Clocker value on a network player?

Synchronous simply means that the audio data is sent to the DAC at the same rate that it is played (e.g. at 44,100 samples per second in the case of standard Redbook CDs). The DAC has no control over the rate at which the data is sent. 

Asynchronous means that the DAC periodically requests a bunch of audio samples as they are needed and the DAC controls when to do these requests. 

Theoretically, an asynchronous USB connection should eliminate any sonic effects from the upstream components because you are just sending digital data and the DAC is in complete control. 

In reality, it usually doesn't work out this way. It appears to be fairly hard to isolate the noise created by the USB interface itself from interfering with the sound quality. Moving the conversion from asynchronous to synchronous outside the DAC often (but not always) results in better sound quality. 

Even DACs with exceptionally good USB circuitry and noise isolation benefit from a cleaner USB signal, although how much depends on the DAC. 

There are no easy answers here (and a lot of differing opinions). 

This is audiophile land. Assume misinformation is rife in almost all cases.

You don't need a precision clock in audio. You need a clock with low jitter over a period of time about about 2-5 times the lowest frequency, or out 0.1 - 0.25 seconds.

Why do I need a 1 part per million accurate clock. Do you think anyone's turntable is this accurate?

Why do I need an oven controlled crystal oscillator, which was developed for frequency stability over long periods of time, not near term jitter. If these designers were really trying for lowest jitter, they would cool the crystal because the phase noise is lower. But people use oven controlled because it is cheap, and they only need stability. Some scientific applications do cool the crystal for lower thermal noise.

I am ranting :-) ... a good clock is essential as many have noted for synchronous signals, but good DACs can eliminate almost all jitter, and certainly audible jitter. Even low cost DACs can do that now. 

Do chord dacs do a good job with jitter?