The filter-chain allows you to create an arbitrary processing graph from LADSPA, LV2 and builtin filters. This filter can be made into a virtual sink/source or between any 2 nodes in the graph.

The filter chain is built with 2 streams, a capture stream providing the input to the filter chain and a playback stream sending out the filtered stream to the next nodes in the graph.

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

Module Name

libpipewire-module-filter-chain

Module Options

node.description: a human readable name for the filter chain
filter.graph = []: a description of the filter graph to run, see below
capture.props = {}: properties to be passed to the input stream
playback.props = {}: properties to be passed to the output stream

Filter graph description

The general structure of the graph description is as follows:

 filter.graph = {
     nodes = [
         {
             type = <ladspa | lv2 | builtin | sofa>
             name = <name>
             plugin = <plugin>
             label = <label>
             config = {
                 <configkey> = <value> ...
             }
             control = {
                 <controlname|controlindex> = <value> ...
             }
         }
         ...
     ]
     links = [
         { output = <portname> input = <portname> }
         ...
     ]
     inputs = [ <portname> ... ]
     outputs = [ <portname> ... ]
     capture.volumes = [
         { control = <portname>  min = <value>  max = <value>  scale = <scale> } ...
     ]
     playback.volumes = [
         { control = <portname>  min = <value>  max = <value>  scale = <scale> } ...
     ]
}

Nodes

Nodes describe the processing filters in the graph. Use a tool like lv2ls or listplugins to get a list of available plugins, labels and the port names.

type is one of ladspa, lv2, builtin or sofa.
name is the name for this node, you might need this later to refer to this node and its ports when setting controls or making links.
plugin is the type specific plugin name.
- For LADSPA plugins it will append .so to find the shared object with that name in the LADSPA plugin path.
- For LV2, this is the plugin URI obtained with lv2ls.
- For builtin and sofa this is ignored
label is the type specific filter inside the plugin.
- For LADSPA this is the label
- For LV2 this is unused
- For builtin and sofa this is the name of the filter to use
config contains a filter specific configuration section. Some plugins need this. (convolver, sofa, delay, ...)
control contains the initial values for the control ports of the filter. normally these are given with the port name but it is also possible to give the control index as the key.

Links

Links can be made between ports of nodes. The portname is given as <node_name>:<port_name>.

You can tee the output of filters to multiple other filters. You need to use a mixer if you want the output of multiple filters to go into one filter input port.

links can be omited when the graph has just 1 filter.

Inputs and Outputs

These are the entry and exit ports into the graph definition. Their number defines the number of channels used by the filter-chain.

The <portname> can be null when a channel is to be ignored.

Each input/output in the graph can only be linked to one filter input/output. You need to use the copy builtin filter if the stream signal needs to be routed to multiple filters. You need to use the mixer builtin plugin if multiple graph outputs need to go to one output stream.

inputs and outputs can be omitted, in which case the filter-chain will use all inputs from the first filter and all outputs from the last filter node. The graph will then be duplicated as many times to match the number of input/output channels of the streams.

Volumes

Normally the volume of the sink/source is handled by the stream software volume. With the capture.volumes and playback.volumes properties this can be handled by a control port in the graph instead. Use capture.volumes for the volume of the input of the filter (when for example used as a sink). Use playback,volumes for the volume of the output of the filter (when for example used as a source).

The min and max values (defaults 0.0 and 1.0) respectively can be used to scale and translate the volume min and max values.

Normally the control values are linear and it is assumed that the plugin does not perform any scaling to the values. This can be changed with the scale property. By default this is linear but it can be set to cubic when the control applies a cubic transformation.

Builtin filters

There are some useful builtin filters available. You select them with the label of the filter node.

Mixer

Use the mixer plugin if you have multiple input signals that need to be mixed together.

The mixer plugin has up to 8 input ports labeled "In 1" to "In 8" and each with a gain control labeled "Gain 1" to "Gain 8". There is an output port labeled "Out". Unused input ports will be ignored and not cause overhead.

Copy

Use the copy plugin if you need to copy a stream input signal to multiple filters.

It has one input port "In" and one output port "Out".

Biquads

Biquads can be used to do all kinds of filtering. They are also used when creating equalizers.

All biquad filters have an input port "In" and an output port "Out". They have a "Freq", "Q" and "Gain" control. Their meaning depends on the particular biquad that is used. The biquads also have "b0", "b1", "b2", "a0", "a1" and "a2" ports that are read-only except for the bq_raw biquad, which can configure default values depending on the graph rate and change those at runtime.

We refer to https://arachnoid.com/BiQuadDesigner/index.html for an explanation of the controls.

The following labels can be used:

bq_lowpass a lowpass filter.
bq_highpass a highpass filter.
bq_bandpass a bandpass filter.
bq_lowshelf a low shelf filter.
bq_highshelf a high shelf filter.
bq_peaking a peaking filter.
bq_notch a notch filter.
bq_allpass an allpass filter.
bq_raw a raw biquad filter. You need a config section to specify coefficients per sample rate. The coefficients of the sample rate closest to the graph rate are selected:

filter.graph = {
    nodes = [
        {
            type   = builtin
            name   = ...
            label  = bq_raw
            config = {
                coefficients = [
                    { rate =  44100, b0=.., b1=.., b2=.., a0=.., a1=.., a2=.. },
                    { rate =  48000, b0=.., b1=.., b2=.., a0=.., a1=.., a2=.. },
                    { rate = 192000, b0=.., b1=.., b2=.., a0=.., a1=.., a2=.. }
                ]
            }
            ...
        }
    }
    ...
}

Convolver

The convolver can be used to apply an impulse response to a signal. It is usually used for reverbs or virtual surround. The convolver is implemented with a fast FFT implementation.

The convolver has an input port "In" and an output port "Out". It requires a config section in the node declaration in this format:

filter.graph = {
    nodes = [
        {
            type   = builtin
            name   = ...
            label  = convolver
            config = {
                blocksize = ...
                tailsize = ...
                gain = ...
                delay = ...
                filename = ...
                offset = ...
                length = ...
                channel = ...
                resample_quality = ...
            }
            ...
        }
    }
    ...
}

blocksize specifies the size of the blocks to use in the FFT. It is a value between 64 and 256. When not specified, this value is computed automatically from the number of samples in the file.
tailsize specifies the size of the tail blocks to use in the FFT.
gain the overall gain to apply to the IR file.
delay The extra delay (in samples) to add to the IR.
filename The IR to load or create. Possible values are:
- /hilbert creates a hilbert function that can be used to phase shift the signal by +/-90 degrees. The length will be used as the number of coefficients.
- /dirac creates a Dirac function that can be used as gain.
- A filename to load as the IR. This needs to be a file format supported by sndfile.
- [ filename, ... ] an array of filenames. The file with the closest samplerate match with the graph samplerate will be used.
offset The sample offset in the file as the start of the IR.
length The number of samples to use as the IR.
channel The channel to use from the file as the IR.
resample_quality The resample quality in case the IR does not match the graph samplerate.

Delay

The delay can be used to delay a signal in time.

The delay has an input port "In" and an output port "Out". It also has a "Delay (s)" control port. It requires a config section in the node declaration in this format:

filter.graph = {
    nodes = [
        {
            type   = builtin
            name   = ...
            label  = delay
            config = {
                "max-delay" = ...
            }
            control = {
                "Delay (s)" = ...
            }
            ...
        }
    }
    ...
}

max-delay the maximum delay in seconds. The "Delay (s)" parameter will be clamped to this value.

Invert

The invert plugin can be used to invert the phase of the signal.

It has an input port "In" and an output port "Out".