Technics SL-P370 CD-player Review

Not long after the introduction of the CD player — it became evident that it was impractical to perform a completely accurate translation of a CD’s 16-bit encoded digital signal into analog form with a 16-bit digital-to-analog (D/A) converter. Theoretically, a 16-bit converter would suffice, but a truly accurate converter chip would require such extremely tight tolerances for its internal resistance values that its price would have been prohibitive.

Fortunately, even the less-than-ideal 16-bit converters used in most CD players are capable of outperforming the finest analog equipment in respect to noise level and most audible distortions. The usual result of conversion errors in these devices is increased distortion at very low signal levels, typically less than —70 dB relative to maximum recording level. Although such levels and the assorted distortions are normally inaudible under actual listening conditions, the potential exists, under special conditions and for very keen-eared and critical listeners, for low-level nonlinearities to mar the near-perfection of the digital recording/playback system.

Until now, the usual approach to the problem has been to use 18- or 20-bit D/A converters in the CD player. With the lowest signal level on a disc (represented by the sixteenth, or least significant, bit) far above the least significant bits of the converter, it is theoretically possible to obtain linear reproduction of every part of a recording. Since this approach is more costly than a basic 16-bit system, it is found largely in expensive CD players. Nevertheless, it is not unusual to find measurable nonlinearities at the lowest signal levels even with these premium-priced converters.

In recent months, we have been hearing about a new “1-bit” conversion system that completely eliminates the problem of low-level nonlinearity. Developed independently in Europe by Philips and in Japan jointly by Matsushita, the parent company of Technics, and NTT (Nippon Telegraph and Telephone), two basic variants of the 1-bit system are now reaching the marketplace. Although the two versions differ in their details, each employs an oversampling process that eventually transforms the 16-bit digital signal from a CD into a train of pulses equivalent to the original analog signal. In the Matsushita system, the signal is effectively sampled at 33.8688 MHz (768 times the original 44.1-kHz rate!), emerging from the process as a pulse-width-modulated (PWM) stream of uniform-amplitude pulses, hence the “1-bit” designation.

Although the actual implementation of this process is much more complex than the above description implies, the result is simply a train of pulses that can be smoothed to an exact replica of the original analog waveform by means of a low-pass filter that removes its ultrasonic components. The resulting audio signal is free of “zero-crossing distortion,” and its linearity is maintained over the full amplitude range. The noise content of the very wide-band pulse signal is relatively high, however, requiring a “noise shaping” process to reduce the noise energy in the audible band (below 20,000 Hz) by shifting it to higher (inaudible) frequencies.

Matsushita refers to its system by the somewhat strained acronym MASH, standing for multistage noise shaping. While 1-bit conversion has appeared in a couple of CD players from other companies, Matsushita has just begun to include MASH in Technics players for the U.S. market. The SL-P370 is the first of these we have had the opportunity to test.

Although the Technics SL-P370 is remarkably inexpensive, it offers a wide complement of convenience features, including programmability for as many as twenty tracks in any order, fast scan and track skipping in both directions, repeat of the entire disc or programmed sequence, and direct access to any numbered track from the front panel or the wireless remote control (which also duplicates almost all other front-panel functions). Taping features include an Auto Cue function, which cues up the following track when the current track is finished and automatically switches the player to pause, and Synchro-Edit, which automatically separates the tracks on a disc into groups that can be dubbed most efficiently onto the sides of a C-46, C-60 or C-90 cassette. There is also a headphone jack with volume control.

The display window is simple yet complete. At start-up it shows the total number of tracks and total playing time of the disc. During play it shows the current track number and its elapsed time as well as the current mode (play, pause, or stop). A row of small numbers along the bottom of the window shows how many tracks are on the disc and which ones have been played already. A time: mode button changes the display to show elapsed time on the disc and remaining time on the track or disc.

The rear apron contains only the audio output jacks and the Synchro-Edit jack, which sends control signals to a compatible Technics cassette deck during dubbing. The SL-P370 measures 17 inches wide, 10-5/8 inches deep, and 3-3/8 inches high, and it weighs 7-3/4 pounds. Price: $219.95.

Lab Tests

Although we had no instruction manual (the test sample was an early unit shipped to us before deliveries to the U.S. market began), we had no problems in using the player since most of its functions and controls are either conventional or self-explanatory.

The output from a 0-dB test signal was 2.31 volts. The open-circuit headphone output was 5 volts at the clipping point, falling to 3.5 volts into 600 ohms and 1.75 volts into 100 ohms. Although the available volume was very good into medium-impedance headphones, the front-panel control could easily be advanced to the point of severe clipping distortion (beyond its 2 o’clock setting). For undistorted playback, we found, the volume should always be set below its midpoint (12 o’clock).

The frequency response was ruler-flat up to 4,000 Hz, rolling off above 10.000 Hz to --0.6 dB at 20,000 Hz. Channel levels were matched within 0.1 dB. The interchannel phase shift varied from —0.2 degree at low frequencies to —0.9 degree at 20,000 Hz. The 0-dB total harmonic distortion (THD) plus noise was 0.0032 percent from 100 to 1,000 Hz, rising to 0.01 percent at 20 and 15,000 Hz. At a fixed 1,000-Hz frequency, the distortion was 0.0024 percent from — 30 to —60 dB, climbing slightly to 0.0032 percent at 0 dB.

Spectral analysis of the low-level signal showed two unusual features First, the linearity of the signals from — 70 to — 90 dB was superb, as we would have expected from the use of MASH circuitry. The amplitude error was a mere —0.5 dB at — 70 and —80 dB, +0.3 dB at —90 dB. Second, the power-line-hum components in the audio outputs of the player were extremely low: The 60-hz component was at a — 125-dB level, and higher-frequency harmonics were undetectable. A full-range noise-spectrum analysis showed that the noise energy (in approximately one-third-octave bands) was at a constant —130 dB up to 500 Hz, rising slowly to — 118 dB at 20.000 Hz.

The channel separation, slightly different for the two channels, was 108 to 118 dB at 100 Hz 96 to 98 dB at 1.000 Hz, and 70 to 73 dB at 20.000 Hz. The maximum de-emphasis frequency-response error was about 0.7 dB at 5,000 Hz, falling to about 0.1 dB at 1.000 and 16,000 Hz. The dynamic range (EIAJ) was 102 dB, with a quantization-noise level of —92 dB. Wideband noise (A-weighted) was —110.4 dB with emphasis and —108.2 dB without emphasis. The frequency (speed) error was —0.0022 percent.

The laser transport slewed rapidly to a selected track, taking only 1.5 seconds to go from Track 1 to Track 15 of the Philips TS4 test disc. The error correction was very good, allowing only a single “tick” on the 2,000-micrometer defect band of the Pierre Verany #2 test disc. Despite its obviously light construction, the SL-P370 was remarkably resistant to impact. A fairly hard slap or blow on the top or side was required to cause audible mistracking or noise bursts.

Comments

During a visit to the Technics factory in Osaka a few months ago, I heard a convincing demonstration of low level D/A converter distortion and of its inaudibility through a MASH-equipped player. Such a demonstration is not easy to duplicate in a normal listening setup— for one thing, you must play back at an unreasonably high gain setting, and I am unwilling to risk my speakers or ears by doing so—but I was left with no doubt that a MASH-equipped player is essentially free of low-level nonlinearities

Nevertheless, it was impressive to see how nearly ideal the very low-price SL-P370 was in that respect. Until now, players that could even come close to its caliber of performance were rare at any price! I had expected that MASH technology would have its impact in inexpensive products eventually, but hardly this soon.

Be that as it may, the SL-P370 is obviously a first-rate product, as nearly “state of the art” as any I know of, and surely one of the top values in home CD players. Even though it is unlikely that most people could actually hear the benefits of MASH under real world listening conditions, this player has so many other virtues that one might almost consider the MASH system to be a bonus.

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