In many audio configurations, the input is a mono source, and may be connected to only one channel of a stereo input or USB audio source. In these cases it is best to only record the channel with a signal (e.g. the left channel).
It is also useful to be able to adjust the volume level up or down at source (i.e. on the CA), to compensate for different microphone and audio hardware configurations.
The attached patch from Adam MacKenzie implements both of these. A new audio producer, ALSAMONOSRC, allows selection of one channel from a stereo source. A new configuration parameter, volume, implements the gstreamer volume element and allows volume adjustment from 0 to 10 (1= 100%, so 0.5 = 50%, 4 = 400%).
UCT has deployed and tested this on a 1.3.x production CA. Here is a sample configuration for a USB audio source (Behringer UCA222 external soundcard) with a left-channel input, volume-adjusted to 4x.