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In Reply to: Datastream jitter posted by audioengr on June 2, 2009 at 22:04:23:
I think the confusion comes because there are really two separate clocks that are involved: the clock that is used at the DAC chip to play out the individual samples and the clock used to actually move data bits across the USB.
Of course there can be many other clocks and many levels of buffering in the complete path that moves PCM samples from the storage media to the DAC chip. Of all of these, only the clock that actually runs the digital to analog conversion has any relevance to the analog waveform output by the DAC. The problem is that with some clock architectures this clock is slaved to earlier clocks and in the process of slaving any jitter in these earlier clocks may be imperfectly attenuated. Another problem is that if the DAC clock is slaved to another clock by some kind of a phase locked loop this clock has to be able to run at a range of frequencies to accommodate a range of input clock rates and adding flexibility to this oscillator adds internal jitter with most designs. Another way out of this problem is to use asynchronous sample rate conversion, but this has other problems as well, having to do with introducing errors in the amplitude dimension even if it eliminates them in the time dimension.
Understanding how clocking really works in digital circuitry is one of the most subtle and difficult parts of design and analysis of computer systems. It gets worse when conversion between analog and digital formats get thrown in. There is little hope that the issues can be understood by non-technical people, let alone most digital designers. So I am inclined to cut most of them a lot of slack in their statements, especially marketing literature which always should be subject to caveat emptor. In the end, it is how a product performs that matters, and not what its manufacturer says.
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: Datastream jitter - Tony Lauck 09:03:19 06/03/09 (1)
- RE: Datastream jitter - audioengr 18:44:26 06/04/09 (0)
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