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Re: Do USB DACs have jitter that originates in the computer?

In a word "yes", you want more detail?

First off USB DACs vary wildly in their implemetations, what effects one strongly may have little effect on others.

A little background that will help understand the following. MOST USB implementations use whats called adaptive mode which has a PLL that tracks the data rate coming over the bus, this means that the jitter of the recovered clock CAN be affected by whats happening on the interface. In adaptive mode the PLL is not directly connected to the datastream (as it is in S/PDIF) so exactly how its affected by the interface is not simple or straight forward. Rules of thumb and theories about S/PDIF jitter do not directly apply here because of the different implementation. There has been very little research into what effects on the bus cause what change in jitter for adaptive mode so its very tough to make predictions such as "this going on in the computer will cause this type of jitter in the recovered clock", we don't know right now. And to make it worse there are several VERY different implementations of adaptive mode which are going to respond differently to external stimulus. For example the TAS1020 and the PCM2706 handle adaptive mode compeltely differently and thus most likely have significantly different jitter responses. Then there are a few DACs that take the clock from the USB chip and run it through a much better PLL such as a VCXO based PLL, these will radically cut down on any jitter effects caused by the interface.

I can think of four main areas that can cause issues:

bus powered PS, many cheap USB DACs draw their power from the USB bus itself, this is a disaster waiting to happen for sonics. ANYTHING that goes on in the computer can wind up affecting this. An external supply for the DAC greatly reduces this.

Actual jitter on the USB bus. This can come from different places, power supply noise on the USB transmitter in the computer will cause jitter on the data, since this is inside the compuetr (one of the electrically noisiest environments on the planet) this can be pretty significant. Jitter in the clock signal fed to the USB transmitter (which is heavily effected by PS noise). An interesting source is the spread spectrum clocks used in most computers today. The frequencies of the oscillators used in most computers is deliberately varied in an attempt to "spread the noise" out over a larger spectrum and make it easier to pass FCC testing. This deliberate clock jitter will also effect the timing on the USB bus, even if its fed from its own stable clock source the jittery input data will still cause jitter on the output.

Noise on the BUS signals. Even if the signal launched from the computer has precise timing on its edges, any noise picked up by the bus wires will cause timing errors when the receiver tries to detect that edge. This can be effected by different amount of shielding on the cable, the environment the cable passes through, but most especially by those infamous power wires in the BUS. Even if your DAC is not powered by the BUS, those wires are still there in the USB cable running right next to the signal wires, no matter how hard you try there will still be some noise from the power wires picked up by the signal wires.

Not directly related to the computer per se, but the USB cable itself can have a significant effect on the signal. The length of the cable can make a significant difference on the noise and jitter of the signal at the receiving end. Even cable construction and materials can effect the jitter at the device end.

Figuring out what effects USB DACs is very much in its infancy right now, I don't think anybody can tell you doing such and such on the computer is going change the sound thusly for USB DACs in general. For a specific model with a specific computer with a specific environment you might be able to do some experiments and come up with a few correlations, but its going to be really tough to come up with genaralizations.

So in a nutshell: yep what goes on in the computer CAN effect jitter in the DAC, what causes what? who knows.

John S.



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