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:
(full code at https://github.com/PaulStoffregen/cores/blob/master/teensy4/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":
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
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:
Code:
#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/cores/blob/master/teensy4/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":
Code:
// 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, // tSamFreqSome 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
