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127 lines
4.1 KiB
Text
127 lines
4.1 KiB
Text
/** \page page_audio Audio
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This document explains how Audio is implemented.
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# Use Cases
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## Audio Devices Are Made Available As Processing Nodes/Ports
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Applications need to be able to see a port for each stream of an
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audio device.
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## Audio Devices Can Be Plugged and Unplugged
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When devices are plugged and unplugged the associated nodes/ports
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need to be created and removed.
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## Audio Port In Canonical Format
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It must be possible to make individual audio channels available
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as a single mono stream with a fixed format and samplerate.
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This makes it possible to link any of the audio ports together
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without doing conversions.
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## Applications Can Connect To Audio Devices
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Applications can create ports that can connect to the audio ports
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so that data can be provided to or consumed from them.
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It should be possible to automatically connect an application to
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a sink/source when it requests this.
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## Default Audio Sink and Sources
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It should be possible to mark a source or sink as the default source
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and sink so that applications are routed to them by default.
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It should be possible to change the default audio sink/source.
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## Application Should Be Able To Move Between Sinks/Sources
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It should be possible to move an application from one device to
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another dynamically.
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## Exclusive Access
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Application should be able to connect to a device in exclusive mode.
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This allows the application to negotiate a specific format with the
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device such as a compressed format.
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Exclusive access means that only one application can access the device
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because mixing is in general not possible when negotiating
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compressed formats.
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# Design
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## SPA
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Audio devices are implemented with an \ref spa_device "SPA Device" object.
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This object is then responsible for controlling the \ref spa_node "SPA Nodes" that
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provide the audio ports to interface with the device.
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The nodes operate on the native audio formats supported by the device.
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This includes the sample rate as well as the number of channels and
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the audio format.
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## Audio Adapter
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An SPA Node called the "adapter" is usually used with the SPA device node as
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the internal node.
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The function of the adapter is to convert the device native format to
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the required external format. This can include format or samplerate
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conversion but also channel remixing/remapping.
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The audio adapter is also responsible for exposing the audio channels
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as separate mono ports. This is called the DSP setup.
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The audio adapter can also be configured in passthrough mode when it
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will not do any conversions but simply pass through the port information
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of the internal node. This can be used to implement exclusive access.
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Setup of the different configurations of the adapter can be done with
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the PortConfig parameter.
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## The Session Manager
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The session manager is responsible for creating nodes and ports for
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the various audio devices. It will need to wrap them into an audio
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adapter so that the specific configuration of the node can be
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decided by the policy mode.
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The session manager should create name and description for the
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devices and nodes.
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The session manager is responsible for assigning priorities to the
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nodes. At least \ref PW_KEY_PRIORITY_SESSION and \ref PW_KEY_PRIORITY_DRIVER
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need to be set.
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The session manager might need to work with other services to gain
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exclusive access to the device (eg. DBus).
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# Implementation
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## PipeWire Media Session (alsa-monitor)
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PipeWire media session uses the \ref SPA_NAME_API_ALSA_ENUM_UDEV plugin
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for enumerating the ALSA devices. For each device it does:
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- Try to acquire the DBus device reservation object to gain exclusive
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access to the device.
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- Create an SPA device instance for the device and monitor this device instance.
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- For each node created by the device, create an adapter with
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an ALSA PCM node in the context of the PipeWire daemon.
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The session manager will also create suitable names and descriptions
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for the devices and nodes that it creates as well as assign session
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and driver priorities.
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The session manager has the option to add extra properties on the
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devices and nodes that it creates to control their behavior. This
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is implemented with match rules.
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*/
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