Maximize master volume

[Chinry]

Implementing #10

When AlsaWriter::open() is run a call to a volume setting function is made. Storing the previous state of the mixer element volume is nontrivial because it can have 9 different channels shown here. I see two options. One option is to assume that there is only mono or stereo. The other would be to store values in a container by getting the channels present. Do you think this is worth pursuing?

[gavv]

Will look into it in upcoming days, thanks!

[gavv]

Thanks for PR, looks great!

Here are my thoughts:

1. I think yes, it makes sense to save per-channel volume and then restore it. It would be annoying if running signal-estimator would always reset sound card volume.

   For example, in some of my tests stands, I often switch between using signal-estimator and playing sound using aplay, on the same card. On some sound cards, I need to set volume to non-default value to use aplay (e.g. on one of my sound card default volume or 100% volume is too loud). It would be quite inconvenient if after each time I run signal-estimator I'll need to open alsamixer and reconfigure volume. I imagine that others may have similar use cases and I'm not the only one :)

2. I guess we could iterate channels numbers from 0 to SND_MIXER_SCHN_LAST-1 and store volumes in container, as you suggested, then restore it.

3. Hopefully this logic will work for most users, however I think we need to provide options to deal with corner cases. I suggest to add two CLI options:

   • --hw-volume FLOAT - override which volume to set for ALSA; default value is 1.0 (which means 100%); float is used for consistency with --volume
   • --no-hw-volume - disable touching ALSA volume

4. Could you please add logging of errors for calls like snd_mixer_attach() and others?

5. Do you know if "Master" element is always present? Maybe we need to add a CLI option for element name as well?

[gavv]

Forgot to mention, assuming mono or stereo is not great because we allow to specify arbitrary channel count in CLI and mostly are not bound to number of channels.

[Chinry]

Okay, will take a look at implementing these changes.

[gavv]

Thanks! FYI: c5ea5ac

[gavv]

Update: since we now have separate options for output and input devices (--out-latency/--in-latency, --out-periods/--in-periods), I think it makes sense to make separate options for volume too. E.g. we can have three options: --out-volume, --in-volume, --no-volume.