ProAudio Review - October 2008 - (Page 19) COMMENTARY ON THE BENCH MEASUREMENTS: PRISMSOUND ORPHEUS FIREWIRE AUDIO INTERFACE The PrismSound Orpheus is a member of an increasing number of multi-channel audio interface units that connect to a computer via a FireWire connection. A software application, the Orpheus control panel applet, included with the unit was used to set up the various modes for performance testing with an Audio Precision System 2722. The control panel applet was run on the same computer that controls the Audio Precision system. Testing was done first in the analog/analog I/O mode using the mixer controls in the control panel applet, then the performance was tested for A to D, D to A, and D to D modes. A/A performance Frequency response for the sampling frequencies of 44.1, 96.0, & 192.0 kHz is shown plotted in Figure 1. With frequencies extended down to 10 Hz, the response at 10Hz was down about 0.08 dB. THD+N in a 22 kHz measuring bandwidth as a function of signal frequency and sampling frequency was little different for sampling frequencies of 44.1, 96.0 & 192.0 kHz. A typical result is plotted in Figure 2 for a 192.0 kHz sample rate for both the 22 & 80 kHz measuring bandwidth. Amount of THD+N with measuring bandwidth increased to 80 kHz was variously greater due to increased noise from the greater bandwidth and was the greatest at the 192 kHz sample rate shown in the figure. THD+N of a 1 kHz test signal as a function of level down from 0 dBFS full scale was pretty much the same for the three sample rates tested. A typical example is shown in Figure 3 at a sample rate of 192 kHz and for both channels. Deviation from linearity was quite good and similar for each of the three tested sample rates. An example of the deviation from linearity is shown in Figure 4 for both channels at the 192.0 kHz sample rate. Channel separation was close to being the same for the two testing directions and for the three sampling rates. A typical result is shown in Figure 5 for both directions at the 44.1 kHz sample rate. A/D performance Frequency response for the three testing sample rates of 44.1, 96.0, & 192.0 kHz was similar in shape and bandwith to that of the Analog/analog response shown in Figure 1. At 10 Hz, response was down about 0.04 dB. THD+N vs. signal frequency and sample rate in a 20 kHz bandwidth, like in the analog/analog mode, was similar in amount at the different sample rates. However, the amount of THD+N over much of the audio band was a bit lower as might be expected – the analog/analog mode being the composite effect of an A/D and D/A in series. This can be seen in Figure 6 for the 96.0 kHz sample rate. As the sample rate went up from 44.1 to 96.0 kHz, the Fs/2 measurement bandwidth THD+N amount went up a little and at 192.0 kHz, went up very significantly to about 0.015%. Looking at how THD+N varied with decreasing signal level from full scale for a 1 kHz tone in a 20 kHz measurement bandwidth, again results were quite similar for the three sample rates. A plot of this for the 192.0 kHz sample rate is shown plotted in Figure 7. Of note, the amount of distortion is lower, again — no surprise — than in figure 3 for the analog/analog mode. Input/output linearity, not shown, was very good for the A/D converter section and, as is quite frequently the case with many measure- ment parameters, was very similar for all three tested sample rates. Channel separation proved to be virtually the same for both tested directions and was better than 124 dB from 20 Hz to 20 kHz for both testing directions and all three sample rates. D/A performance Frequency response in the D/A mode looked like Figure 1 in general shape with the exception that there was the onset of a much sharper rolloff starting at about 88 kHz. At the low end, response was down about 0.04 dB at 10 Hz. This, along with the 0.04 dB loss in the A/D more would account for the 0.08 dB overall loss at 10 Hz in the A/A mode. THD+N vs. frequency and sample rate at full scale, as in the other modes, was essentially the same in a 20 kHz measurement bandwidth. The appearance of the plot, not shown, is similar to Figure 2 for the A/A mode. Surprisingly, in the 80 kHz measurement bandwidth, the highest THD+N was at the 44.1 kHz sample rate. THD+N of a 1 kHz signal with decreasing level below full scale was similar in appearance to Figure 3 for the A/A mode with the amount of THD+N being a couple of dB lower below – 10 dBFS. Again, THD+N in a 20 kHz measurement bandwidth was about the same for all three sample rates. Deviation from linearity was similar in nature to Figure 4 but with less deviation below –120 dBFS. Channel separation in the D/A mode was very similar to that in Figure 5 for the A/A mode suggesting that the majority of crosstalk PRISM BENCH TEST continues on page 37 ® Fig. 4 A/A mode deviation from linearity of a 1 kHz test signal at a sampling frequency of 192.0 kHz. Red/Magenta = Ch1/Ch2. Fig. 5 A/A mode typical channel separation vs. frequency at 44.1 kHz sample rate. Red/Magenta = Ch1 > Ch2/Ch2 > Ch1. Fig. 6 A/D mode THD+N at 96.0 kHz sample rate as a function of signal frequency and measurement bandwidth. Red/Blue = Ch 1/Ch2 at 22 kHz measurement BW, Magenta/Cyan = Ch1/Ch2 at 80 kHz measurement BW. www.proaudioreview.com October 2008 | ProAudio Review | 19 http://www.proaudioreview.com
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