[alsa-devel] oss emul: powerbook5,3 microphone to mono

[Robert T. Drury]

On machine powerbook5,3 the built-in microphone is attached to 
the right channel of the sound chip.  In the alsa oss emulation 
in linux-2.6.23.9 (alsa-*-1.0.12) the plugin layer copies the 
left input channel to a one-channel buffer when  calling ioctl 
SNDCTL_DSP_CHANNELS with one channel.  So no mic to mono on the 
powerbook5,3.

In the kernel source: linux-2.6.23.9/sound/core/oss/pcm_plugin.c:

The function snd_pcm_plug_read_transfer copies from the two 
channel hardware input buffer to the one channel user space 
buffer.   The left source channel appears to be selected for 
copy in function snd_pcm_plug_format_plugins.  One approach 
is to query line/mic input there for powerbook5,3 and select 
right channel source for copy.  Any ideas?