Usb Audio Device Descriptor(8) Type I Format Type Descriptor

Type I Format Type Descriptor

struct usb_format_type_1
{
U8 bLength; /*Size of this descriptor, in bytes: 8+(ns*3)*/
U8 bDescriptorType; /*CS_INTERFACE descriptor type*/
U8 bDescriptorSubtype; /*FORMAT_TYPE descriptor subtype.*/
U8 bFormatType; /*FORMAT_TYPE_I. Constant identifying
the Format Type the AudioStreaming
interface is using.*/
U8 bNrChannels; /*Indicates the number of physical
channels in the audio data stream.*/
U8 bSubframeSize; /*The number of bytes occupied by one
audio subframe. Can be 1, 2, 3 or 4.*/
U8 bBitResolution; /*The number of effectively used bits from
the available bits in an audio subframe.*/
U8 bSamFreqType; /*Indicates how the sampling frequency
can be programmed:
0: Continuous sampling frequency
1..255: The number of discrete sampling
frequencies supported by the
isochronous data endpoint of the
AudioStreaming interface (ns)*/
U8 tLowerSamFreq[3]; /*Lower bound in Hz of the sampling
frequency range for this isochronous data
endpoint.*/
U8 tUpperSamFreq[3]; /*Upper bound in Hz of the sampling
frequency range for this isochronous data
endpoint.*/
};

bLength = 8+(ns*3); 当为continous frequecy 时, ns = 2;

bDescriptorType = 0x24; class-specific interface;

bDescriptorSubtype = 0x02; 同input terminal->bDescriptorSubtype, 此时要通过bLength来区别，input terminal->bLength = 12;

bFormatType = 0x01; FORMAT_TYPE_I;

bNrChannels: audio streaming 物理声道数；

bSubframeSize: 对于一个subframe, 需要多少bytes；

bBitResolution: bit resolution，即多少位的pcm数据；

bSamFreqType: 0 表示 continuous sampling frequecy; 1…ns(max=255)表示离散的sampling freqency；

tLowSamFreq/tUpperSamFreq: Sampling Frequency的高低位；