Basic Commands for ALSA

If you're already familiar with ALSA utilities, feel free to skip this section. For newcomers, familiarize yourself with the following commands to manage sound devices effectively. First, ensure that you have the ALSA utilities installed:

sudo apt install alsa-utils

Next, use these commands to list the audio devices recognized by your operating system:

• List all playback devices:

aplay -l

• List all recording devices:

arecord -l

These commands help you identify the names, hardware IDs, and channel counts of your audio devices. They also confirm that any external devices, particularly USB interfaces, are properly connected.

To access the ALSA mixer, which allows you to adjust input/output levels and settings for each device, run the following command:

alsamixer

Creating Loopback Interfaces

Now that you have the basics down, let’s proceed to creating virtual loopback interfaces. To do this, you need to load the snd-aloop module into your system. This module enables audio to be sent to one side of an interface and received on the other—essentially acting as a virtual audio cable.

With these virtual interfaces, you can perform advanced audio routing. For example, you can send audio to an interface that captures or processes the sound, or simply duplicate it. The possibilities are vast with multiple virtual interfaces at your disposal.

Load the loopback module with the following command:

sudo modprobe snd-aloop

After loading the module, running arecord -l or aplay -l should show the new loopback devices:

card 1: Loopback [Loopback], device 0: Loopback PCM [Loopback PCM]

Subdevices: 7/8

...

The first device (device 0) serves as the input for audio routing, while the second device (device 1) is used for output. Anything sent to the first device will be available on the second.

For instance, to route audio to the first interface, you would use:

hw:1,0,0

To listen to it from the second interface, use:

hw:1,1,0

You can utilize both arecord and aplay commands to test the setup by playing a WAV file into the first interface and recording it on the second.

Customizing Loopback Interfaces

If you wish to create more than one pair of loopback interfaces, you can do this by creating a configuration file. Create the following file at /etc/modprobe.d/alsa-loopback.conf and populate it as follows:

options snd-aloop enable=1,1,1,1 index=0,1,2,3

This configuration will generate four pairs of loopback interfaces. You can adjust the number of enabled interfaces by modifying the values accordingly.

To ensure the snd-aloop module loads automatically at startup, execute this command:

echo "snd_aloop" > /etc/modules-load.d/snd-aloop.conf

Now, every time you reboot, the snd-aloop module will load with your specified number of interfaces.

Testing Your Setup

To test your new virtual interfaces, you can open your preferred audio application and select them as input/output devices. Alternatively, you can use the command line for testing with these commands:

If you need test audio files, consider downloading free WAV files from freesound.org.

To play a test file into the input loopback, use:

aplay -D hw:1,0,0 test_input.wav

To record from the output loopback, run:

arecord -D hw:1,1,0 -f S16_LE -c 2 -r 48000 test_output.wav

By following these steps, you should have successfully set up virtual audio routing without the complications of more elaborate audio server frameworks. For further information on ALSA, check out the official documentation available online.

Thanks for reading! Explore more with these related articles: 7 GitHub Repositories with Free Software, Tools, and Tips; 10 Common Mistakes Developers Make When Reviewing Code.

How to Set Up ALSA Utilities

Creating Multiple Loopback Interfaces