A while back I bought one of those $10 tiny class D amplifiers that can drive my bookshelf speakers with up to 30W (depending on power supply voltage, between 12V and 24V). An old laptop supply supplies this. It can be fed from a 3.5 audio socket or from bluetooth. Works like a charm. May not be audiophile but good enough for me.
I've been feeding it from an older mobile phone that mounts a SMB share from my workhorse computer. But as I would like to automate it so that I can use music to wake me up, I bought a bluetooth USB dongle from one of many AliExpress sellers. It contains a Realtek chipset that I checked is supported in Linux and supports BT 5.3 profile.
How to set the dongle up? Plugging it in elicited messages in the syslog, as dmesg showed. I used the KDE desktop widget (probably bluedevil) to connect to it and command it to pair and connect to amplifier. A prerequisite is that bluetoothd should be running. I discoved that the configuration can also be done from the CLI using bluez-tools. But once discovered and paired, nothing more needs to be done on this front as the setup is static.
How to send audio to it? I thought there might be a PCM device under /dev like for sound cards. But no. It seems that you need a service like pulseaudio to drive it. So I got stuck into pulseaudio documentation. Along the way I discovered that my distro prefers pipewire. So I installed the pipewire packages and this also obsoleted the corresponding pulseaudio packages. One of the packages pipewire-pulseaudio supplies compatible functionality.
Pipewire is a process that should keep running to handle all this. How to fire it off? It turns out that it's preferable to run it with user permissions, using the user instance of systemd, e.g.
systemctl --user start pipewire.service
How to make sure it runs when the user logs in? You activate the systemd units like this:
systemctl --user enable pipewire.service pipewire.socket
Then start both the service and the socket the first time.
How to send audio to pipewire? I struggled with this under pulseaudio, it wouldn't show up in players like vlc or amarok. But once I switched to pipewire, the bluetooth audio sink was just there and I could control the volume and make it the default audio playback.
How to specify this device to mplayer so that I can play from a cron job? I just had to specify it to the -ao option.
mplayer -ao pulse audio.mp3
Notice that mplayer sees pipewire the same as pulseaudio. I can put this in a profile in .mplayer/config named after the amplifier ID:
[xinyi]
ao=pulse
then I can specify:
mplayer -profile xinyi audio.mp3
The advantage is that I can specify other settings in the profile.
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