This from the Tweek Geek site. I started thinking about the fact that the HiFace has 2 precision clocks on board (2.5ppm) and the slight delay in the interface of iTunes when using the HiFace led me to believe it is in fact Async. This is what I found:
28) Does the hiFace work in asynchronous mode?
Sure, the transfer of data over the USB bus happens in a totally asynchronous fashion, with respect to their retransmission to the DAC, in short packets which use all the available bus bandwidth. Actually, hiFace is master in the transmission from PC to interface. As it uses its low jitter oscillators to time the S/PDIF transmission, line jitter is totally uncorrelated to data transmission from PC to interface.
28) Does the hiFace work in asynchronous mode?
Sure, the transfer of data over the USB bus happens in a totally asynchronous fashion, with respect to their retransmission to the DAC, in short packets which use all the available bus bandwidth. Actually, hiFace is master in the transmission from PC to interface. As it uses its low jitter oscillators to time the S/PDIF transmission, line jitter is totally uncorrelated to data transmission from PC to interface.