USB音频设备规范1.0版

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"USB Audio Class Specification 1.0 涵盖了USB音频设备类的详细定义，适用于USB开发者学习和参考。这份文档是1998年3月18日发布的1.0版本，由多个公司和专家共同贡献完成。" USB Audio Class Specification 1.0 是一个重要的技术文档，它定义了USB接口上音频设备的操作规范。这个规范旨在为不同类型的音频设备，如音箱、麦克风、耳机等，提供统一的通信标准，确保它们能够与各种主机系统（如个人电脑）无缝协作。在1.0版本中，规范涵盖了以下几个关键方面： 1. **设备分类**：文档将USB音频设备分为几个类别，包括音频功能设备（Audio Function Device）和音频接口设备（Audio Interface Device）。音频功能设备是实际执行音频处理的硬件，而音频接口设备则负责设备与主机之间的通信。 2. **音频流**：规范详细描述了如何传输音频数据，包括同步问题和数据包格式。这确保了音频流在传输过程中的实时性和低延迟。 3. **属性请求**：定义了特定的属性请求，允许主机查询和设置音频设备的各种参数，如采样率、位深度等。 4. **子类划分**：根据设备功能，如数字信号处理器（DSP）、混音器等，规范将设备划分为不同的子类，以便于管理和实现。 5. **通道定义**：规范包含了对不同音频通道（如主声道、环绕声道、辅助声道）的定义，以及如何控制和混合这些通道。 6. **属性字段**：文档指出属性字段从8位扩展到了16位，增加了设备描述的详细程度，使得设备可以携带更多的元数据和控制信息。 7. **贡献者**：IBM、微软、Altec Lansing、Dolby Laboratories、Logitech和Philips等公司的专家参与了该规范的制定，确保了行业内的广泛支持和兼容性。 8. **修订历史**：从0.1到0.6的版本迭代显示了规格的逐步完善，每次更新都解决了错误，添加了新的特性或改进了现有内容。通过理解和遵循USB Audio Class Specification 1.0，开发者能够创建符合标准的USB音频设备，提高产品的互操作性和用户体验。对于硬件制造商和软件开发者来说，这份文档是构建USB音频解决方案的关键参考资料。

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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