PipeWire: Loopback

PipeWire 1.2.7

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Loopback

The loopback module passes the output of a capture stream unmodified to a playback stream. It can be used to construct a link between a source and sink but also to create new virtual sinks or sources or to remap channel between streams.

Because both ends of the loopback are built with streams, the session manager can manage the configuration and connection with the sinks and sources.

# Module Name

`libpipewire-module-loopback`

# Module Options

- `node.description`: a human readable name for the loopback streams
- `target.delay.sec`: delay in seconds as float (Since 0.3.60)
- `capture.props = {}`: properties to be passed to the input stream
- `playback.props = {}`: properties to be passed to the output stream

# General options

Options with well-known behavior. Most options can be added to the global configuration or the individual streams:

- PW_KEY_REMOTE_NAME
- PW_KEY_AUDIO_RATE
- PW_KEY_AUDIO_CHANNELS
- SPA_KEY_AUDIO_POSITION
- PW_KEY_MEDIA_NAME
- PW_KEY_NODE_LATENCY
- PW_KEY_NODE_DESCRIPTION
- PW_KEY_NODE_GROUP
- PW_KEY_NODE_LINK_GROUP
- PW_KEY_NODE_VIRTUAL
- PW_KEY_NODE_NAME : See notes below. If not specified, defaults to 'loopback-PID-MODULEID'.

Stream only properties:

- PW_KEY_MEDIA_CLASS
- PW_KEY_NODE_NAME : if not given per stream, the global node.name will be prefixed with 'input.' and 'output.' to generate a capture and playback stream node.name respectively.

# Channel handling

Channels from the capture stream are copied, in order, to the channels of the output stream. The remaining streams are ignored (when capture has more channels) or filled with silence (when playback has more channels).

When a global channel position is set, both capture and playback will be converted to and from this common channel layout. This can be used to implement up or downmixing loopback sinks/sources.

# Example configuration of a virtual sink

This Virtual sink routes stereo input to the rear channels of a 7.1 sink.

# ~/.config/pipewire/pipewire.conf.d/my-loopback-1.conf
context.modules = [
{ name = libpipewire-module-loopback
args = {
node.description = "CM106 Stereo Pair 2"
#target.delay.sec = 1.5
capture.props = {
node.name = "CM106_stereo_pair_2"
media.class = "Audio/Sink"
audio.position = [ FL FR ]
}
playback.props = {
node.name = "playback.CM106_stereo_pair_2"
audio.position = [ RL RR ]
target.object = "alsa_output.usb-0d8c_USB_Sound_Device-00.analog-surround-71"
node.dont-reconnect = true
stream.dont-remix = true
node.passive = true
}
}
}
]

# Example configuration of a virtual source

This Virtual source routes the front-left channel of a multi-channel input to a mono channel. This is useful for splitting up multi-channel inputs from USB audio interfaces that are not yet fully supported by alsa.

# ~/.config/pipewire/pipewire.conf.d/my-loopback-2.conf
context.modules = [
{ name = libpipewire-module-loopback
args = {
node.description = "Scarlett Focusrite Line 1"
capture.props = {
audio.position = [ FL ]
stream.dont-remix = true
node.target = "alsa_input.usb-Focusrite_Scarlett_Solo_USB_Y7ZD17C24495BC-00.analog-stereo"
node.passive = true
}
playback.props = {
node.name = "SF_mono_in_1"
media.class = "Audio/Source"
audio.position = [ MONO ]
}
}
}
]

# Example configuration of an upmix sink

This Virtual sink has 2 input channels and 6 output channels. It will perform upmixing using the PSD algorithm on the playback stream.

# ~/.config/pipewire/pipewire.conf.d/my-loopback-3.conf
context.modules = [
{ name = libpipewire-module-loopback
args = {
node.description = "Upmix Sink"
audio.position = [ FL FR ]
capture.props = {
node.name = "effect_input.upmix"
media.class = Audio/Sink
}
playback.props = {
node.name = "effect_output.upmix"
audio.position = [ FL FR RL RR FC LFE ]
node.passive = true
stream.dont-remix = true
channelmix.upmix = true
channelmix.upmix-method = psd
channelmix.lfe-cutoff = 150
channelmix.fc-cutoff = 12000
channelmix.rear-delay = 12.0
}
}
}
]

# See also

`pw-loopback` : a tool that loads the loopback module with given parameters.