Excluding devices from pipewire control

In my previous blog post I described how I used pipewire to add a bluetooth dongle to the audio outputs of my computer.

Unfortunately activating pipewire had the result of it taking control of all the sound interfaces on my computer. This caused a problem with a line-in port which I use for recording audio at scheduled times. It turns out that now and then pipewire will reset the port for maximum gain, wrecking the recording.

I searched for how to exclude this port from pipewire's influence and this was the best answer. I discovered the PCI bus id of the sound card (actually onboard sound device) in question and wrote an override script just like the one described with my card's id. Unfortunately the whole device has to be disabled, not just the line-in port, but that's ok for me. I still have an HDMI port which I can connect to my amplifier instead of using the analog line-out port. So thanks to the author of that ZenLinux blog for the insight. It's a pity such a simple task has to be so complicated, maybe future wireplumber developments will make this simpler.

sox, rec, and play stopped working with ALSA?

I upgraded my openSUSE installation from 12.1 to 12.2 recently and to my chagrin, my audio recording script stopped working. I use references to ALSA devices such as hw:1. The error message was cryptic:

rec FAIL formats: can't open output file `hw:1': can not open audio device: Invalid argument

After a lot of searching, I found the answer. The base cause is that in libsox2, which openSUSE 12.2 uses instead of libsox1, has separated out the Linux specific ALSA support into a separate shared object which is not loaded unless you set the environment variable AUDIODRIVER=alsa. So this is what I had to do in my script:

export AUDIODRIVER=alsa
export AUDIODEV=hw:1
rec ... program.wav

You are supposed to be able to use -t alsa in the options of rec but I had no luck with that.

Similarly for the play command which is just a link to sox like rec.

If you are just using the default sound device, you may not hit this bug as it may fall back to pulseaudio support. It's only when you must target a specific ALSA sound card that this bug manifests itself.

Here is the bug report that was the crucial clue.