ALSA configuration

One of the components of WirePlumber is the ALSA monitor. This monitor is responsible for creating PipeWire devices and nodes for all the ALSA cards that are available on the system. It also manages the configuration of these devices.

The ALSA monitor is enabled by default and can be disabled using the monitor.alsa feature in the configuration file.

The monitor, as with all device monitors, is implemented as a SPA plugin and is part of PipeWire. WirePlumber merely loads the plugin and lets it do its work. The plugin then monitors UDev and creates device and node objects for all the ALSA cards that are available on the system.

Note

One thing worth remembering here is that in ALSA, a "card" represents a physical sound controller device, and a "device" is a logical access point that represents a set of inputs and/or outputs that are part of the card. In PipeWire, a "device" is the direct equivalent of an ALSA "card" and a "node" is almost equivalent (close, but not quite) of an ALSA "device".

Properties

The ALSA monitor SPA plugin (api.alsa.enum.udev) supports properties that can be used to configure it when it is loaded. These properties can be set in the monitor.alsa.properties section of the WirePlumber configuration file.

Example:

monitor.alsa.properties = {
  alsa.use-acp = true
}

alsa.use-acp: A boolean that controls whether the ACP (alsa card profile) code is to be the default manager of the device. This will probe the device and configure the available profiles, ports and mixer settings. The code to do this is taken directly from PulseAudio and provides devices that look and feel exactly like the PulseAudio devices.

The remaining monitor properties are listed under "ALSA PROPERTIES / Monitor properties" in pipewire-props(7).

Rules

When device and node objects are created by the ALSA monitor, they can be configured using rules. These rules allow matching the existing properties of these objects and updating them with new values. This is the main way of configuring ALSA device settings.

These rules can be set in the monitor.alsa.rules section of the WirePlumber configuration file.

Example:

monitor.alsa.rules = [
  {
    matches = [
      {
        # This matches the value of the 'device.name' property of the device.
        device.name = "~alsa_card.*"
      }
    ]
    actions = {
      update-props = {
        # Apply all the desired device settings here.
        api.alsa.use-acp = true
      }
    }
  }
  {
    matches = [
      # This matches the value of the 'node.name' property of the node.
      {
        node.name = "~alsa_output.*"
      }
    ]
    actions = {
      # Apply all the desired node specific settings here.
      update-props = {
        node.nick              = "My Node"
        priority.driver        = 100
        session.suspend-timeout-seconds = 5
      }
    }
  }
]

Where the properties are documented

The names that appear inside update-props are PipeWire object properties. They are not defined by WirePlumber: they are implemented by the ALSA SPA plugin and by the audio adapter that PipeWire attaches to every audio node. Their complete reference is the pipewire-props(7) man page, which is also available online at https://docs.pipewire.org/page_man_pipewire-props_7.html

That man page covers the common device properties, the common node properties (identifying, classifying, scheduling, session manager and format properties), the audio adapter properties (merger, resampler and channel mixer, including all the channelmix.* and resample.* properties) and the ALSA-specific device and node properties. Note that only a part of it is ALSA-specific: the common and audio adapter properties apply to Bluetooth nodes and to application streams as well.

The two sections below are a selection only - the properties that are most commonly changed on ALSA devices, plus the bits whose behavior is specific to WirePlumber. For anything that is not listed here, refer to pipewire-props(7).

Device properties

A selection of the properties that are commonly set on devices created by the monitor:

api.alsa.use-acp

Use the ACP (alsa card profile) code to manage this device. This will probe the device and configure the available profiles, ports and mixer settings. The code to do this is taken directly from PulseAudio and provides devices that look and feel exactly like the PulseAudio devices.

Default value:: true
Type:: boolean

api.alsa.use-ucm

When ACP is enabled and a UCM configuration is available for a device, by default it is used instead of the ACP profiles. This option allows you to disable this and use the ACP profiles instead.

This option does nothing if api.alsa.use-acp is set to false.

Default value:: true
Type:: boolean

api.alsa.soft-mixer

Setting this option to true will disable the hardware mixer for volume control and mute. All volume handling will then use software volume and mute, leaving the hardware mixer untouched. The hardware mixer will still be used to mute unused audio paths in the device.

Type:: boolean

api.alsa.ignore-dB

Setting this option to true will ignore the decibel setting configured by the driver. Use this when the driver reports wrong settings.

Type:: boolean

device.profile-set

This option can be used to select a custom ACP profile-set name for the device. This can be configured in UDev rules, but it can also be specified here. The default is to use "default.conf".

Type:: string

device.disabled

Disables the device. WirePlumber will not create it at all, so it will not appear in the list of cards or devices.

Type:: boolean

Note

Profile and route (ACP "port") selection is handled by WirePlumber itself, which is why the api.acp.auto-profile and api.acp.auto-port properties described in pipewire-props(7) are disabled by default. They are only useful in custom configurations where the relevant WirePlumber components are disabled. To influence WirePlumber's own choice of profile, see Profile priorities below.

Node properties

A selection of the properties that are commonly set on nodes created by the monitor:

node.description

A user-friendly name for the node. This is what most user interfaces show as the name of the sink or source.

Type:: string

node.disabled

Disables the node. WirePlumber will not create it at all, so it will not appear in the list of nodes.

Type:: boolean

priority.session

This configures the priority of the node when selecting a default node (default sink/source as a link target for streams). Higher priority nodes will be more likely candidates for becoming the default node.

Type:: integer

Note

By default, sources have a priority.session value around 1600-2000 and sinks have a value around 600-1000. If you are increasing the priority of a sink, it is not advised to use a value higher than 1500, as it may cause a sink's monitor to be selected as the default source.

Important

Priorities only decide the default node as long as there is no saved user selection. A node that was selected with wpctl set-default — or by any other client that sets the default, such as a desktop volume applet — is remembered in the default-nodes state file and outranks every priority.session value on all subsequent starts, which makes changes to this property appear to have no effect at all. The saved selections are listed in the "Default Configured Devices" section of wpctl status; wpctl clear-default forgets them and lets the priorities decide again.

session.suspend-timeout-seconds

This option configures a different suspend timeout on the node. By default this is 5 seconds. For some devices (HiFi amplifiers, for example) it might make sense to set a higher timeout because they might require some time to restart after being idle.

A value of 0 disables suspend for a node and will leave the ALSA device busy. The device can then be manually suspended with pactl suspend-sink|source.

Type:: integer

audio.position

The position of the channels. By default the number of channels and their position are determined by the selected device profile. You can override this setting here and optionally swap or reconfigure the channel positions.

Type:: array of strings (example: ["FL", "FR", "LFE", "FC", "RL", "RR"])

ALSA buffer properties

PipeWire by default uses a timer to consume and produce samples to/from ALSA devices. After every timeout, it queries the hardware pointers of the device and uses this information to set a new timeout. This works well for most devices, but there is a class of devices, so called "batch" devices, that need extra buffering and timing tweaks to work properly. This is because batch devices only get their hardware pointers updated after each hardware interrupt. When the hardware interrupt frequency and the timer frequency are aligned, it is possible for the hardware pointers to be updated just after the timer has expired, resulting in sometimes wrong timing information being returned by the query. In contrast, non-batch devices get pointer updates independent of the interrupt.

This means that for batch devices we need to set the interrupt at a sufficiently high frequency, at the cost of CPU usage, while for non-batch devices we want to set the interrupt frequency as low as possible to save CPU. For batch devices we also need to take the extra buffering into account caused by the delayed updates of the hardware pointers.

Note

Most USB devices are batch devices and will be handled as such by PipeWire by default.

There are 2 tunable parameters to control the buffering and timeouts in a device:

api.alsa.period-size

This sets the device interrupt to every period-size samples for non-batch devices and to half of this for batch devices. For batch devices, the other half of the period-size is used as extra buffering to compensate for the delayed update. So, for batch devices, there is an additional period-size/2 delay. It makes sense to lower the period-size for batch devices to reduce this delay.

Type:: integer (samples)

api.alsa.headroom

This adds extra delay between the hardware pointers and software pointers. In most cases this can be set to 0. For very bad devices or emulated devices (like in a VM) it might be necessary to increase the headroom value.

Type:: integer (samples)

api.alsa.period-num

This configures the number of periods in the hardware buffer, which controls its size. Note that this is multiplied by the period of the device to determine the size, so for batch devices, the total buffer size is effectively period-num * period-size/2.

Type:: integer

In summary, this is the overview of buffering and timings:

Property	Batch	Non-Batch
IRQ Frequency	api.alsa.period-size/2	api.alsa.period-size
Extra Delay	api.alsa.headroom + api.alsa.period-size/2	api.alsa.headroom
Buffer Size	api.alsa.period-num * api.alsa.period-size/2	api.alsa.period-num * api.alsa.period-size

Finally, it is possible to disable the batch device tweaks with:

api.alsa.disable-batch

This disables the batch device tweaks. It removes the extra delay added of period-size/2 if the device can support this. For batch devices it is also a good idea to lower the period-size (and increase the IRQ frequency) to get smaller batch updates and lower latency.

Type:: boolean

ALSA extra latency properties

Extra internal delay in the DAC and ADC converters of the device itself can be set with the latency.internal.* properties:

latency.internal.rate = 256
latency.internal.ns = 0

You can configure a latency in samples (relative to rate with latency.internal.rate) or in nanoseconds (latency.internal.ns). This value will be added to the total reported latency by the node of the device.

You can use a tool like jack_iodelay to get the number of samples of internal latency of your device.

This property is also adjustable at runtime with the ProcessLatency param. You will need to find the id of the Node you want to change. For example: Query the current internal latency of an ALSA node with id 58:

$ pw-cli e 58 ProcessLatency
Object: size 80, type Spa:Pod:Object:Param:ProcessLatency (262156), id Spa:Enum:ParamId:ProcessLatency (16)
  Prop: key Spa:Pod:Object:Param:ProcessLatency:quantum (1), flags 00000000
    Float 0.000000
  Prop: key Spa:Pod:Object:Param:ProcessLatency:rate (2), flags 00000000
    Int 0
  Prop: key Spa:Pod:Object:Param:ProcessLatency:ns (3), flags 00000000
    Long 0

Set the internal latency to 256 samples:

$ pw-cli s 58 ProcessLatency '{ rate = 256 }'
Object: size 32, type Spa:Pod:Object:Param:ProcessLatency (262156), id Spa:Enum:ParamId:ProcessLatency (16)
  Prop: key Spa:Pod:Object:Param:ProcessLatency:rate (2), flags 00000000
    Int 256
remote 0 node 58 changed
remote 0 port 70 changed
remote 0 port 72 changed
remote 0 port 74 changed
remote 0 port 76 changed

IEC958 (S/PDIF) passthrough

iec958.codecs

S/PDIF passthrough will only be enabled when the accepted codecs are configured on the ALSA device. This can be done by setting the list of supported codecs on this property.

Note that it is possible to also configure this property at runtime, either with tools like pavucontrol or with the pw-cli tool, like this: pw-cli s <node-id> Props '{ iec958Codecs : [ PCM ] }'

Type:: array of strings (example: [ "PCM", "DTS", "AC3", "EAC3", "TrueHD", "DTS-HD" ])

ALSA MIDI

The ALSA MIDI monitor bridges the ALSA sequencer into PipeWire as a single node. It is enabled by the monitor.alsa-midi feature; see Well-known features.

monitor.alsa-midi.properties

Properties for the MIDI bridge node.

monitor.alsa-midi.properties = {
  ## Name set for the node with ALSA MIDI ports
  node.name = "Midi-Bridge"

  ## Removes longname/number from MIDI port names
  api.alsa.disable-longname = false
}

The properties that are accepted here are listed under "ALSA MIDI PROPERTIES" in pipewire-props(7).

Profile priorities

When no profile has been stored for a device and none is forced by other means, WirePlumber picks the available profile with the highest priority. The device.profile.priority.rules section allows overriding that choice by naming an explicit order of preferred profiles for matching devices.

The rules are matched against device properties, and the update-props action sets a priorities property holding a JSON array of profile names, most preferred first:

device.profile.priority.rules = [
  {
    matches = [
      { device.name = "~alsa_card.*" }
    ]
    actions = {
      update-props = {
        priorities = [ "pro-audio", "output:analog-stereo" ]
      }
    }
  }
]

The first profile in the list that is actually available on the device is selected. If none of them is available, WirePlumber falls back to picking the highest priority profile as usual.

Note

This section applies to all devices, not only ALSA ones. For Bluetooth devices, the bluetooth.profile-preference setting provides a simpler way of expressing a quality-versus-latency preference; see Well-known settings and Bluetooth configuration.