Sony DTC-75ES DAT Deck Review

Sony’s DTC-75ES digital audio tape deck gives Americans their first opportunity to purchase a full-featured DAT recorder designed for home use. It officially introduces the Serial Copy Management System (SCMS) supported by tape and equipment manufacturers and the American and European recording industries. This “one-generation” arrangement allows a user to make digital-to-digital copies of CD’s but not to make digital-to-digital copies of the copies. The new Sony DAT deck also incorporates the latest in 1-bit design developments in its digital-to-analog (d/a) and analog-to-digital (a/d) converters. And, perhaps the most welcome news of all, the DTC-75ES digital recorder actually costs less than a number of today’s best analog cassette decks.

At first glance, indeed, it would be easy to mistake the DTC-75ES for a regular topflight cassette deck. Its cassettes, though smaller than their familiar analog counterparts, measuring only 2-7/8 x 3/8 x 2-1/16 inches, go into a similar-looking cassette well, which is equipped with a power-assisted door. Interior illumination and a transparent door panel give full visibility to the tape reels and label.

Since the research and tooling needed to produce the high-precision mechanical drives and heads required by the DAT format is very costly, the ability to borrow in this area from existing VCR drive/head technology is an important consideration in holding down the cost of home DAT machines such as the DTC-75ES. The transport mechanism of the DTC-75ES uses a standard head drum, 30 millimeters (mm) in diameter, which spins the two audio heads at 2,000 rpm (1,000 rpm in the lower-fidelity long-play mode). The heads are mounted at an angle to the drum, and as the tape is slowly pulled around it, they are alternately exposed to the tape over a 90-degree arc. They can thus read or write the long, diagonally recorded (“helical-scan”) tracks that contain both the digital audio signal and the subcode information.

The subcode fields in DAT recordings hold such important information as absolute time codes, pro-gram-selection numbers, and start, skip, and end codes. Unlike analog cassettes, in which blank spaces between selections can be tolerated, DAT machines must preserve the continuity of the subcode information even in the absence of an audio signal. For this reason, then, the fast-forward button of the DTC-75ES does not run the tape directly to the end but instead automatically stops the machine at the last recorded “end id” marker. At that point a new selection can be recorded immediately, or an intervening silent space can safely be added by using the record-mute button.

“Start id” codes can be written automatically or manually, and start, skip, and end codes can be written or erased manually either during or after the initial recording. Using the DTC-75ES’s editing facilities, the exact locations of these codes can be shifted backward or forward by up to 70 seconds, in increments as small as 0.3 second. Further, since the originally assigned selection numbers will often need to be changed because of subsequent editing or the addition of new selections, the DTC-75ES provides a renumber button that updates all of the program numbers in a single pass.

The DTC-75ES supports all three of the DAT format’s sampling rates: 48 kHz for analog-input standard recordings, 44.1 kHz for digital-input recordings of CD’s and commercially recorded DAT’s, and 32 kHz for analog-input recordings in long-play mode. In long-play mode, a C-120 cassette will hold 4 rather than 2 hours of music, at the cost of limiting high-frequency response to about 14,500 Hz.

The low-level inaccuracies inherent in even the best 16-bit linear d/a converters have recently given rise to the idea of 1-bit conversion, which employs an ultra-high-speed stream of pulses at a constant amplitude to express the 65,536 (216) amplitude values that can be represented by the 16-bit signals used in digital audio recording. This 1-bit technology is now finding its way into various CD and DAT machines and is incorporated in the Sony DTC-75ES. Its advantage is that pulse intervals that are precisely controlled by a quartz clock can be maintained far more accurately than can the transistor/resistor-dependent analog voltages with which digital amplitude values must normally be associated in d/a conversion. The result is better handling of very low-level signals. And such low-level signals are not limited to musical passages marked pianissimo; they occur in the transition area between the positive and negative halves of every audio waveform.

Unfortunately, however, to implement pure 1-bit conversion would theoretically require over-sampling the recorded signal not at eight or sixteen times the normal rate, as is commonly done in high-quality CD players, but at 65,536 times the normal rate. Since the standard sampling rate for CD’s and for DAT decks in their digital dubbing mode is 44,100 times a second, the rate needed to generate the range of pulse-widths required for straight 1-bit operation would rise to an awesome 2,890,093,500 samples a second, or roughly 3 gigahertz.

While operating at such frequencies is beyond the capacity of current integrated-circuit technology, “noise-shaping” and “bit-compression” techniques can be (and are) employed by various companies to obtain high performance from a 1-bit design in a considerably reduced bandwidth. In the Sony DTC-75ES, for example, these techniques are used in conjunction with a high-density linear converter that is itself remarkable in being able to operate at up to 50 MHz. Large-scale integration (LSI) of the various DAT circuits actually helps make higher-frequency operations possible, and at the same time it helps bring down the production cost.

The display window of the DTC-75ES manages to present an enormous amount of information without visual clutter. In addition to a linear tape-counter mode, the absolute time, program time, and remaining time on the tape are shown by successive presses of the COUNTER mode button. (The same button, in conjunction with various program-number keys, also controls the brightness of the display.)

The deck has a twenty-two-segment-per-channel peak-holding level indicator that is calibrated from — 60 to 0 dB. When making a digital recording it is imperative never to “run into the red” (a common practice with analog decks). Once all 16 binary bits have been turned on, at 0 dB, any larger input will create severe distortion. For that reason, the DTC-75ES’s level indicator is supplemented by a two-digit numeric margin readout, which displays the difference, to the nearest 0.5 dB, between the 0-dB point and the maximum signal level encountered in either channel. If the record level goes too high even momentarily, the margin indicator will flash until the margin reset button is pressed. In addition to the clutched record-level controls (which affect only analog input signals), adjustable, digitally controlled fader facilities are provided.

The deck’s remote control can be used to program up to sixty selections for playback in any order. It has the usual scan and automatic search functions as well as a repeat function for individual selections, the whole tape, or specific portions of a single selection. In conjunction with a number of Sony CD players, the remote control can also be used to synchronize dubbing operations.

If pressed during play mode, the fast-forward and rewind buttons provide audible cue-and-review facilities. Entered from the stop mode, fast-winding runs at two hundred times normal tape speed. A front-panel switch is used to select among the rear-mounted analog, fiber-optic digital, and coaxial digital input jacks. A switch for timer-controlled operations is also provided, as is a front-panel headphone jack with its own level control.

The Sony DTC-75ES measures 16-5/8 inches wide, 13-3/4 inches deep, and 5-1/4 inches high. It weighs about 16-1/2 pounds. Price: $950.

Lab Tests

We checked the frequency response of the DTC-75ES both with the Sony TY-7551 calibrated test DAT and by making overall record-playback frequency sweeps with our Audio Precision System One test instrument. In no case did the deck’s response deviate from the ideal by more than 0.16 dB.

Predictably, total harmonic distortion (THD) of a 1,000-Hz tone at 0 dB was roughly twice as high on an overall record-playback basis as it was in playback-only mode. Again, however, as the figures show, in both cases the distortion was measurable only in the third decimal place. Our crosstalk measurements revealed excellent channel separation as well.

The graph of total harmonic distortion plus noise (THD + N) vs. analog input levels shows the importance of not exceeding 0 dB when recording a DAT. Given the 94-dB A-weighted signal-to-noise ratios (overall record-playback) we measured, there is ample margin available to encourage erring on the safe side when setting record levels, though this may mean changing the habits of a lifetime. The separate left- and right-channel low-level-linearity measurements (see graph) disclosed no nonlinearities of consequence down to a level of approximately — 90 dB.

In an attempt to get at least some indication of the measurable difference between digital-to-digital dubbing and copying a CD via the player’s regular analog outputs, we also measured the deck’s THD + N in decibels at various levels with both analog and digital inputs (see graph). The approximately 6-dB difference (a 2:1 voltage ratio) between the analog and digital curves across most of the dynamic range shows the clear advantage of recording dig-ital-to-digital if possible. Still, the overall distortion was low enough in both cases that the difference should not be glaringly audible.

Comments

The DTC-75ES is certainly an easy deck to use, and if you confine your taping to the full-digital mode, you won’t even have to set the record-level controls. Buttons and displays seemed intuitively well placed. If we have any criticism in terms of recording ease, it is that the deck supplies only an over marking on its record-level indicators (together with the flashing margin display) when you push your recording level too high. There’s no way to know how much above 0 dB your level is currently set, and on a number of occasions we had to make several trials before arriving at the correct setting.

As to the vexing question of just how much difference it makes to dub digital-to-digital rather than by way of a CD player’s analog output jacks, we’re not ready to pass judgment. Certainly we heard no clear and repeatable differences between the sound of the original CD’s and either their analog or digitally dubbed copies on the DTC-75ES. Yet our experience with other advances in state-of-the-art audio leads us to expect that using a number of DAT machines over a period of time may disclose differences that were by no means obvious at the start. We can say that in terms of wide frequency response, inaudible noise, solid imaging, and CD-quality clarity, the recordings we made on the Sony DTC-75ES sounded as good as any others we have heard to date.

For me, the arrival of the DTC-75ES stimulated some personal recollections. My own love affair with tape began some thirty years ago with open-reel machines that could not begin to match the sonic quality of the Sony DTC-75ES. For years my articles in this magazine were principally oriented toward readers who were willing to lug those behemoths to churches, school auditoriums, and the like to capture the sound of live music on tape.

While the arrival of the cassette deck vastly widened the market, its technical limitations largely restricted its use as a recorder to copying already recorded material. Most DAT’s will also be used primarily for dubbing, of course. But the arrival of the DTC-75ES also opens to a new generation of audio enthusiasts the possibility of realizing the joys of live recording, of becoming an active participant in the realization of music. Few pleasures are more sublime.

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