USB 24bits audiostream descriptors RRS feed

  • Question

  • I am working on a demo for an USB headset and I have a question about the audio endpoint descriptors of the USB. I have a working 8-48kHz 16bit audio stream and I can select any sample frequency in the windows properties of an audio device. I use the following descriptors:


    // Interface 2: Speaker, alternate setting 1. Type 1 format descriptor. 
    static const UsbAudioStreamingType1DescriptorType UsbIfd2Format = 
      sizeof(UsbAudioStreamingType1DescriptorType),                                       // uint8 bLength; 
      UDESC_CS_INTERFACE,                                                                 // uint8 bDescriptorType; 
      UA_FORMAT_TYPE,                                                                     // uint8 bDescriptorSubtype; 
      UA_FORMAT_TYPE_I,                                                                   // uint8 bFormatType; 
      AUDIO_LSR_NOC,                                                                      // uint8 bNrChannels; 
      AUDIO_LSR_SAMPLE_SIZE,                                                              // uint8 bSubFrameSize; 
      AUDIO_LSR_SAMPLE_SIZE << 3,                                                         // uint8 bBitResolution; 
      0x00,                                                                               // uint8 bSamFreqType; 
      (uint8)((AUDIO_LSR_MIN_SAMPLE_FREQUENCY) & 0xFF),                                   // uint8 first byte minumum sample frequency  
      (uint8)((AUDIO_LSR_MIN_SAMPLE_FREQUENCY >> 8) & 0xFF),                              // uint8 second byte minumum sample frequency                            
      (uint8)(((0x10000000 | AUDIO_LSR_MIN_SAMPLE_FREQUENCY) >> 16) & 0xFF),              // uint8 third byte minumum sample frequency  
      (uint8)((AUDIO_LSR_MAX_SAMPLE_FREQUENCY) & 0xFF),                                   // uint8 first byte maximum sample frequency  
      (uint8)((AUDIO_LSR_MAX_SAMPLE_FREQUENCY >> 8) & 0xFF),                              // uint8 second byte maximum sample frequency  
      (uint8)(((0x10000000 | AUDIO_LSR_MAX_SAMPLE_FREQUENCY) >> 16) & 0xFF),              // uint8 third byte maximum sample frequency  
    // Interface 2: Speaker, alternate setting 1. Audio endpoint descriptor. 
    static const UsbAudioEndpointDescriptorType UsbIfd2StdEndpoint = 
      sizeof(UsbAudioEndpointDescriptorType),                                             // uint8 bLength; 
      USB_DT_ENDPOINT,                                                                    // uint8 bDescriptorType; 
      USB_DIR_OUT | USB_EP_AUDIO_RX,                                                      // uint8 bEndpointAddress; 
      0x01,                                                                               // uint8 bmAttributes; 
      ((AUDIO_LSR_MAX_SAMPLE_FREQUENCY / 1000) * AUDIO_LSR_SAMPLE_SIZE) * AUDIO_LSR_NOC,  // uint16 wMaxPacketSize; 
      0x01,                                                                               // uint8 bInterval; 
      0x00,                                                                               // uint8 bRefresh; 
      0x00,                                                                               // uint8 bSynchAddress; 

    With these settings:

    // Sample frequencies 
    #define AUDIO_LSR_MIN_SAMPLE_FREQUENCY 0x01F40  // 8kHz 
    #define AUDIO_LSR_MAX_SAMPLE_FREQUENCY 0x0BB80  // 48kHz 
    // Sample size 
    #define AUDIO_LSR_SAMPLE_SIZE 0x02 // in bytes 
    // Defines for mono/stereo. 
    #define AUDIO_LSR_NOC 0x02 // Number Of Channels (stereo)

    Now I want to change it to a 8-48kHz 24bit stream. By changing the AUDIO_LSR_SAMPLE_SIZE to 0x03. When I do this the sample frequency selection box in the windows properties window is grayed out at 48kHz so I cannot select any other frequency. When I play something to the device the stream is 24bit though. I already tried uninstalling the drivers but this does not change anything.

    Does anyone encounter this problem as well or does anyone have an idea what might cause this problem?

    Friday, December 13, 2013 9:54 AM


  • It turned out that the problem was not in these descriptors but in the Input Terminal Descriptor where the channel configuration was wrong.
    Wednesday, January 22, 2014 11:54 AM