How to use the command 'pw-loopback' (with examples)

The pw-loopback command is a utility from PipeWire, a server and user-space API to manage multimedia pipelines. It allows you to create loopback devices that facilitate audio routing between input and output nodes in a software-defined manner. This command is handy for creating virtual audio devices that can be configured to automatically connect to audio sources or sinks, such as speakers or microphones, thus enabling flexible audio configurations and testing environments.

Use case 1: Creating a loopback device with the default behavior

Code:

pw-loopback

Motivation:
Creating a basic loopback device is essential when you want to test or use the default audio routing capabilities of PipeWire without any custom configurations. It provides a straightforward setup where audio can be looped back from an input to an output.

Explanation:
This invocation of pw-loopback doesn’t include any additional parameters, making it straightforward and indicative of default behavior. It automatically routes any captured audio back as output, helpful for basic testing or initial setup verification.

Example Output:
Upon execution, you will notice a loopback device is created and visible under the PipeWire manager’s utilities or any application that shows audio devices.

Use case 2: Creating a loopback device that connects to the speakers automatically

Code:

pw-loopback -m '[FL FR]' --capture-props='media.class=Audio/Sink'

Motivation:
This scenario is useful for users who want specific audio regions such as speaker channels [Front Left (FL) Front Right (FR)] controlled and sent to the speakers automatically. This setup bypasses the need for manual connection making real-time sound checks efficient.

Explanation:

• -m '[FL FR]' specifies the channel map indicating the audio should be routed from the Left and Right Front channels.
• --capture-props='media.class=Audio/Sink' assigns the device to act like a speaker (audio sink), which means it simulates an output device receiving audio data.

Example Output:
The loopback device will appear as a speaker in your sound settings, and any audio sent to it will automatically be linked to the physical or pre-configured speakers.

Use case 3: Creating a loopback device that connects to the microphone automatically

Code:

pw-loopback -m '[FL FR]' --playback-props='media.class=Audio/Source'

Motivation:
This setup is pivotal for users aiming to use virtual microphones, where audio input needs to be automatically directed to an application expecting a microphone input, such as a VoIP app.

Explanation:

• -m '[FL FR]' as before, this defines the audio channel coordination.
• --playback-props='media.class=Audio/Source' signifies the device as a source (microphone), aligning the playback stream as an input.

Example Output:
Applications that require microphone input will recognize this loopback device as a legitimate source of audio input, providing flexibility in input routing for recording or communication purposes.

Use case 4: Creating a dummy loopback device

Code:

pw-loopback -m '[FL FR]' --capture-props='media.class=Audio/Sink' --playback-props='media.class=Audio/Source'

Motivation:
This setup allows for testing or development where automatic connections aren’t desired. A standalone virtual device can be beneficial when manual control over input and output connections are needed for sophisticated audio routing tests.

Explanation:

• -m '[FL FR]' specifies the standard channel map.
• --capture-props='media.class=Audio/Sink' and --playback-props='media.class=Audio/Source' delineate the device can act as both an audio source and sink, but connections aren’t automatically managed by this configuration.

Example Output:
A dummy loopback device appears in your device manager list without auto-connections. You’ll need to use an advanced tool to manually route its input to and from this device.

Use case 5: Loopback device with channel swapping towards speakers

Code:

pw-loopback --capture-props='media.class=Audio/Sink audio.position=[FL FR]' --playback-props='audio.position=[FR FL]'

Motivation:
Channel swapping may be required in testing environments to validate speaker connections against specifications or for creative audio manipulation.

Explanation:

• --capture-props='media.class=Audio/Sink audio.position=[FL FR]' establishes the device as a sink and defines the standard channel positions.
• --playback-props='audio.position=[FR FL]' swaps the audio channels for playback, meaning audio from the Front Left ends up on the Right and vice versa.

Example Output:
Sounds intended for the Left speaker would unintendedly play through the Right speaker, illustrating channel swapping that can be beneficial for testing wire connections or sound checks.

Use case 6: Loopback device with channel swapping towards the microphone

Code:

pw-loopback --capture-props='audio.position=[FR FL]' --playback-props='media.class=Audio/Source audio.position=[FL FR]'

Motivation:
Channel swap testing can also be applicable to microphones, where feeding swapped audio inputs can help detect incorrect stereo recordings setup or for intentional audio manipulations.

Explanation:

• --capture-props='audio.position=[FR FL]' symbolizes that the input will experience a channel swap.
• --playback-props='media.class=Audio/Source audio.position=[FL FR]' confirms the device acts as a source with the supposed swapped channel inputs.

Example Output:
Applications using this loopback microphone will see channels swapped, aiding diverse troubleshooting or creative configurations.

Conclusion:

The pw-loopback command offers substantial flexibility for configuring audio routing in a PipeWire-enabled environment. By cultivating multiple configurations, it serves as an invaluable tool for both developers and end-users in achieving precise and customized audio management strategies.