----以下摘自USB總線音頻設備規范
Universal Serial Bus Device Class Definition for Audio Devices --- Release 3.0-Errata， April 05, 2019
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 are not synchronized to a start of frame (SOF).
3.11.2 SYNCHRONOUS
The clock system of synchronous isochronous audio endpoints can be controlled externally through SOF or Bus Interval synchronization. Such an endpoint shall lock its sample clock to the SOF tick marking the start of a new Bus Interval.
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 shall run an internal process that allows them to match their natural data rate to the data rate that is imposed at their interface.
----翻譯：
3.3音頻同步類型
在音頻流接口中使用的每個同步音頻端點都使用了USB規范第5節中定義的同步類型。以下各節簡要介紹可能的同步類型。
3.3.1異步
異步的同步音頻端點以一定的速率生成或者丟棄音頻數據來匹配USB，這個速率是鎖定到本地時鍾或者說和USB設備的本地時鍾相關的（本地時鍾即USB音頻芯片外部時鍾或內部時鍾）。這些端點沒有同步到幀開始（SOF），也就是沒有同步到USB的主機時鍾。
3.3.2同步
同步的同步音頻端點的時鍾系統可以受外部控制，可以是SOF或者總線的同步間隔。這樣的端點需要鎖定它的采樣時鍾到幀頭來標記總線間隔的開始。
3.3.3自適應
自適應等時音頻端點能夠在其操作范圍內以任何速率發送或接收數據。這意味着這些端點必須運行一個內部進程，該進程允許它們將其本地數據速率與在其接口上施加的數據速率相匹配。
老版本
Universal Serial Bus Device Class Definition for Audio Devices --- Release 1.0 --- March 18, 1998
3.3 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.
3.3.1 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.
3.3.2 Synchronous
The clock system of synchronous isochronous audio endpoints can be controlled externally through SOF synchronization. Such an endpoint must do one of the following:
· Slave its sample clock to the 1ms SOF tick.
· Control the rate of USB SOF generation so that its data rate becomes automatically locked to SOF.
3.3.3 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.