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.

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.

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.

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
        }
    }
}
]