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In Reply to: RE: How can USB performance impact audio quality? posted by audioengr on June 13, 2011 at 11:17:26:
You didn't get the distinction I made between jitter coupled from the input signal and jitter in the local oscillator. This distinction is the whole point of the asynchronous USB clock architecture and other clock architectures such as AES/EBU where the DAC sends the clock to the transport. Once one has this architecture it becomes possible to work separately at minimizing these two causes of jitter. With this clock architecture there is no physical necessity for any jitter coupling from the input signal, but there still is physical necessity for jitter in the local clock and the associated clock circuitry up to and including the actual switches that comprise the DAC. If one fails to make this distinction one might end up using the inherent physical limitations of oscillators as an excuse for failing to engineer sufficient isolation from the input signal, but this will be an excuse not an explanation.
Even if the local clock has significant jitter, it will still be possible to measure (and presumably hear) the effect of coupled jitter in the input signal. Measurement can be done, for example, by deliberately injecting jitter that is synchronized to a signal and then measuring the jitter sidebands in the output of the DAC using synchronous detection.
While I have no doubt that poorly designed or implemented isolation stages may fail to isolate jitter, they certainly do not add jitter in a proper clock architecture. (With improper clock architectures this will not be the case. Thus if clock is fed forward from the source and jitter suppressed using phase locked loops or digital variations thereof, there will be eventual buildup of jitter and unstable system operation. This is a familiar problem in communications systems, for example I have worked on system designs that ran into there problems using both analog PLLs and digital PLLs when these were used in series. Even with systems where signal is reclocked at each stage, such as in Ethernet repeaters and the FDDI token ring, there is a build up of timing errors which result in system failure when too many stages of reclocking are used. This can be avoided by the appropriate clock topology.)
I agree with the emphasis on simplicity. If one must start with the USB and all of its baroque complexity (in the classic Intel engineering philosophy) the complex logic must be logically, electrically, and physically isolated from the sensitive analog and mixed signal electronics that are the heart of any DAC. Furthermore, the higher the quality of the DAC the less opportunity its own jitter will have to mask any coupled jitter, and hence the necessity for greater jitter isolation.
When measuring jitter, it's the jitter spectrum that matters not the total jitter energy. Specifically it's the jitter spectrum in and near the audible band that is going to produce sidebands that will be perceived as audible distortion. Jitter at RF frequencies may be important in telecommunications applications and jitter at minute fractions of a Hz may be important in instrumentation applications, but neither of these is very important in audio applications. A single number that comprises jitter is not relevant unless it's very low, thereby ensuring that any possible spectrum with this jitter power will produce inaudible distortion, but this mandates a signal number that is below 10 psec, possibly even below 1 psec and hence probably not obtainable.
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: How can USB performance impact audio quality? - Tony Lauck 12:31:04 06/13/11 (1)
- RE: How can USB performance impact audio quality? - audioengr 10:33:11 06/14/11 (0)
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