Computer Audio Asylum

Music servers and other computer based digital audio technologies.

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RE: If USB JItter specs were Superior to Spdif

Posted by Tony Lauck on April 12, 2012 at 13:28:07:
The issue isn't the wire type, it's the method of flow control. The right way is for the DAC to pull data out of the transport rather than have the transport push a stream of timed samples to the DAC. With USB the right way is to use a proprietary protocol and driver for block mode transfer or to use the async protocol, which can use drivers provided by the operating system. With SPDIF the right way is to run an extra wire from the DAC back to the transport to slave the transport to the DAC's clock. If the wrong way is used, heroic measures will be needed to get good results. However, even with the right way results won't be excellent without careful implementation. A good system architecture is useless unless it is well implemented.

It's the entire system that counts and the way it is put together. It is not possible to break a system down into individual components and optimize each of these separately if one wants the best possible results. There is far too much interaction between components. A great SPDIF installation will beat a poor USB implementation and conversely. Note that when a "push" architecture is used the source, the cable and the sink are all in the signal path and hence critical to sound quality. When a "pull" architecture is used only the sink is in the signal path. The source and cable are not in the signal path and if they have an effect on sound quality it is only because of second order effects at the sink, i.e. various forms of leakage such as ground bounce and poor clock design.

Note that with both technologies, jitter on the cable is typically nearly as large as it can be to avoid data errors, since jitter is typically one of the factors limiting the maximum possible data rate. If the wire is conveying audio timing information because it is being run in push mode then the cable jitter will be 10 - 100 times larger than the jitter needed for full resolution reproduction out of a DAC. Jitter down at the level needed for audio quality is not readily measurable with any test equipment, and it certainly can't be seen on the screen of even an expensive scope. DACs that run in push mode must have a phase locked loop or other heroic measures to reduce the incoming jitter down to levels that will produce tolerable sound quality and these measures are neither cheap nor completely effective.