrofit filters provided an appreciable increase in 1630 performance, the limitations of the 16-bit system, particularly at low levels, could not be overcome.

And though DAT certainly seemed convenient, I found the shelf life to be rather limited. After a few months, many tapes developed sections that played back sounding much like a fax transmission! I wouldn't want to archive to such a format.

Happily, we now have systems that can deliver a superbly clean 24-bit capture at 192 kHz.
When I first heard digital back in early 1983, my feeling was that it took vinyl 100 years to get to where it was, and that in another 100 years, digital will have arrived too. ;-}
I feel most fortunate that we didn't have to wait anywhere near that long.

Best regards,
Barry
www.soundkeeperrecordings.com

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