Что, самому лень найти и прочесть? Пожалуйста:
"PACKET NOISE: In a DAC the data packets coming in on the USB bus are not continuous—there is significant time in-between each packet. Thus the processing of these packets produces noise on the power supply and ground plane that come in bursts, and we refer to this as "packet noise". Since the rate of USB packets is 8KHz there are strong components of this noise in the audio band. This noise can cause jitter in clock oscillators, re-clocking flops, and DAC chips. It can also go directly into noise on the output of DAC chips."

ЗЫ Внутри пакетов находятся гораздо более высокочастотные импульсы, но сами пакеты идут со скоростью 8кГц.

И кстати, жду объяснений по поводу изменения потенциала на корпусе Сотм.