This page describes the modifications and setup required to get a linux box (a Raspberry Pi) working as an aprs iGate.
Alsa is used for the sound system. To check the proper input levels use the command:
arecord -Dhw:0 -c2 -d 0 -fdat /dev/null -vvv
This will show a rolling output of the recorded sound levels. More alsa options: alsa
Alsamixer and aumix can both set the sound levels and the following settings are near optimal for me:
PCM Capture: Line
Line capture input level: 70
Everything else in alsamixer can be set to mute
Btw, if F4 doesn't toggle the recording view, try the TAB key
Recording device: Line
Pcm level: 100%
All other levels (including line, as it's line-out) can be set to zero
test soundmodem output
a program called netcat (or ncat) can be used to send single character to the soundmodem ip address (if using MKISS).
netcat -u 18.104.22.168 5000
when issuing the above command user input is expected. Simply type a letter, followed by <ENTER> and the soundmodem should start making AX.25 noises.