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In Reply to: GENERAL RESPONSE posted by Archimago on April 21, 2013 at 16:55:55:
The Jtest signal should not be used for testing asynchronous USB DACs. It was specifically designed to detect probable faults in SPDIF signals and a DACs rejection thereof. The faults in a USB connection are different, so a different test ought to be used.
SPDIF is a continuous stream that contains sample clock data mixed with the signal. In Julian Dunn's Jtest signal, the low order bit is toggled to exercise the codes, thereby creating intersymbol interference caused by the encoded bit transitions passing through a cable that has limited bandwidth. This intersymbol interference shifts the transition times of the clock transitions, thereby creating jitter in the recovered sample clock. The DAC can defend against this to a certain extent through the use of a PLL or other "flywheel" mechanism, but the result will be imperfect and the resulting sonic pollution will show up in the analog output.
USB is a packet based system, and the exact timing of the signals on the cable is not related to the audio clock. Furthermore, in the case of asynchronous connections, the relationship between the USB data clock and the sample clock is dynamic, because the DAC clock requests data from the computer under control of its local clock. If the DAC hardware were perfect, the timing and jitter on the input USB signal would be irrelevant. In reality, the DAC hardware is not perfect and the timing and noise on the incoming signal indirectly affect the DAC clock. However, the coupling does not happen in the signal path, it happens through parasitic coupling, typically through the power and ground wiring. If one wants to assess these effects, one will need a different kind of test signal. Also, because they are much lower in magnitude, they will be much more difficult to measure. (Adaptive USB should never be used at the 44.1 family of sample rates, because it has a brutal amount of packet jitter caused by beats from the USB clock and the audio sample clock.)
With either SPDIF or async USB any sonic differences are relatively hard to measure (or hear). This is not the case with 320 mp3 encoding, where the differences are trivial to measure, and are audible on many music recordings when played on a decent system.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
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Follow Ups
- RE: GENERAL RESPONSE - Tony Lauck 10:56:08 04/22/13 (0)
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