USB AUDIO_INTERFACE explanations please!

[sonaben]

Hello,

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:

#ifdef AUDIO_INTERFACE
	// 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 https://github.com/PaulStoffregen/co...sy4/usb_desc.c)

- 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!

Ben

[PaulStoffregen]

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.

https://www.usb.org/sites/default/files/audio10.pdf

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.