Brian
Current DACs use asynchronous USB control, which means the DAC controls the timing of pulling data from the PC. Before async USB, the PC controlled the timing as it pushed the data to the DAC, which was far less accurate. A PC is just not good at delivering bits in real time. Before aysnc USB, memory playback was one way to try to make the timing more accurate, since the PC did not have to go out to disk, which could interrupt the timing on the usb. With asynch USB output timing is not really an issue. Only after asych USB became the norm, did people start to worry about the electrical noise, rather than the USB timing.