Computer Audio Asylum

Music servers and other computer based digital audio technologies.

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RE: Cable effects same regardless of USB mode

Posted by John Swenson on December 2, 2009 at 12:11:57:
Essentially yes. Both cases have two clocks in the DAC, the "USB clock" which runs at the USB bus frequency and gets data off the bus, and the "audio clock" which pulls audio data out of the buffer. In adaptive mode the audio clock is adjustable so it can be set to the average of the audio data rate. In async mode the audio clock is fixed and the computer adjusts the rate at which it sends the data so the buffer does not over/under flow.
As far as jitter is concerned the difference is in how well you can do between a fixed and an adjustable clock. Most adaptive implementations do not use a particularly good implementation. It is possible to implement an adjustable clock that is better than what is in most adaptive USB chips, and this has been undertaken by a couple companies. But you can always get lower jitter with a good fixed frequency clock.
There is actually a good example of this case of its the implementation of the clock thats important, not the asyncness itself that is important. The recent inexpensive Musiland devices use an asynchronous protocol but then use a frequency synthesizer to generate the local clock rather than use a fixed frequency oscillator. The result is jitter that is actually worse than some of the better adaptive implementations!
John S.