Trying to configure mono (one channel) output on emu10k1 hardware using the alsa version that comes with kernel 3.9 is broken. Instead of expecting mono samples, the alsa callback expects twice as many stereo samples. Not getting the required number of samples then causes a buffer underrun, and sound gets interrupted or messed up. In fact, what the audio hardware does is that it plays not only the buffer, but also the data "behind the buffer", causing wierd noises.

The channel count is setup as

channels = 1;

snd_pcm_hw_params_malloc(&HWParms);

snd_pcm_open(&SoundStream,CardName,SND_PCM_STREAM_PLAYBACK,SND_PCM_NONBLOCK | SND_PCM_ASYNC);

snd_pcm_hw_params_set_channels_min(SoundStream,HWParms,&channels);

snd_pcm_hw_params_set_channels_max(SoundStream,HWParms,&channels);

snd_pcm_hw_params_set_channels_first(SoundStream,HWParms,&channels);

plus error checking. Then, a bit later, the number of configured channels is verified:

snd_pcm_hw_params_get_channels(HWParms,&channels);

channels is at this point still 1, i.e. the driver accepted the mono output, but does then not play correctly.

Note that the same call sequence works for stereo, and it also works for mono for other underlying hardware drivers, i.e. mono output works on snd-hda-intel, for example. It does not on emu10k1.