USB音频设备类定义3.0修订版-错误更正

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"Audio30-Errata.pdf 是一份关于Universal Serial Bus (USB) Audio Device Class Definition的Release 3.0修订版错误集，发布于2019年4月5日。这份文档主要包含了对UAC（USB音频设备类）3.0规范的更新和修正，由多个业界知名公司的专家共同贡献，包括AMD、Apple、C-Media、Cirrus Logic、Conexant Systems、DisplayLink、Dolby Laboratories、Google、Intel、Logitech、MCCI、MediaTek、Microsoft、NXP Semiconductors、Qualcomm和Synopsys等。" 在USB Audio Device Class Definition Release 3.0中，主要关注的是音频设备如何通过USB接口进行通信和数据传输。USB音频设备类（UAC）定义了一套标准，允许音频设备如耳机、麦克风、音频接口等与计算机和其他USB兼容设备无缝配合。UAC3.0作为这一标准的最新版本，旨在提高音频质量和效率，同时引入了新的功能和改进。此修订版错误集（Errata）是针对UAC3.0规格书中已知问题的集合，可能包含规范的澄清、错误修复、遗漏的补充等内容。这确保了开发者和制造商在设计和实现USB音频设备时能够遵循最新的、最准确的指导，以减少潜在的兼容性和性能问题。修订历史部分列出了不同修订版本的时间、文件名以及描述，这些修订可能涉及新增特性、修改现有规范或者修复已知问题。例如，虽然具体的修订内容没有给出，但可以推测每个修订都可能对音频设备的兼容性、性能、功耗管理等方面进行了优化。对于USB音频设备开发的专业人士来说，这份Errata文档至关重要，因为它提供了对UAC3.0标准的深入理解和应用的必要信息。它帮助开发者避免在实现过程中遇到已知的问题，从而提升产品的质量和用户体验。同时，这个文档也是USB音频设备行业标准发展的见证，展示了各公司间的技术交流和合作。

Release 3.0-Errata April 05, 2019 16

synchronous to a second master clock, which is either free running

with respect to the first master clock (asynchronous sampling rate

conversion) or synchronous to the first master clock (synchronous

sampling rate conversion).

Selector Unit (SU) Selects from a number of input Clusters.

Service Interval The period between consecutive transfers on a periodic endpoint,

specified by the value in the bInterval field of the endpoint

descriptor. This is not the same as the Bus Interval.

SUD Acronym for Selector Unit descriptor.

TD Acronym for Terminal descriptor.

Terminal Addressable logical object inside an Audio Function that represents

a connection to the Audio Function’s outside world.

Unit Addressable logical object inside an Audio Function that represents

a certain audio sub-functionality.

UD Acronym for Unit descriptor.

XUD Acronym for Extension Unit descriptor.

Release 3.0-Errata April 05, 2019 17

2 MANAGEMENT OVERVIEW

The USB is very well suited for transport of audio, ranging from low fidelity voice connections to high quality, multi-

channel audio streams. The USB has become a ubiquitous connector on modern PCs and is well-understood by

most consumers today. As such, it has become the connector of choice for many peripherals and is indeed the

simplest and most pervasive digital audio connector available today. Consumers can count on this medium to meet

all of their current and future audio needs. Many applications from communications, to entertainment, to music

recording and playback, can take advantage of the audio features of the USB.

In principle, a versatile bus specification like the USB provides many ways to propagate and/or control digital

audio. For the industry, however, it is very important that audio transport mechanisms be well defined and

standardized on the USB. Only in this way can interoperability be guaranteed among the many possible Audio

Devices on the USB. Standardized audio transport mechanisms also help to keep software drivers as generic as

possible. The Audio Device Class described in this document satisfies those requirements. It is written and revised

by experts in the audio field. Other device classes that address audio in some way should refer to this document

for their audio interface specification.

An essential issue in audio is synchronization of the data streams. Indeed, the smallest artifacts are easily detected

by the human ear. Therefore, a robust synchronization scheme on isochronous transfers has been developed and

incorporated in the USB Specification and the USB 3.1 Specification. The Audio Device Class definition adheres to

these synchronization schemes to transport audio data reliably over the bus.

This document contains all necessary information for a designer to build a USB-compliant device that incorporates

Audio Functionality. It specifies the standard and class-specific descriptors that shall be present in each USB Audio

Function. It further explains the use of class-specific requests that allow for full Audio Function control. A number

of predefined data formats are listed and fully documented. Each format defines a standard way of transporting

audio over the USB. Provisions have been made so that vendor-specific audio formats and compression schemes

can be handled as well.

Many of the changes introduced in this version of the USB Specification for Audio Devices are inspired by the

desire to use USB Audio in modern portable devices. Special attention has been paid to make the Audio Device

Class more power-friendly by providing new tools to selectively enable and disable parts of the Audio Function and

also by supporting burst mode data transfers for longer sleep times in between data transfers. In addition, the

specification supports new CODEC types and data formats for consumer audio applications, provides numerous

clarifications of the original specification and extensions to support various changes in the core specification.

2.1 OVERVIEW OF KEY DIFFERENCES BETWEEN ADC V2.0 AND V3.0

The following list is not an exhaustive list of all changes that have been introduced. For complete information,

refer to the full specification.

• Dolby Processing Unit removed

• Encoder and Decoder support removed

• Copy protection (S/PDIF-style) removed

• bmAttributes (MaxPacketsOnly bit) field in class-specific isochronous endpoint descriptor removed

• Type II and Extended Type II Audio Data Formats removed

• New Power Domains

• New Multi-Function Processing Unit

• Additional Type III formats introduced

Release 3.0-Errata April 05, 2019 19

3 FUNCTIONAL CHARACTERISTICS

3.1 INTRODUCTION

In many cases, Audio Functionality does not exist as a standalone device. It is one capability that, together with

other functions, constitutes a “composite” device. An example of this is a DVD-ROM player, which can incorporate

video, audio, data storage, and transport control. The Audio Function is thus located at the interface level in the

Device Class hierarchy. The following figure provides details.

Figure 3-1: Multiple Configurations and their Audio Interface Associations and Interfaces

An Audio Function is considered to be a ‘closed box’ that has very distinct and well defined interfaces to the

outside world. Audio Functions are described through an Audio Interface Association (AIA). The AIA groups all USB

interfaces that together provide access to the Audio Function for control and streaming purposes.

A USB Device may express different AIAs representing the same underlying hardware by using multiple USB Device

Configuration descriptors. These AIAs may be used to provide access to the Audio Function at different compliance

levels. For example, one AIA might provide access to the Audio Function in ADC 1.0 mode, whereas another AIA

might provide access to the Audio Function in ADC 3.0 mode, as defined by this document. Note that the Audio

Function may take on a different topology and even different functionality when accessed through different AIAs.

Because only one Configuration may be active at a time, only one AIA shall be active at a time for the same Audio

Function. Host software should choose the most desirable Configuration and use the Standard

SET_CONFIGURATION command to select the Configuration containing the most desired AIA or AIAs (if there are

multiple independent Audio Functions residing in the same USB Device).

An AIA shall have a single AudioControl interface and can have zero or more AudioStreaming and zero or more

MIDIStreaming interfaces. The AudioControl (AC) interface is used to access the AudioControls inside the Audio

Function whereas the AudioStreaming (AS) interfaces are used to transport audio streams into and out of the

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