Arcam Delta Black Box D/A processor Measurements
Sidebar 4: Measurements
On the test bench, the Arcam Black Box 2 performed better than the Musical Fidelity Digilog that I also reviewed in this issue in several respects. Fig.1 shows the spectral analysis of the Black Box 2's output when playing the dithered 90dB signal on the CBS test disc. The unit exhibited a slight deviation from linearity, as shown by the amplitude at 1kHz not quite reaching the 90dB level. More revealing, however, is what is present at the output in addition to the signal read from the disc. This test shows the amplitude (vs frequency) of the signal at the player's output when playing a dithered 90dB, 1kHz sinewave, measured using the Audio Precision set-up with a swept bandpass filter. Ideally, only the 1kHz signal should be present. The Black Box 2's small 120Hz peak at 110dB is indicative of a full-wave rectified power supply whose ripple frequency is double the line frequency, while harmonics can be seen at 2kHz, 3kHz, 5kHz, and 7kHz.
Fig.1 Arcam Black Box 2, 1/3-octave spectrum of dithered 1kHz tone at 90dB.
The response error with preemphasized data is shown in fig.2. The Black Box 2 exhibited a slight difference in deemphasis accuracy between left and right channels.
Fig.2 Arcam Black Box 2, de-emphasis error (right channel dashed; 0.2dB/vertical div.)
In the linearity department, the Black Box 2 outperformed the Digilog, the difference at 90dB being 2dB and 3dB respectively. Linearity error is the difference between the actual level recorded on the test disc and the measured output of the player. As the signal level decreases, DACs become less linear. Fig.3 shows the fade to noise with dither track on the CDS test disc. This is a signal whose amplitude varies continuously from 60dB to 120dB over a period of 30 seconds. A perfect DAC (which doesn't exist) would produce a straight line.
Fig.3 Arcam Black Box 2, fade to noise with dither (10dB/vertical div.)
Another way of looking at linearity is shown in fig.4. The vertical axis is error, and the horizontal axis is signal level. The Black Box's output starts to deviate below perfect linearity at 80dBFS and plateaus at 2dB until the point where the output starts to become dominated by noise.Robert Harley
Fig.4 Arcam Black Box 2, linearity error (2dB/vertical div.)