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68 lines
1.8 KiB
Markdown
68 lines
1.8 KiB
Markdown
# ALSA
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This explains the mapping between alsa cards and streams and session manager
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objects.
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## ALSA Cards
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An ALSA card is exposed as a PipeWire device
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## Streams
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Each alsa PCM is opened and a Node is created for each PCM stream.
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# Session Manager
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## ALSA UCM
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The mapping of the PipeWire object hierarchy to the ALSA object hierarchy is the following:
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One PipeWire device is created for every ALSA card.
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For each UCM verb, a Node is created for the associated PCM devices.
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For each UCM verb, an Endpoint is created.
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In a first step: For each available combination of UCM device and modifier,
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a stream is created. Streams are marked with compatible other streams.
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Streams with the same modifier and mutually exclusive devices are grouped
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into one stream and the UCM devices are exposed on the endpoint as destinations.
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## ALSA fallback
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Each PCM stream (node) becomes an endpoint. The endpoint references the
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alsa device id
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Each endpoint has 1 stream (for now) called HiFi Playback / HiFi Capture.
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More streams can be created depending on the format of the node.
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## ALSA pulse UCM
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Using the alsa backend of pulseaudio we can create the following streams
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## ALSA pulse fallback
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The pulse alsa backend will use the mixer controls and some probing to
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create the following nodes and endpoints
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# PulseAudio
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PulseAudio uses the session manager API to construct cards with profiles
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and sink/source with ports.
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If an Endpoint references a Device, a card object is created for the device.
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Each Endpoint becomes a sink/source.
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Each Stream in the endpoint becomes a profile on the PulseAudio card. Because
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only one profile is selected on the device, only 1 stream is visible on
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the endpoint. This clashes with the notion that multiple streams can be
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active at the same time but is a pulseaudio limitation.
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Each Endpoint destination becomes a port on the sink/source.
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