Avoid Claiming Audio Interfaces: Use Interface 4 for Control

[schlupmann]

Thank you very much for initiating this project.

You may already be aware of this, but there is no need to claim the audio interfaces or disconnect the audio streams. To enable concurrent operation between the Audio Stream (ALSA/PipeWire) and MixiD, control traffic should be routed through the dedicated Interface 4, leaving the audio interfaces untouched :
* Formula: wIndex = Unit_ID | 0x0004
* Example: Matrix Mixer Unit 0x3c00 becomes 0x3c04.
* Example: Monitor Unit 0x3600 becomes 0x3604.

The usb interface layout is something like this :
| Interface | Class | Driver | Usage |
| 0 | Audio Control | snd-usb-audio | OS Audio Configuration |
| 1 | Audio Stream | snd-usb-audio | PCM Playback/Capture |
| 2 | Audio Stream | snd-usb-audio | PCM Playback/Capture |
| 3 | HID | usbhid | Hardware Buttons / Scroll Control |
| 4 | Vendor Specific | None | DSP Control (MixiD Target) |

This works very nicely on my audient id14 ;-)...

[TheOnlyJoey]

Thanks for the report,

I have played around with this in the past but had some varying results, hence going for the claim method that worked consistently for me.
Will give it another shot soon after i round up my current set of patches.

[schlupmann]

Targetting with wIndexes ending in 04 works flawlessly for me and that is most certainly the way the original software is controlling the device without interfering with the audio stream...

Attached find my full notes on the iD 14 protocol if that helps.

audient_id14_protocol.md

[TheOnlyJoey]

Thanks for the notes!
Documentation is still on my todo list, so this helps filling in the gaps in my own notes.

I am aware of the control interface (my initial implementation used that), but as mentioned, I had some inconsistent results with it before (some distro's not being able to communicate at all without disconnecting from the kernel interface, or inconsistent behavior). I will try to revisit this after finishing my current set of patches.

[schlupmann]

I’ve sent you an invite to the repo with my expanded, somewhat messy although fully working version of MixiD (on debian 13). I kept it private since it needs cleanup and may have diverged from your goals, but hopefully there are pieces you can use.

[TheOnlyJoey]

It seems like you have been busy!
I can use some things from your branch for sure, overlaps with some of the work I have already been doing.
Thanks!

Feel free to always propose PR's ofc!