playback: fall back to raw-range mapping for Alsa mixers without dB info - #1731
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playback: fall back to raw-range mapping for Alsa mixers without dB info#1731qianc wants to merge 1 commit into
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Some Alsa mixer controls expose no dB information at all, most notably the Alsa pulse plugin's `Master`, which maps onto the PulseAudio sink volume. AlsaMixer::open() failed with "Could not get Alsa softvol dB range" for such controls, making them unusable as `--mixer alsa` targets. Instead of failing, map the volume linearly onto the control's raw range and force `VolumeCtrl::Linear`, since without dB information other taper curves cannot be applied faithfully. The softvol antilog compensation is also skipped for these controls: it counteracts Alsa's internal linear-to-dB conversion, which does not happen when the raw volume is applied directly. For PulseAudio a linear raw mapping is the desired behavior anyway, as its raw volume scale is already perceptually (cubically) tapered. This allows `--mixer alsa --alsa-mixer-device pulse --alsa-mixer-control Master` to control the PulseAudio sink (master) volume from Spotify Connect while playing back through the pulseaudio backend. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Some Alsa mixer controls expose no dB information at all — most notably the Alsa pulse plugin's
Master, which maps onto the PulseAudio sink volume.AlsaMixer::open()currently fails hard for such controls:making them unusable as
--mixer alsatargets.What this PR does
When
snd_ctl_get_dB_rangereports that no dB information is available, fall back to mapping the volume linearly onto the control's raw volume range instead of erroring out:VolumeCtrlis forced toLinear, since without dB information other taper curves cannot be applied faithfully.For PulseAudio specifically, a linear raw mapping is the desired behavior: PA's raw volume scale is already perceptually (cubically) tapered (e.g. raw 50% = −18.06 dB), so the Connect volume slider tracks the sink volume 1:1.
Motivation / use case
Running librespot with
--backend pulseaudioon a headless device (raspotify on a Pi 4 with an IQaudIO DAC), where the DAC's hardwareDigitalcontrol spans −103.5..0 dB — too wide a range for a usable taper, and separate from the PulseAudio sink volume that other audio sources go through. With this change,lets Spotify Connect control the PulseAudio sink (master) volume directly, affecting all streams on the sink with a sensible taper.
Testing
Alsa mixer control has no dB information ..., falling back to linear mapping on the raw volume range), the Connect volume slider tracks the PulseAudio sink volume (pactl/alsamixer -D pulseMaster) ~1:1 in both directions, and playback through the pulseaudio backend is unaffected.🤖 Generated with Claude Code