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；