As I understand it, asynchronous USB uses internal DAC's clock
for the D/A converter. This clock is independent
(asynchronous) to USB data rate. For that reason data has to
be buffered. DAC has to request different number of samples
each frame to keep data buffer between underflow and overflow.
Since D/A converter uses data from the buffer (memory) with
internal clock, then quality of the cable lies only in good
shielding to prevent electrical noise contamination of the
DAC. In synchronous USB D/A converter clock comes from the
computer. Usually D/A converter runs from internal clock that
is synchronized with incoming USB clock by PLL (Phase Lock
Loop) but it is less than perfect. This scheme is used in
pretty much every CDP, except here incoming USB clock can be
very jittery.
for the D/A converter. This clock is independent
(asynchronous) to USB data rate. For that reason data has to
be buffered. DAC has to request different number of samples
each frame to keep data buffer between underflow and overflow.
Since D/A converter uses data from the buffer (memory) with
internal clock, then quality of the cable lies only in good
shielding to prevent electrical noise contamination of the
DAC. In synchronous USB D/A converter clock comes from the
computer. Usually D/A converter runs from internal clock that
is synchronized with incoming USB clock by PLL (Phase Lock
Loop) but it is less than perfect. This scheme is used in
pretty much every CDP, except here incoming USB clock can be
very jittery.