Raspberry Pi: Downmixing From Stereo to Mono Sound Output

First, let’s choose which device to use. Enter the command cat /proc/asound/modules to list all available sound devices whether it’s a PWM audio or USB sound device.

The command you just entered should output something like:

0 snd_bcm2835 << this is the PWM audio I configured on GPIO18 & GPIO13.
1 snd_usb_audio << this is my USB sound device.

Enter the command sudo nano /etc/asound.conf and paste the following code:

# convert stereo to mono LEFT output

pcm.monocard{
  slave.pcm "hw:N"
  slave.channels 2
  type route
  ttable {
    # Copy both input channels to output channel 0 (Left).
    0.0 1
    1.0 1
    # Send nothing to output channel 1 (Right).
    0.1 0
    1.1 0
  }
}

pcm.!default monocard

Change the N on the line slave.pcm "hw:N" to  the number that corresponds to the audio device you want to use. I will be using my USB audio device so mine should read slave.pcm "hw:1".

The above code uses both left and right audio input to output to LEFT channel. If you want the output to the RIGHT then you can just edit the line 8 – 14 to:

# Copy both input channels to output channel 1 (Right).
0.1 1
1.1 1
# Send nothing to output channel 0 (Left).
0.0 0
1.0 0

Restart ALSA by sudo /etc/init.d/alsa-utils restart or you can reboot your Raspberry Pi using sudo reboot.