4/6 Multi-Channel Playback
The recent CM8738 chips support for the 4/6 multi-channel playback function. This is useful especially for AC3 decoding.
When the multi-channel is supported, the driver name has a suffix "-MC" such like "CMI8738-MC6". You can check this name from /proc/asound/cards.
When the 4/6-ch output is enabled, the front DAC accepts up to 6 (or 4) channels. This is different from the dual DACs described in the previous section. While the dual DAC supports two different rates or formats, the 4/6-ch playback supports only the same condition for all channels.
For using 4/6 channel playback, you need to specify the PCM channels as you like and set the format S16LE. For example, for playback with 4 channels,
snd_pcm_hw_params_set_access(pcm, hw, SND_PCM_ACCESS_RW_INTERLEAVED);
// or mmap if you like
snd_pcm_hw_params_set_format(pcm, hw, SND_PCM_FORMAT_S16_LE);
snd_pcm_hw_params_set_channels(pcm, hw, 4);
and use the interleaved 4 channel data.
There is a control switch, "Line-In As Bass". As you can imagine from its name, the line-in jack is used for the bass (5th and 6th channels) output.