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.

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
      }
    }
  }
]

Device properties

The following properties can be configured 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.profile

The initial active profile name. The default is to start from the “Off” profile and then let WirePlumber select the best profile based on its policy.

Type:: string

api.acp.auto-profile

Automatically select the best profile for the device. Normally this option is disabled because WirePlumber will manage the profile of the device. WirePlumber can save and load previously selected profiles. Enable this in custom configurations where the relevant WirePlumber components are disabled.

Type:: boolean

api.acp.auto-port

Automatically select the highest priority port that is available (“port” is a PulseAudio/ACP term, the equivalent of a “Route” in PipeWire). This is by default disabled because WirePlumber handles the task of selecting and restoring Routes. Enable this in custom configurations where the relevant WirePlumber components are disabled.

Type:: boolean

api.acp.probe-rate

Sets the samplerate used for probing the ALSA devices and collecting the profiles and ports.

Type:: integer

api.acp.pro-channels

Sets the number of channels to use when probing the “Pro Audio” profile. Normally, the maximum amount of channels will be used but with this setting this can be reduced, which can make it possible to use other samplerates on some devices.

Type:: integer

Some of the other properties that can be configured on devices:

device.nick: A short name for the device.

device.description: A longer, user-friendly name of the device. This will show up in most user interfaces as the device’s name.

device.disabled

Disables the device. PipeWire will remove it from the list of cards or devices.

Type:: boolean

Node properties

The following properties can be configured on nodes created by the monitor:

priority.driver

This configures the node driver priority. Nodes with higher priority will be used as a driver in the graph. Other nodes with lower priority will have to resample to the driver node when they are joined in the same graph. The default value is set based on some heuristics.

Type:: integer

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.

node.pause-on-idle

Pause the node when nothing is linked to it anymore. This is by default false because some devices make a “pop” sound when they are opened/closed. The node will normally pause and suspend after a timeout (see below).

Type:: boolean

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.format

The sample format of the device. By default, PipeWire will use a 32 bits sample format but a different format can be set here.

Type:: string ("S16LE", "S32LE", "F32LE", …)

audio.rate

The sample rate of the device. By default, the ALSA device will be configured with the same samplerate as the global graph. If this is not supported, or a custom value is set here, resampling will be used to match the graph rate.

Type:: integer

audio.channels

The number of channels of the device. By default the channels and their position are determined by the selected device profile. You can override this setting here.

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"])

api.alsa.use-chmap

Use the channel map as reported by the driver. This is disabled by default because it is often wrong and the ACP code handles this better.

Type:: boolean

api.alsa.disable-mmap

Disable the use of mmap for the ALSA device. By default, PipeWire will access the memory of the device using mmap. This can be disabled and force the usage of the slower read and write access modes, in case the mmap support of the device is not working properly.

Type:: boolean

channelmix.normalize

Normalize the channel volumes when mixing & resampling, making sure that the original 0 dB level is preserved so that nothing sounds wildly quieter/louder. This is disabled by default.

Type:: boolean

channelmix.mix-lfe

Creates a “center” channel for X.0 recordings from the front stereo on X.1 setups and pushes some low-frequency/bass from the “center” of X.1 recordings into the front stereo on X.0 setups. This is disabled by default.

Type:: boolean

monitor.channel-volumes

By default, the volume of the sink/source does not influence the volume on the monitor ports. Set this option to true to change this. PulseAudio has inconsistent behaviour regarding this option, it applies channel-volumes only when the sink/source is using software volumes.

Type:: boolean

node.disabled

Disables the node. Pipewire will remove it from the list of the nodes.

Type:: boolean

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

Startup tweaks

api.alsa.start-delay

Some devices need some time before they can report accurate hardware pointer positions. In those cases, an extra start delay can be added to compensate for this startup delay. This sets the startup delay in samples. The default is 0.

Type:: integer (samples)

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" ])