UAC规范（USB音频）
UAC 输入终端描述符
2021-02-04
5540
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UAC输入终端描述符全称：Input Terminal Descriptor，简称ITD.
输入终端描述符用于向主机提供输入终端的相关信息，这个概念和UVC摄像头的输入终端描述符类似。
UAC输入终端描述符定义-UAC1.0
typedef struct _USB_AC_IT_DESCRIPTOR
{
UINT8 bLength;
UINT8 bDescriptorType;
UINT8 bDescriptorSubtype;
UINT8 bTerminalID;
UINT16 wTerminalType;
UINT8 bAssocTerminal;
UINT8 bNrChannels;
UINT16 wChannelConfig;
UINT8 iChannelNames;
UINT8 iTerminal;
} USB_AC_IT_DESCRIPTOR;
- bLength:该描述符的结构体大小，固定为12字节长度。
- bDescriptorType：描述符类型,描述符类型CS_INTERFACE，值为0x24
- bDescriptorSubtype:描述符的子类型 INPUT_TERMINAL = 0x02.
- bTerminalID：输入终端的ID，惟一。
- wTerminalType：输入终端的类型。
| Terminal Type | Code | I/O | Description |
|---|---|---|---|
| USB Undefined | 0x0100 | I/O | USB Terminal, undefined Type. |
| USB streaming | 0x0101 | I/O | A Terminal dealing with a signal carried over an endpoint in an AudioStreaming interface. The AudioStreaming interface descriptor points to the associated Terminal through the bTerminalLink field. |
| USB vendor specific | 0x01FF | I/O | A Terminal dealing with a signal carried over a vendor-specific interface. The vendor-specific interface descriptor must contain a field that references the Terminal. |
| Input Undefined | 0x0200 | I | Input Terminal, undefined Type. |
| Microphone | 0x0201 | I | A generic microphone that does not fit under any of the other classifications. |
| Desktop microphone | 0x0202 | I | A microphone normally placed on the desktop or integrated into the monitor. |
| Personal microphone | 0x0203 | I | A head-mounted or clip-on microphone. |
| Omni-directional microphone | 0x0204 | I | A microphone designed to pick up voice from more than one speaker at relatively long ranges. |
| Microphone array | 0x0205 | I | An array of microphones designed for directional processing using host-based signal processing algorithms. |
| Processing microphone array | 0x0206 | I | An array of microphones with an embedded signal processor. |
更多详见：http://www.usbzh.com/article/detail-802.html 或官方文档 https://www.usb.org/sites/default/files/termt10.pdf
- bAssocTerminal：关联的输出终端ID.
- bNrChannels: 音频输出集的逻辑通道个数，详见音频通道集描述符
- wChannelConfig：音频通道的位置标识，详见音频通道集描述符
- iChannelNames：未预设的通道名字符串起始索引，详见音频通道集描述符
- iTerminal： 该输入终端的字符串索引。
Audio Class-Specific AC Interface Descriptor Subtypes
| Descriptor Subtype | Value |
|---|---|
| AC_DESCRIPTOR_UNDEFINED | 0x00 |
| HEADER | 0x01 |
| INPUT_TERMINAL | 0x02 |
| OUTPUT_TERMINAL | 0x03 |
| MIXER_UNIT | 0x04 |
| SELECTOR_UNIT | 0x05 |
| FEATURE_UNIT | 0x06 |
| PROCESSING_UNIT | 0x07 |
| EXTENSION_UNIT | 0x08 |
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