This type of USB hand-shaking makes the computer bus “master” which then controls the rate at which the data are sent to the “downstream” audio device. The audio device becomes a “slave” to the “upstream” computer. This device has to try and “track” or adapt its internal clocking, in real-time, to that of the source.

This is usually accomplished with phase-locked loops (PLLs) and other inherently jittery clock extraction methods, instead of more stable, dedicated fixed clocks. A superior technique is asynchronous USB, in which the “downstream” device becomes “master” and “slaves” the computer to the USB device.