Hi folks,
I just spent a while figuring out how to solve a particular need and want to save it in the mailing list for posterity.
I'm an amateur radio operator, and we make extensive use of external sound cards for interfacing computers with radios for digital communication modes. I was experiencing a problem where I really wanted to record received audio (RX) to the left channel of a file, and transmitted audio (TX) to the right channel. Doing so would let me hear exactly what the modem program heard and sent.
After much fussing, I found an arrangement that worked: set up a null-sink to represent the rx-tx stream I wanted, then use two loopbacks, one each for RX and TX. Once that was set up, recording the file was simple.
# BurrBrown chipset used in SignaLink USB
SOUND_CARD="BurrBrown"
RADIO_RX=$(pacmd list-sources | egrep "input.*$SOUND_CARD" | grep -oP "<\K[^ >]+")
RADIO_TX=$(pacmd list-sources | egrep "output.*$SOUND_CARD.*monitor" | grep -oP "<\K[^ >]+")
RADIO_TX=$(pacmd list-sources | egrep "output.*$SOUND_CARD.*monitor" | grep -oP "<\K[^ >]+")
pactl load-module module-null-sink sink_name=rxtx sink_properties=device.description=RXTX
pactl load-module module-loopback source=$RADIO_RX sink=rxtx channels=1 channel_map=left
pactl load-module module-loopback source=$RADIO_TX sink=rxtx channels=1 channel_map=right
Hopefully this will help someone in the future looking to record both the input and output to a sound card!
Chris Keller, K0SWE
