USB视频类设备定义1.1版

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"USB_Video_Class_1.1.pdf 是一份关于USB Video Class（UVC）规范的文档，修订于2005年6月1日。这份文档由多个公司的贡献者共同编写，包括Intel、Canon、Microsoft、STMicroelectronics、Texas Instruments、Logitech、Fujifilm等知名公司。" USB Video Class（UVC）是USB设备类定义的一部分，旨在标准化视频设备与主机之间的通信。这个规范允许USB设备，如网络摄像头、视频捕获设备、视频输出设备等，能够无缝地与兼容的计算机系统进行交互，无需额外的驱动程序或复杂的安装过程。该规范的版本1.1是在早期版本的基础上进行了更新和改进，以适应不断发展的技术需求。文档的核心内容可能包括以下几个关键部分： 1. **类定义**：详细阐述了USB Video Class的架构，包括其数据传输模式、控制管道和媒体传输接口。这确保了设备和主机之间的互操作性。 2. **设备配置**：描述了如何配置USB视频设备，包括设备描述符、配置描述符、接口描述符和端点描述符，这些是设备与主机通信的基础。 3. **视频流格式**：定义了各种视频流的编码格式，如YUV、RGB等，以及帧率、分辨率等参数的规范。 4. **控制接口**：说明了设备如何通过控制管道实现对摄像头参数的调整，如焦距、曝光时间、白平衡等。 5. **数据传输**：详细规定了如何高效、实时地传输视频流，包括批量传输和中断传输的不同使用场景。 6. **电源管理**：涵盖USB设备的电源状态管理，确保设备在不牺牲性能的情况下节省能源。 7. **兼容性和测试**：提供了测试协议和准则，以确保设备符合UVC标准，并能在不同的操作系统上正常工作。 8. **安全性和隐私**：可能包括对视频设备安全性的考虑，如防止未授权访问和保护用户隐私的措施。这份文档对于开发USB视频设备的硬件工程师、软件开发者以及系统集成商来说具有重要参考价值，他们可以依据此规范来设计和实现符合标准的USB视频设备，确保其能被广泛接受和兼容。同时，对于理解USB视频设备的工作原理和技术细节的用户来说，也是极好的学习资料。

USB Device Class Definition for Video Devices

isochronous endpoints, a Payload Transfer is contained in the data

transmitted during a single (micro)frame: up to 1023 bytes for a full-

speed endpoint; up to 1024 bytes for a high-speed endpoint; and up to

3072 bytes for a high-speed/high-bandwidth endpoint. For bulk

endpoints, a Payload Transfer is contained in the data transmitted in a

single bulk transfer (which may consist of multiple bulk data

transactions).

Payload Data Format-specific data contained in a Payload Transfer (excluding the

Payload Header).

Payload Header A header at the start of each Payload Transfer that provides data framing

and encapsulation information.

PU Processing Unit.

Request A mechanism supported by the video function for the host software to

interact with a Control within an Entity.

Sample Transfer A sample transfer is composed of one or more payload transfer(s)

representing a video sample.

STC Source Time Clock. The clock used by the data source that governs the

sampling of video (or related) data.

SU Selector Unit.

TD Terminal Descriptor.

Terminal An Entity representing a source (Input Terminal) or sink (Output

Terminal) for data flowing into or out of a video function.

UD Unit Descriptor.

Unit An Entity representing a transformation of data flowing through a video

function.

USB Universal Serial Bus.

USB Transaction See USB 2.0 Chapter 5.

USB Transfer See USB 2.0 Chapter 5.

UVC USB Video Class.

VC VideoControl; refers to the interface used for video function control.

VIC Video Interface Collection; refers to the collection of VideoControl and

VideoStreaming interfaces within the same video function.

VS VideoStreaming; refers to the interface(s) used for video stream

transport.

XU Extension Unit.

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USB Device Class Definition for Video Devices

2 Functional Characteristics

The video function is 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 video

function.

Video functions are addressed through their video interfaces. Each video function has a single

VideoControl (VC) interface and can have several VideoStreaming (VS) interfaces. The

VideoControl (VC) interface is used to access the device controls of the function whereas the

VideoStreaming (VS) interfaces are used to transport data streams into and out of the function.

The collection of the single VideoControl interface and the VideoStreaming interfaces that

belong to the same video function is called the Video Interface Collection (VIC). An Interface

Association Descriptor (IAD) is used to describe the Video Interface Collection.

2.1 Video Interface Class

The Video Interface class groups all functions that can interact with USB-compliant video data

streams. All functions that convert between analog and digital video domains can be part of this

class. In addition, those functions that transform USB-compliant video data streams into other

USB-compliant video data streams can be part of this class. Even analog video functions that are

controlled through USB belong to this class.

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

VideoControl Interface. No further interaction with the function is mandatory, although most

functions in the video interface class will support one or more optional VideoStreaming

interfaces for consuming or producing one or more video data streams.

The Video Interface class code is assigned by the USB. For details, see section A.1 "Video

Interface Class Code".

2.2 Video Interface Subclass and Protocol

The Video Interface class is divided into subclasses as identified by the Interface Subclass code.

The following two interface subclasses are defined in this specification.

• VideoControl Interface

• VideoStreaming Interface

The following Function Subclass is used in the Interface Association Descriptor (see section 3.6,

“Interface Association Descriptor”):

• Video Interface Collection

The Interface Protocol is not used and must be set to 0x00.

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USB Device Class Definition for Video Devices

The assigned codes can be found in sections A.2, "Video Interface Subclass Codes" and A.3,

"Video Interface Protocol Codes" of this specification. All other subclass codes are unused and

reserved except code 0xFF, which is reserved for vendor-specific extensions.

2.3 Video Function Topology

To be able to manipulate the physical properties of a video function, its functionality must be

divided into addressable entities. The following two generic entities are identified:

• Units

• Terminals

Units provide the basic building blocks to fully describe most video functions. Video 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 data streams inside the video

function. Units are wired together by connecting their I/O Pins according to the required

topology. A single Output Pin can be connected to one or more Input Pins (fan-out allowed).

However, a single Input Pin can only be connected to one Output Pin (fan-in disallowed). Loops

or cycles within the graph topology are not allowed.

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

Terminal (IT) is an entity that represents a starting point for data streams inside the video

function. An Output Terminal (OT) represents an ending point for data streams. From the video

function’s perspective, a USB endpoint is a typical example of an Input Terminal or Output

Terminal. It either provides data streams to the video function (IT) or consumes data streams

coming from the video function (OT). Likewise, a Charge Coupled Device (CCD) sensor, built

into the video function is represented as an Input Terminal in the video function’s model.

Connection to a Terminal is made through its single Input Pin 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 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 possible to use

the Unit model to describe fully analog or even hybrid video 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 video 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 video function. In addition, these

descriptors provide all necessary information about the topology of the video function. They

fully describe how Terminals and Units are interconnected.

The descriptors are further detailed in section 3, "Descriptors" of this document.

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USB Device Class Definition for Video Devices

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

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

• Input Terminal

• Output Terminal

• Selector Unit

• Processing Unit

• Extension Unit

Also, there are certain special Terminals that extend the functionality of the basic Input and

Output Terminals. These special Terminals support additional Terminal Descriptor fields and

Requests that are specific to the extended features these Terminals provide. These include:

• Media Transport Terminal (defined in USB Device Class Definition for Video Media

Transport Terminal specification)

• Camera Terminal

The types of Units defined in this specification could be extended in future revisions, or via

companion specifications. For example, a Tuner Unit could be added as a companion

specification to accommodate devices with TV Tuners.

Inside a Unit or Terminal, functionality is further described through Video Controls. A Control

typically provides access to a specific video property. Each Control has a set of attributes that

can be manipulated or that present additional information about the behavior of the Control.

Controls have attributes, which might include:

• Current setting

• Minimum setting

• Maximum setting

• Resolution

• Size

• Default

Consider a Brightness Control inside a Processing Unit. By issuing the appropriate requests, the

Host software can obtain values for the Brightness Control’s attributes and, for instance, use

them to correctly display the Control in a User Interface. Setting the Brightness Control’s current

setting attribute allows the Host software to change the brightness of the video that is being

streamed.

The ensemble of Unit Descriptors, Terminal Descriptors and Video Controls provide a full

description of the video function to the Host. A generic class driver shall be able to fully control

the video function. When functionality is represented by Extension Units, the class driver shall

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USB Device Class Definition for Video Devices

permit access to vendor-specific extensions via a pass-through mechanism. The implementation

details of such a class driver are beyond the scope of this specification.

2.3.1 Input Terminal

The Input Terminal (IT) is used as an interface between the video function’s "outside world" and

other Units inside the video function. It serves as a receptacle for data flowing into the video

function. Its function is to represent a source of incoming data after this data has been extracted

from the data source. The data may include audio and metadata associated with a video stream.

These physical streams are grouped into a cluster of logical streams, leaving the Input Terminal

through a single Output Pin.

An Input Terminal can represent inputs to the video function other than USB OUT endpoints. A

CCD sensor on a video camera or a composite video input is an example of such a non-USB

input. However, if the video stream is entering the video function by means of a USB OUT

endpoint, there is a one-to-one relationship between that endpoint and its associated Input

Terminal. The class-specific Output Header descriptor contains a field that holds a direct

reference to this Input Terminal (see section 3.9.2.2, “Output Header Descriptor”). The Host

needs to use both the endpoint descriptors and the Input Terminal descriptor to get a full

understanding of the characteristics and capabilities of the Input Terminal. Stream-related

parameters are stored in the endpoint descriptors. Control-related parameters are stored in the

Terminal descriptor.

The symbol for the Input Terminal is depicted in the following figure.

Figure 2-1 Input Terminal Icon

outgoing data. The video data stream enters the

ints.

2.3.2 Output Terminal

The Output Terminal (OT) is used as an interface between Units inside the video function and

the "outside world". It serves as an outlet for video information, flowing out of the video

unction. Its function is to represent a sink of f

Output Terminal through a single Input Pin.

An Output Terminal can represent outputs from the video function other than USB IN endpo

A Liquid Crystal Display (LCD) screen built into a video device or a composite video out

connector are examples of such an output. However, if the video stream is leaving the video

function by means of a USB IN endpoint, there is a one-to-one relationship between that

endpoint and its associated Output Terminal. The class-specific Input Header descriptor contain

a field that holds a direct reference to this Output Terminal (see section 3.9.2.1, “Input Header

Descriptor”). The Host needs to use both the endpoint descriptors and the Output Terminal

Revision 1.1 June 1, 2005 7

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