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In Reply to: RE: Length of usb cable affects SQ posted by Presto on July 1, 2015 at 12:04:01:
In the (unlikely) event that the source, source connector, cable, destination connector and sink all have the same impedance there won't be any reflections to worry about. However, there will still be cable losses and these will be frequency dependent, limiting the bandwidth of the cable, with a greater limitation for longer cable runs. The effect of longer cable runs will be reduced signal amplitude at the receiver and increased rise time. This will spread out the transition time between "0" and "1" and this will make determining the exact time of the transition more dependent on any noise at the receiver which adds/subtracts to the point at which the receiver decides whether it saw a 0 or a 1. That is to say, more jitter...If there are reflections, particularly reflections at both ends or from some discontinuity in the middle of the cable, then the reflections will affect the receivers decision making process if they take place around the time of transitions. The reflections also depend on the past data bits that have been sent, making the process difficult to understand.
With SPDIF/AES as normally used the sample clock is derived from the incoming waveform and thus can affect the timing of digital to analog conversions. With async USB the sample clock comes from a local oscillator and (theoretically) is not affected by the timing of events on the USB cable. However, in practice, the required processing by the USB receiver creates noise and this noise can (and does) couple into the DAC sample clock circuitry, so USB signal quality can still affect results.
With a sufficiently narrow bandwidth phase lock loop an SPDIF receiver can (theoretically) filter out jitter in the incoming clock so that the sample clock is not polluted. In practice this doesn't work perfectly. For example, the designers of the SABRE chip explain in their white paper that the SPDIF receiver circuitry on the input part of the chip creates noise and this noise can couple into the clock circuitry on the output side of the chip. The result may be a second order effect, but it can be measured and heard.
The USB receiver circuitry is vastly more complex and runs at much higher speed than lowly AES/EBU/SPDIF signaling. This means that the USB receiver is likely to create more noise. This will have to be isolated from the critical DAC circuitry and this is difficult and costly.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
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- RE: Length of usb cable affects SQ - Tony Lauck 07/1/1513:17:46 07/1/15 (0)
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