USB Device Class Definition for Audio Devices clock domains

3.10 Clock Domains

A Clock Domain is defined as a zone within which all sampling clocks are derived from the same master

clock. Therefore, within the same Clock Domain, all sampling clocks are synchronous and their timing

relationship is constant. However, the sampling clocks can be at different sampling frequencies. The master

clock can be generated in many different ways. An internal crystal could be the master clock, the USB start

of frame (SOF) could be used or even an externally supplied clock could serve as a master clock.

In general, multiple different Clock Domains can exist within the same audio function.

3.11 Audio Synchronization Types

Each isochronous audio endpoint used in an AudioStreaming interface belongs to a synchronization type as

defined in Section 5 of the USB Specification. The following sections briefly describe the possible

synchronization types.

3.11.1 Asynchronous

Asynchronous isochronous audio endpoints produce or consume data at a rate that is locked either to a

clock external to the USB or to a free-running internal clock. These endpoints cannot be synchronized to a

start of frame (SOF) or to any other clock in the USB domain.

3.11.2 Synchronous

The clock system of synchronous isochronous audio endpoints can be controlled externally through SOF

synchronization. Such an endpoint must lock its sample clock to the 1ms SOF tick. Optionally, a high-

speed endpoint could lock its clock to the 125 µs SOF that occurs at the beginning of every microframe to

improve accuracy.

3.11.3 Adaptive

Adaptive isochronous audio endpoints are able to source or sink data at any rate within their operating

range. This implies that these endpoints must run an internal process that allows them to match their natural

data rate to the data rate that is imposed at their interface.