USB音频设备规范1.0版

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"USB Audio Devices.pdf" 本文档是“USB音频设备”的1.0版本发布，由IBM、微软、Altec Lansing、Dolby Laboratories、Logitech等公司的代表共同参与编写，旨在定义和规范用于处理音频、语音和声音相关功能的USB设备或嵌入式功能。该文档覆盖了1998年3月18日发布时的标准。 USB音频设备类定义适用于所有操纵音频数据（模拟和数字）以及控制音频环境的功能，如音量和音调控制。然而，它不包括与音频数据再现相关的传输机制操作，如磁带传输机制或CD-ROM驱动器控制。MIDI数据流的处理，作为音频的一部分，也在本文档中涉及。文档的主要内容可能包括以下几个方面： 1. **设备类定义**：详细说明了USB音频设备的分类，包括音频接口、音频函数和其他相关组件的定义，确保设备在USB总线上的互操作性。 2. **协议和接口**：描述了设备如何通过USB接口与主机通信，包括传输协议、数据包结构和控制管道的使用。 3. **功能描述**：涵盖各种音频功能，如输入和输出端口、混音器、音量控制器、均衡器等，以及它们在USB框架下的工作原理。 4. **MIDI支持**：详细阐述了如何通过USB传输MIDI数据，包括MIDI接口的定义、MIDI消息的封装和解封。 5. **同步问题**：讨论了实时音频数据传输中的同步问题，确保音频流的连续性和无损性。 6. **子类划分**：可能列出了不同的音频设备子类，如基本音频设备、多通道音频设备、合成器设备等，以及它们各自的特性。 7. **属性请求**：描述了如何通过特定的属性请求来控制和查询设备的参数，如音量设置、采样率等。 8. **错误处理和恢复**：提供了在USB通信中遇到问题时的错误处理策略，以及如何恢复设备的正常运行。 9. **兼容性和测试**：可能包含了确保设备与不同操作系统和USB主机兼容的测试方法和指南。这个1.0版本的发布标志着USB音频设备标准的初步形成，为后续的开发和标准化工作奠定了基础，推动了USB音频设备的广泛应用和发展。随着技术的进步，后续版本可能会包含更多的增强和扩展，以适应不断变化的音频技术需求。

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USB Device Class Definition for Audio Devices

Release 1.0 March 18, 1998 16

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 subfunctionality.

XUD Acronym for Extension Unit Descriptor.

USB Device Class Definition for Audio Devices

Release 1.0 March 18, 1998 17

2 Management Overview

The USB is very well suited for transport of audio (voice and sound). PC-based voice telephony is one of

the major drivers of USB technology. In addition, the USB has more than enough bandwidth for sound,

even high-quality audio. Many applications related to voice telephony, audio playback, and recording can

take advantage of the USB.

In principle, a versatile bus specification like the USB provides many ways to propagate and 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. The Audio Device Class definition adheres to this

synchronization scheme 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 must 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 USB. However, provisions have been made so that

vendor-specific audio formats and compression schemes can be handled.

USB Device Class Definition for Audio Devices

Release 1.0 March 18, 1998 18

3 Functional Characteristics

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. A perfect example of this is a CD-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. It consists of a number of interfaces grouping related

pipes that together implement the interface to the audio function.

Audio functions are addressed through their audio interfaces. Each audio function has a single

AudioControl interface and can have several AudioStreaming and MIDIStreaming interfaces. The

AudioControl (AC) interface is used to access the audio Controls of the function whereas the

AudioStreaming (AS) interfaces are used to transport audio streams into and out of the function. The

MIDIStreaming (MS) interfaces are used to transport MIDI data streams into and out of the audio

function. The collection of the single AudioControl interface and the AudioStreaming and

MIDIStreaming interfaces that belong to the same audio function is called the Audio Interface Collection

(AIC). A device can have multiple Audio Interface Collections active at the same time. These Collections

are used to control multiple independent audio functions located in the same composite device.

Note: All MIDI-related information is grouped in a separate document, Universal Serial Bus Device

Class Definition for MIDIStreaming Interfaces that is considered part of this specification.

3.1 Audio Interface Class

The Audio Interface class groups all functions that can interact with USB-compliant audio data streams.

All functions that convert between analog and digital audio domains can be part of this class. In addition,

those functions that transform USB-compliant audio data streams into other USB-compliant audio data

streams can be part of this class. Even analog audio functions that are controlled through USB belong to

this class.

In fact, for an audio function to be part of this class, the only requirement is that it exposes one

AudioControl interface. No further interaction with the function is mandatory, although most functions in

the audio interface class will support one or more optional AudioStreaming interfaces for consuming or

producing one or more isochronous audio data streams.

The Audio Interface class code is assigned by the USB. For details, see Section A.1, “Audio Interface

Class Code.”

3.2 Audio Interface Subclass and Protocol

The Audio Interface class is divided into Subclasses that can be further qualified by the Interface Protocol

code. However, at this moment, the Interface Protocol is not used and must be set to 0x00. All audio

functions are part of a certain Subclass. The following three Subclasses are currently defined in this

specification:

• AudioControl Interface Subclass

• AudioStreaming Interface Subclass

• MIDIStreaming Interface Subclass

The assigned codes can be found in Sections A.2, “Audio Interface Subclass Codes” and A.3, “Audio

Interface Protocol Codes” of this specification. All other Subclass codes are unused and reserved except

code 0xFF which is by specification reserved for vendor-specific extensions.

USB Device Class Definition for Audio Devices

Release 1.0 March 18, 1998 19

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.

3.4 Inter Channel Synchronization

An important issue when dealing with audio, and 3-D audio in particular, is the phase relationship

between different physical audio channels. Indeed, the virtual spatial position of an audio source is

directly related to and influenced by the phase differences that are applied to the different physical audio

channels used to reproduce the audio source. Therefore, it is imperative that USB audio functions respect

the phase relationship among all related audio channels. However, the responsibility for maintaining the

phase relation is shared among the USB host software, hardware, and all of the audio peripheral devices

or functions.

To provide a manageable phase model to the host, an audio function is required to report its internal delay

for every AudioStreaming interface. This delay is expressed in number of frames (ms) and is due to the

fact that the audio function must buffer at least one frame worth of samples to effectively remove packet

jitter within a frame. Furthermore, some audio functions will introduce extra delay because they need time

to correctly interpret and process the audio data streams (for example, compression and decompression).

However, it is required that an audio function introduces only an integer number of frames of delay. In the

case of an audio source function, this implies that the audio function must guarantee that the first sample

it fully acquires after SOFn (start of frame n) is the first sample of the packet it sends over USB during

frame (n+δ). δ is the audio function’s internal delay expressed in ms. The same rule applies for an audio

sink function. The first sample in the packet, received over USB during frame n, must be the first sample

that is fully reproduced during frame (n+δ).

By following these rules, phase jitter is limited to ±1 audio sample. It is up to the host software to

synchronize the different audio streams by scheduling the correct packets at the correct moment, taking

into account the internal delays of all audio functions involved.

USB Device Class Definition for Audio Devices

Release 1.0 March 18, 1998 20

3.5 Audio Function Topology

To be able to manipulate the physical properties of an audio function, its functionality must be divided

into addressable Entities. Two types of such generic Entities are identified and are called Units and

Terminals.

Units provide the basic building blocks to fully describe most audio functions. Audio functions are built by

connecting together several of these Units. A Unit has one or more Input Pins and a single Output Pin,

where each Pin represents a cluster of logical audio channels inside the audio function. Units are wired

together by connecting their I/O Pins according to the required topology.

In addition, the concept of a Terminal is introduced. There are two types of Terminals. An Input Terminal

(IT) is an Entity that represents a starting point for audio channels inside the audio function. An Output

Terminal (OT) represents an ending point for audio channels. From the audio function’s perspective, a

USB endpoint is a typical example of an Input or Output Terminal. It either provides data streams to the

audio function (IT) or consumes data streams coming from the audio function (OT). Likewise, a Digital to

Analog converter, built into the audio function is represented as an Output Terminal in the audio

function’s model. Connection to the Terminal is made through its single Input or Output Pin.

Input Pins of a Unit are numbered starting from one up to the total number of Input Pins on the Unit. The

Output Pin number is always one. Terminals only have one Input or Output Pin that is always numbered

one.

The information, traveling over I/O Pins is not necessarily of a digital nature. It is perfectly possible to use

the Unit model to describe fully analog or even hybrid audio functions. The mere fact that I/O Pins are

connected together is a guarantee (by construction) that the protocol and format, used over these

connections (analog or digital), is compatible on both ends.

Every Unit in the audio function is fully described by its associated Unit Descriptor (UD). The Unit

Descriptor contains all necessary fields to identify and describe the Unit. Likewise, there is a Terminal

Descriptor (TD) for every Terminal in the audio function. In addition, these descriptors provide all

necessary information about the topology of the audio function. They fully describe how Terminals and

Units are interconnected.

This specification describes the following seven different types of standard Units and Terminals that are

considered adequate to represent most audio functions available today and in the near future:

• Input Terminal

• Output Terminal

• Mixer Unit

• Selector Unit

• Feature Unit

• Processing Unit

• Extension Unit

The ensemble of UDs and TDs provide a full description of the audio function to the Host. A generic

audio driver should be able to fully control the audio function, except for the functionality, represented by

Extension Units. Those require vendor-specific extensions to the audio class driver.

The descriptors are further detailed in Section 4, “Descriptors” of this document.

Inside a Unit, functionality is further described through audio Controls. A Control typically provides

access to a specific audio property. Each Control has a set of attributes that can be manipulated or that

present additional information on the behavior of the Control. A Control can have the following four

attributes:

• Current setting attribute

• Minimum setting attribute

• Maximum setting attribute

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