Alesis MasterLink ML-9600 Hard Disk/CD-R Recorder Measurements

Sidebar 3: Measurements

Starting with the Alesis MasterLink ML-9600's A/D section, it took an unbalanced analog input of 1.79V RMS at 1kHz to give an accurate 0dBFS. As expected, the balanced input required 6.9V. The input impedances at 1kHz were 10k ohms and 29k ohms, respectively. The level metering appeared to be accurate, the "-1" light illuminating at 6.2V RMS balanced input; ie, exactly -1dBFS. The Alesis' overall analog/digital and digital/analog frequency response is shown in fig.1, taken at 48kHz and 96kHz sample rates, and is flat within the audioband.

Fig.1 Alesis MasterLink ML-9600, A/D/A frequency response at -12dBFS, 48kHz and 96kHz sampling (right channel dashed, 2dB/vertical div.).

With the output word length set to an indicated 24 bits, the ML-9600's ADCs demonstrate superbly low linearity error down to below -120dBFS (fig.2), suggesting at least 18-bit performance. This is borne out by spectral analysis of the digital data representing a balanced 1kHz tone captured at -1dBFS, again with the word length set to 24 bits (fig.3). The noise floor is about 145dB down from peak level, and the THD associated with the 1kHz tone lies at -116.5dB (0.00015%), this mainly the third harmonic. (The THD figure was calculated using the PrismSound DScope II to ignore the contribution of noise.)

Fig.2 Alesis MasterLink ML-9600, A/D converter departure from linearity, 24-bit word length (right channel dashed, 2dB/vertical div.).

Fig.3 Alesis MasterLink ML-9600, digital-domain spectrum of 1kHz sinewave, DC-22kHz, at -1dBFS, 44.1kHz sampling, 24-bit A/D word length (linear frequency scale).

Increasing the analog input level to produce data at -0.1dBFS doubled the calculated THD to a still negligible 0.00035% (-109dB), but unlike other inexpensive ADCs—such as the one in the RME Digi96/8 PAD soundcard, reviewed in March—it appears safe to use the top dB of the Alesis' dynamic range. Dropping the input level to produce 24-bit digital data at -90dBFS gave the spectrum shown in fig.4. The calculated THD level rises to 1.41% (-37dB), which is superb performance for this very low signal level. However, note that both fig.3 and 4 show the presence of some enharmonic spuriae, though it is fair to note that these are all below -125dBFS and are revealed only by the very low noise floor of the Alesis' analog inputs.

Fig.4 Alesis MasterLink ML-9600, digital-domain spectrum of 1kHz sinewave, DC-22kHz, at -90dBFS, 44.1kHz sampling, 24-bit A/D word length (linear frequency scale).

Turning to the ML-9600's D/A section, I recorded my usual suite of test signals on the Alesis' hard drive, driving its AES/EBU data input from a CD transport for the 16-bit data, and from the RME Digi96/8 Pro soundcard fitted to my PC for the 24-bit data. The analog output impedance was a low 150 ohms across the band and the signal was noninverting from both unbalanced and balanced jacks, the latter wired with pin 2 hot. The frequency response for 44.1kHz-sampled data is shown as the lower pair of traces in fig.5, and doesn't need comment. (This graph was plotted from the balanced jacks; the response from the unbalanced jacks was identical.)

Fig.5 Alesis MasterLink ML-9600, D/A balanced frequency response at -12dBFS, without emphasis (top) and with emphasis (bottom). (Right channel dashed, 0.5dB/vertical div.)

However, when decoding pre-emphasized data, the Alesis didn't apply the correct equalization, leading to a 9dB boost at 20kHz (top pair of traces in fig.5). To check that the emphasis flag was correctly set in the data, I repeated the test playing back a pre-emphasized CD-R in the ML-9600's CD drive. There was no change in the result.

Channel separation was superb from both sets of analog outputs (fig.6), almost reaching 120dB in the lower midrange. It did rise with frequency, due to the usual capacitive coupling somewhere in the circuitry, but was still around an excellent 90dB at 20kHz.

Fig.6 Alesis MasterLink ML-9600, D/A balanced channel separation (1dB/vertical div.).

Fig.7 shows spectral analyses of the ML-9600's analog output while it decoded data on its hard drive representing a dithered 1kHz tone at -90dBFS with both 16- and 24-bit word lengths. Not only is the output free from harmonic or power-supply-related spuriae, but the increase in bit depth results in the noise floor in the midrange and above dropping by almost 18dB, suggesting around 19-bit performance for the D/A converters, which is superb.

Fig.7 Alesis MasterLink ML-9600, 1/3-octave spectrum of dithered 1kHz tone at -90dBFS, with noise and spuriae (from top to bottom): 16-bit data, 24-bit data (right channel dashed).

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