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

Robert T. Drury rtdrury at earthlink.net
Thu Apr 9 23:34:14 CEST 2009

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- (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 

In the kernel source: linux-

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?

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