----以下摘自USB总线音频设备规范
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.
----翻译：
3.3音频同步类型
在音频流接口中使用的每个同步音频端点都使用了USB规范第5节中定义的同步类型。以下各节简要介绍可能的同步类型。
3.3.1异步
异步的同步音频端点以一定的速率生成或者丢弃音频数据来匹配USB，这个速率是锁定到本地时钟或者说和USB设备的本地时钟相关的（本地时钟即USB音频芯片外部时钟或内部时钟）。这些端点没有同步到帧开始（SOF），也就是没有同步到USB的主机时钟。
3.3.2同步
同步的同步音频端点的时钟系统可以受外部控制，可以是SOF或者总线的同步间隔。这样的端点需要锁定它的采样时钟到帧头来标记总线间隔的开始。
3.3.3自适应
自适应等时音频端点能够在其操作范围内以任何速率发送或接收数据。这意味着这些端点必须运行一个内部进程，该进程允许它们将其本地数据速率与在其接口上施加的数据速率相匹配。
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.