# Automatic Software DSP

## Introduction

WirePlumber provides a mechanism for transparently handling oddball and embedded devices that require software DSP to be done in userspace. Devices such as smartphones, TVs, portable speakers, and even some laptops implement an audio subsystem designed under the assumption that the hardware sink/source will be “backed” by some sort of transparent DSP mechanism. That is, the hardware device itself should not be directly accessed, and expects to be sent preprocessed/pre-routed samples. Often, especially with Android handsets, these samples are preprocessed or pre-routed by the vendor’s proprietary userspace.

WirePlumber’s automatic software DSP mechanism aims to replicate this functionality in a standardised and configurable way. The target device sink/source is hidden from other PipeWire clients, and a virtual node is linked to it. This virtual node is then presented to clients as the node, allowing implementers to specify any custom processing or routing in a way that is transparent to users, the kernel, and the hardware.

## Activating

In addition to the `node.software-dsp.rules` section, the `node.software-dsp` feature must be enabled in the desired profile(s).

## Matching a node

Matching rules are specified in `node.software-dsp.rules`. The `create-filter` action specifies behaviour at node insertion. All node properties can be matched on, including any type-specific properties such as `alsa.id`.

## Configurable properties

filter-graph

  SPA-JSON object describing the software DSP node. This is passed as-is as an argument to `libpipewire-module-filter-chain`. See the filter-chain documentation for details on what options can be set in this object.

  Note

  The `target.object` property of the virtual node should be configured statically to point to the node matched by the rule.

filter-path

  Absolute path to a file on disk storing a SPA-JSON object as plain text. This will be parsed by WirePlumber into a WpConf object with a single section called `node.software-dsp.graph`, then passed as-is into `libpipewire-module-filter-chain`.

  Note

  `filter-graph` and `filter-path` are mutually exclusive, with the former taking precedence if both are present in the matched rule.

hide-parent

  Boolean indicating whether or not the matched node should be hidden from clients. `node/software-dsp.lua` will set the permissions for all clients other than WirePlumber itself to `'-'`. This prevents use of the node by any userspace software except for WirePlumber itself.

## Examples

wireplumber.conf.d/99-my-dsp.conf

    node.software-dsp.rules = [
      {
        matches = [
          { "node.name" = "alsa_output.platform-sound.HiFi__Speaker__sink" }
          { "alsa.id" = "~WeirdHardware*" } # Wildcard match
        ]

        actions = {
          create-filter = {
            filter-graph = {} # Virtual node goes here
            filter-path = "/path/to/spa.json"
            hide-parent = true
          }
        }
      }
    ]

    wireplumber.profiles = [
      main = {
        node.software-dsp = required
      }
    ]

This will match any sinks with the UCM HiFi Speaker profile set that are associated with cards containing the string “WeirdHardware” at the start of their name.