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.
