Some tech documentation and snippets, finally organized.

X11 screencast (with audio)

Screenrecording of a 1024x768 pixel area, with an x/y-offset of 80/120px. Sound is recorded at a bitrate of 128kb/s from the analog input of the default ALSA sound card, the outline of the captured region is displayed on screen:

avconv -f x11grab -show_region 1 \
       -s 1024x768 -i :0.0+80,120 \
       -f alsa -i hw:0 \
       -b:v 1200k -b:a 128k \
       -y ~/screencast.mpg

Don't forget to check the mixer settings!

Recording sound from another application instead of an external source requires to modprobe the snd_aloop kernel module and a line of ALSA configuration:

### File: ~/.asoundrc

pcm.!default { type plug slave.pcm "hw:Loopback,0,0" }

Additionally, the avconv audio input parameters need to be changed to

-f alsa -i hw:Loopback,1,0

The configuration of an optional output channel for audio monitoring is described here at