How can I switch my sound output between mono and stereo in an app using the ALSA API?

Question

There are possibilities to do this in the asoundrc config file, but I need an application-level solution without the need to restart the app. So far, I've had this line of code in the init part of my app:

snd_pcm_hw_params_set_channels (cd::output_handle, cd::output_params, (unsigned int) 2);

The audio data stream comes from the CD drive, so that it always features 2 channels (stereo). Can I simply set the channel count to 1 on the fly? Thank you.

Answer 1

You can change the hw parameters only when the PCM stream is not running. See: https://www.alsa-project.org/alsa-doc/alsa-lib/pcm.html. You need to fill first the snd_pcm_hw_params_t structure with the desired configuration, and then apply this configuration with snd_pcm_hw_params(). For instance:

int err;
snd_pcm_t *handle;
snd_pcm_hw_params_t *params;
snd_pcm_hw_params_alloca(&params);

err = snd_pcm_open(&handle, "default" //, ...
//...
err = snd_pcm_hw_params_any(handle, params);
//...

if((err = snd_pcm_hw_params_set_channels(handle, params, 2)) == 0) {
    err = snd_pcm_hw_params(handle, params);
}