Sony DTC-2000ES DAT recorder Measurements

Sidebar 3: Measurements

Because the DTC-2000 is the first consumer product with Super Bit Mapping, I was eager to look at its technical performance and investigate Sony's published claims and technical graphs of what SBM does. But first, let's look at the DTC-2000's D/A and A/D converter performance.

The unit's maximum output level was 2.5V when fed 0dBFS, 1kHz sinewave data. Output impedance was a moderate 295 ohms at any audio frequency. The DC levels at the output were very low, measuring just 0.9mV (left channel) and 0.1mV (right).

The line input impedance measured 43.6k ohms at 1kHz—slightly below the 50k nominal value. The line input required a voltage of 1.3V RMS to produce a digital signal of 0dBFS with the input level control exactly halfway up, suggesting that the DTC-2000 will interface well with a variety of source levels. The DTC-1000 doesn't invert absolute polarity from analog input to output, or in its D/A section. I also found the DTC-2000's digital meters to be very accurate, precisely tracking those of the Audio Precision System One.

Fig.1 shows the DTC-2000's D/A frequency response (top traces), de-emphasis error (middle traces), and overall record/replay response from analog input to analog output (bottom traces). Although the responses are flat, the DTC-2000 has a mild de-emphasis error that will be audible as a slight darkening of the sound with pre-emphasized tapes. Although the 0.7dB maximum rolloff isn't severe, it covers a broad enough band to be just audible. Channel separation was excellent, measuring 106dB at 1kHz, rising gently to 90dB at 20kHz. The DTC-2000 produced low levels of intermodulation products when driven by a code representing an equal-level mix of 19kHz and 20kHz tones. The 1kHz difference product lay below –100dB—excellent performance.

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Fig.1 Sony DTC-2000ES, D/A frequency response (top), de-emphasis error (middle), and A/D-D/A frequency response (bottom) (right channel dashed, 0.5dB/vertical div.).

Fig.2 is a spectral analysis of the DTC-2000's output when decoding a –90dB, 1kHz undithered sinewave. The spectrum is free from power-supply noise (except a trace of 60Hz in the right channel), and the overall noise level is low. (Compare this plot with that of the Bel Canto Aida, also reviewed in this issue.) Fig.3 is the DTC-2000's noise-modulation plot. This is superb performance, and among the best I've measured. The noise level is low, and the traces are very tightly grouped, almost appearing as a single trace above about 3kHz.

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Fig.2 Sony DTC-2000ES, spectrum of dithered 1kHz tone at –90.31dBFS, with noise and spuriae (1/3-octave analysis, right channel dashed).

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Fig.3 Sony DTC-2000ES, D/A noise modulation, –60 to –100dBFS (10dB/vertical div.).

The DTC-2000's D/A converter linearity was excellent (bottom traces in fig.4), as would be expected from Sony's 1-bit type DAC. The A/D converter linearity (top traces in fig.4) was also good, but the apparent positive "linearity error" below –90dB is more likely noise swamping the signal. Nonetheless, these are both excellent linearity plots.

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Fig.4 Sony DTC-2000ES, A/D departure from linearity (top) and D/A departure from linearity (bottom) (right-channel dashed, 2dB/vertical div.).

We can see this difference in noise performance between the D/A and A/D stages in figs.5 and 6. Fig.5 is the DTC-2000's reproduction of a –90dB, 1kHz undithered sinewave with a digital input driving the DTC-2000. This excellent waveshape is overlaid with very little audioband noise, and the transitions are uniform. For comparison, fig.6 shows a low-level waveform as reproduced by the DTC-2000 with a very-low-level analog input signal (50µV). We can see that the A/D converter adds a fair amount of noise to the signal, as would be expected; A/D converter technology is way behind D/A converter engineering.

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Fig.5 Sony DTC-2000ES, waveform of undithered 1kHz sinewave, digital input, at –90.31dBFS.

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Fig.6 Sony DTC-2000ES, waveform of undithered 1kHz sinewave at –90dBFS, analog input, no SBM.

Fig.6 was made with the Super Bit Mapping turned off. Fig.7 is the same test signal, but with SBM turned on. The much higher noise level seen with SBM is a result of the noise floor's spectral energy being shifted higher in frequency. The noise energy is so frequency-specific that you can almost count the cycles of the noise. (I count between 18 and 20 cycles overlaying each 1kHz wave, which suggests that the noise energy is concentrated at the upper end of the audio band—exactly what's expected from knowing what SBM does.)

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Fig.7 Sony DTC-2000ES, waveform of undithered 1kHz sinewave at –90dBFS, analog input, with SBM.

There's been some debate over potential audible problems when so much noise energy is concentrated over a narrow band of frequencies. When noise energy is concentrated over too narrow a frequency band, it begins to be heard as a specific pitch. At the 1990 AES convention in Montreux, for example, JA attended a demonstration of an early noise-shaping technique. He immediately noticed a high-frequency "whistle" overlaying the music, the shaped noise acquiring too much character. [To be fair, I was listening to very-low-level music considerably louder than the noise-shaper algorithm's designers ever anticipated. It could be argued that, at lower playback levels, the shaped noise would have dropped below my hearing's high-frequency threshold and thus have lost its audible pitch.—Ed.]

Looking again at fig.7, we can see the potential to hear that signal as a tone overlaid with a second tone; the signal almost looks like a twin tone of 1kHz and 20kHz. Note, however, that the ear's sensitivity drops at 20kHz, making the high-frequency noise component less audible than is suggested in this graph.

We can look more closely at the noise floor's shape by performing a 1/3-octave spectral analysis of the DTC-2000's output with no input signal. I did this with and without SBM engaged, producing the plots in fig.8. The curve made with SBM (solid trace) shows a lower noise floor throughout most of the audioband, but vastly higher noise in the top octave. We saw this high-frequency noise overlaying the waveform in fig.7. Note that noise-shaping techniques such as SBM can't lower the overall noise level within the audioband. Instead, they merely shift it around, as seen in this graph.

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Fig.8 Sony DTC-2000ES, spectrum of digital silence, 500Hz–50kHz, with noise and spuriae, with SBM (solid) and without (dashed) (1/3-octave analysis).

Sony's promotional graphs for SBM show apparently improved resolution with SBM on low-level waveforms—see Stereophile, Vol.15 No.8, p.55. This is somewhat misleading, however. Sony's engineers invoked a 16kHz low-pass filter for their measurements, removing the concentration of high-frequency noise from the waveform. It could be argued, however, that human hearing acts as a low-pass filter (reduced sensitivity at very high audio frequencies), making the filtered waveforms subjectively appropriate.

At any rate, what matters is how SBM affects our musical impressions. And JGH did indeed think highly of its effect.—Robert Harley

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COMMENTS
jimtavegia's picture

I still use my DTC 690 to record WABE FM NPR  broadcasts for listening later, but not much else.  I also had a Portable 7, but once it quit for the 2nd time and the repair was over $300 I was done.  My 690 is still rolling long nicely to this day. And yes, tapes are still available. I also still use my Sony Minidisc 330 as well when quality is not a real issue. 

I must admit that my 3 Tascam SDHC recorders; 2 DR-2ds that do 2496 and my older DR-07 that is redbook are now my go to recorders for everything that I want to transfer to CD or DVD - or + R. 

Poor Audiophile's picture

Nice to hear from you again!

CHARLES S LUDWIG's picture

I purchased a pair of Sony 75ES decks and later a 87ES deck as well as a PCM 300. These are gone now as none quite satisfied. But, I still have and use a pair of Sony PCM--7010F DAT Recorders connected to a Sony RM-D7200 Dual Remote Controller. The remote controller allows for auto edits with 3 frame accuracy. I think my PCM-7010F's represent DAT recording performance as good as it ever got. I do not have any more modern technology which sounds better.

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