plietar
ALSA: support selection of ALSA mixer - for (hardware-based) volume control
Some DAC cards support hardware-based volume control. Example: HiFiBerry DAC+ pro on Raspberry. Using librespot (v20161230-7fd8503) from: https://github.com/herrernst/librespot/releases
On raspberry pi + OSMC (raspbian) + above mentioned DAC. Works very well out of the box.
Running the client with:
/home/osmc/librespot/librespot --name Raspberry --cache /tmp --bitrate 320 --backend alsa --device hw:0
I notice using alsamixer and librespot in verbose mode that changing that volume changes in the client causes some events in librespot,
DEBUG:librespot::spirc: kMessageTypeVolume ... ...
the audio volume does change, increase/decrease, however alsamixer doesn't show any actual mixer volume change at all in any of its mixers, which seems to indicate that the volume change is done by librespot itself as a "software volume change". Higher end DACs can control volume via hardware which is preferable. In the case of the HiFiBerry DAC pro, the Alsa mixer is called "Digital" (shows up as one of the mixers displayed by the "alsamixer" tool)
When using spotify-connect-web, running it as:
spotify-connect-web --key /home/your-user/your-key.key --name Raspberry --bitrate 320 --playback_device hw:0 --mixer Digital
...any volume change in the spotify client causes the alsamixer "Digital" mixer to reflect the change (value between 100 and 0), which indicates that the volume change is being done via hardware. Note that spotify-connect-web also supports "software-based volume control" if desired
This is definitely a frequent request (#128 and #75). Originally, librespot only supported PortAudio, which doesn't have any sort of volume control.
@joerg-krause had an implementation for ALSA at some point. It would need to be adjusted to at least fallback to software volume control for other backends
Noticed that today's binary release for raspberry by @herrernst supports a "--mixer" option! Tried it & librespot crashed like so:
/home/osmc/librespot# /home/osmc/librespot/librespot --name Raspberry --cache /tmp --bitrate 320 --backend alsa --device hw:0 --mixer Digital INFO:librespot: librespot 6f1b785 (2017-03-26). Built on 2017-03-26. thread 'main' panicked at 'Invalid mixer', /buildslave/rust-buildbot/slave/stable-dist-rustc-cross-host-linux/build/src/libcore/option.rs:715 note: Run with
RUST_BACKTRACE=1for a backtrace.
Amixer shows that the "Digital" mixer exists:
... Simple mixer control 'Digital',0 Capabilities: pvolume pswitch Playback channels: Front Left - Front Right Limits: Playback 0 - 207 Mono: Front Left: Playback 207 [100%] [0.00dB] [on] Front Right: Playback 207 [100%] [0.00dB] [on] ...
Tried with other mixers shown by alsamixer & amixer, and they all crash librespot
Wondering if the --mixer command line switch is not yet ready for use, or if it's meant to be used differently
It's meant to use different. For now, there is only one mixer supported: softmixer. --mixer is not the ALSA mixer device name.
