UAC规范（USB音频）
UAC 输出终端描述符
2021-02-04
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UAC输出终端描述符：Output Terminal Descriptor，简称OTD,用于向主机提供输出终端的相关信息。
UAC输出终端描述符定义-UAC1.0
typedef struct _USB_AC_OT_DESCRIPTOR
{
UINT8 bLength;
UINT8 bDescriptorType;
UINT8 bDescriptorSubtype;
UINT8 bTerminalID;
UINT16 wTerminalType;
UINT8 bAssocTerminal;
UINT8 bSourceID;
UINT8 iTerminal;
} USB_AC_OT_DESCRIPTOR;
- bLength:该描述符的结构体大小，固定为9字节长度。
- bDescriptorType：描述符类型CS_INTERFACE，值为0x24
- bDescriptorSubtype:描述符的子类型 OUTPUT_TERMINAL = 0x03.
- bTerminalID：输出终端的ID，惟一。
- wTerminalType：输出终端的类型。
| 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. |
| Output Undefined | 0x0300 | O | Output Terminal, undefined Type. |
| Speaker | 0x0301 | O | A generic speaker or set of speakers that does not fit under any of the other classifications. |
| Headphones | 0x0302 | O | A head-mounted audio output device. |
| Head Mounted Display Audio | 0x0303 | O | The audio part of a VR head mounted display. The Associated Interfaces descriptor can be used to reference the HID interface used to report the position and orientation of the HMD. |
| Desktop speaker | 0x0304 | O | Relatively small speaker or set of speakers normally placed on the desktop or integrated into the monitor. These speakers are close to the user and have limited stereo separation. |
| Room speaker | 0x0305 | O | Larger speaker or set of speakers that are heard well anywhere in the room. |
| Communication speaker | 0x0306 | O | Speaker or set of speakers designed for voice communication. |
| Low frequency effects speaker | 0x0307 | O | Speaker designed for low frequencies (subwoofer). Not capable of reproducing speech or music. |
- bAssocTerminal：关联的输入终端ID.如果不存在，设为0。
- bSourceID:该输出终端连接的单元/终端ID.
- iTerminal:该输出终端的字符串描述符索引。
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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