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Thread: USB AUDIO_INTERFACE explanations please!

  1. #1
    Junior Member
    Join Date
    Feb 2021

    USB AUDIO_INTERFACE explanations please!


    I'm working with a Teensy 4.0 with the USB as AUDIO (USB_AUDIO in usb_desc.h) trying to update the descriptors (final goal to increase the number of output channels), and I'm struggling to understand how the AUDIO_INTERFACE is configured and structured in usb_desc.c:

    	// configuration for 480 Mbit/sec speed
            // interface association descriptor, USB ECN, Table 9-Z
            8,                                      // bLength
            11,                                     // bDescriptorType
            AUDIO_INTERFACE,                        // bFirstInterface
            3,                                      // bInterfaceCount
            0x01,                                   // bFunctionClass
            0x01,                                   // bFunctionSubClass
            0x00,                                   // bFunctionProtocol
            0,                                      // iFunction
    (full code at

    - Why do we have 3 audio interfaces? Could you please describe each of them?
    - Why do we have 2 "Input Terminal Descriptor" and 3 "Output Terminal Descriptor"? How do they relate to each interface?
    - What is the logic in the succession of items?

    I managed to update bNrChannels on line 1574 for "Type I Format Descriptor":

    // Type I Format Descriptor
    	// USB DCD for Audio Data Formats 1.0, Section 2.2.5, Table 2-1, page 10
    	11,					// bLength
    	0x24,					// bDescriptorType = CS_INTERFACE
    	2,					// bDescriptorSubtype = FORMAT_TYPE
    	1,					// bFormatType = FORMAT_TYPE_I
    	4,					// bNrChannels: WAS 2 I UPDATED TO 4
    	2,					// bSubFrameSize = 2 byte
    	16,					// bBitResolution = 16 bits
    	1,					// bSamFreqType = 1 frequency
    	LSB(44100), MSB(44100), 0,		// tSamFreq
    Some softwares on Windows recognize the update and I can play 4 tracks (eg Reaper), Windows can also manage to play stereo, but other softwares like VLC crash probably because they cannot understand the descriptor. (got some logs from VLC: "directsound debug: Windows speaker config: Unknown and stream has 2 channels, using 2 channels").

    I tried also to update the "wChannelConfig" param to 0x00FF which seems to be 7.1 (instead of 0x0003 = Left & Right Front) but this doesn't make a difference.

    Also if I've compared the usb descriptor with another 8 tracks audio interface I have (using Thesycon USB descriptor dumper) and it is much simpler than the Teensy one.

    Just understanding the main logic around the descriptors for Teensy would help a lot, thanks!


  2. #2
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    All this stuff comes from the USB audio class specification. Sorry, I don't have time to write a lengthy explanation, but here is a direct link to the PDF spec.

    It's lengthy, like most of these specs, but a lot of it is long lists of optional stuff. The key to reading it is skipping over all that stuff we're not using and just focus on the important parts.

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