Creating UCM2 profiles for devices to split multichannel streams

[brndd]

Hey. Apologies for this question-issue, but I wasn't sure where to ask and figured it was better to do it here in public (so that others may benefit from the discussion) than ask by email or something.

I randomly stumbled upon #121 which is about creating a UCM2 profile for the TC-Helicon GoXLR device to split some multichannel streams to better match the physical layout of the device.

I had no idea this was possible with UCM2 (in fact I looked at UCM2 for this a few months ago and got the impression that it was not possible), and would be interested in creating a similar profile for my device, a Behringer UMC204HD. This device has a similar problem of reporting a stereo input stream when the physical layout is two mono inputs, and a 4-channel output stream when the physical layout is two stereo outputs. Thus far I've been using PulseAudio/PipeWire remappings to get around the issue, but a UCM2 profile would be a far superior solution, if indeed it is possible.

So, my questions are:

• Is being able to split streams like this a new feature of ALSA/UCM2? How do I know if my system (Fedora 34) supports this feature?
• Is there documentation somewhere to get me started with writing a config for my device? I have no idea what half the stuff in 97beee6 means so I'm expecting to fall into some pits during the process ;)

[perexg]

• Is being able to split streams like this a new feature of ALSA/UCM2? How do I know if my system (Fedora 34) supports this feature?

The 1.2.5 has support for the private alsa library configuration. The previous UCM may rely on the global alsa-lib configuration. It's nothing new, but the pieces are improved to describe the special hardware cases.

• Is there documentation somewhere to get me started with writing a config for my device? I have no idea what half the stuff in 97beee6 means so I'm expecting to fall into some pits during the process ;)

I'm working slowly on a comprehensive UCM2 documentation (interrupted by other things), it will be part of the alsa-lib documentation since 1.2.6 (https://www.alsa-project.org/alsa-doc/alsa-lib/) - it's not released yet.

The used alsa-lib PCM plugins are documented here: https://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html

The alsa-lib PCM devices referred by the GoXLR UCM configurations are defined using the standard alsa-lib configuration syntax (LibraryConfig blocks).

[perexg]

The split is supported since a3cedf9 commit. Closing now.