Trying to understand the relationship between Bluetooth headphones, virtual_oss, and the mixer

[arnelson]

I recently set up a FreeBSD desktop, and I've managed to get Bluetooth headphones working using the following command:

virtual_oss -T /dev/sndstat -C 2 -c 2 -r 48000 -b 16 -s 20ms -P /dev/bluetooth/headphones -R /dev/null -w vdsp.ctl -d dsp -l mixer

Admittedly, I have only a cursory understanding of what that does. I'm more used to how Linux does things, and I don't understand OSS very well yet. This setup works for audio, and I can adjust the headphones' volume with their built-in volume keys. But the Bluetooth headphones don't seem to exist in any audio mixer or settings, and I can't adjust them with the volume keys on my keyboard or with the MATE desktop's sound settings GUI.

I can list my audio devices from /dev/sndstat:

Installed devices:
pcm0: <ATI R6xx (HDMI)> (play) default
pcm1: <ATI R6xx (HDMI)> (play)
pcm2: <ATI R6xx (HDMI)> (play)
pcm3: <ATI R6xx (HDMI)> (play)
pcm4: <ATI R6xx (HDMI)> (play)
pcm5: <ATI R6xx (HDMI)> (play)
pcm6: <Realtek ALCS1200A (Rear Analog 5.1/2.0)> (play/rec)
pcm7: <Realtek ALCS1200A (Front Analog)> (play/rec)
pcm8: <Realtek ALCS1200A (Rear Digital)> (play)
Installed devices from userspace:
dsp: <Virtual OSS> (play/rec)
mixer: <Virtual OSS> (play/rec)

dsp and mixer were created by virtual_oss, and they appear in their own category. But they don't appear anywhere else. They're not listed in the GUI sound settings, and I can't select dsp as my main audio device. It's not a "PCM device". I've seen that term come up repeatedly when googling this and in other forum threads, but I haven't found a good explanation of the difference between PCM devices and these virtual devices, or why /dev/bluetooth/<addr>, which doesn't even exist as a file, isn't a PCM device.

I can start YouTube videos in Firefox (after setting the necessary about:config flag to make it use OSS) and they will play on the Bluetooth headphones. But MATE claims that my selected audio device is my monitor, and its volume controls adjust the monitor volume. mixer also thinks the monitor is the current audio device:

pcm0:mixer: <ATI R6xx (HDMI)> on hdaa0 (play) (default)
    vol       = 1.00:1.00     pbk
    pcm       = 1.00:1.00     pbk

So the Bluetooth headphones are selected, but nothing in the system thinks they're selected... even if it works, this is a very strange, buggy state for the system to be in. Surely this isn't the only way to do Bluetooth audio?

I only partially understand what -d dsp is doing in the virtual_oss command, but it's probably related. I know /dev/dsp is the main OSS audio device, and writing to it will output sound. So virtual_oss has... replaced it with its own copy, rendering mixer's output meaningless? Or is something else going on?

The only GUI control for virtual_oss I'm aware of is virtual_oss_ctl, which is equally confusing. It has VOL+ and VOL- keys, but they don't seem to do anything. I can't find any way to adjust the Bluetooth headphones' volume from any command-line or GUI program.

Am I missing something, or is this just the state of Bluetooth audio in FreeBSD? I'm open to writing my own GUI to handle volume control for Bluetooth devices, but, if that's necessary, where should I start looking to figure out how to interact with virtual_oss?

 

[richardtoohey2]

I can't answer your questions, but sure the people who can help will be asking - which version of FreeBSD?

14.1 has just been released and that has some audio changes, but don't know if relevent to anything you are asking about:

FreeBSD 14.1-RELEASE Release Notes

FreeBSD is an operating system used to power modern servers, desktops, and embedded platforms.

www.freebsd.org

The snd_clone framework has been removed from the sound subsystem, including related sysctls, simplifying the system. The per-channel nodes (/dev/dspX.Y) are no longer created, just the primary device (/dev/dspX). e6c51f6db8d7 (Sponsored by The FreeBSD Foundation)

Audio now supports asynchronous device detach. This greatly simplifies hot plugging and unplugging of things such as USB headsets, and eases use of PulseAudio in cases that require operating system sleep and wake (suspend and resume). d692c314d29a (Sponsored by The FreeBSD Foundation)

 

[arnelson]

Ah, yes, sorry about that. It's 14.1.

 

[Tieks]

I know /dev/dsp is the main OSS audio device, and writing to it will output sound. So virtual_oss has... replaced it with its own copy, rendering mixer's output meaningless?

Device /dev/dsp is considered the default audio device. In your case virtual_oss indeed overrides that with -d dsp. Try setting it to something non-existent like -d dsp9 and tell your musicplayer to send its output to dsp9.