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doc: better output formatting for the midi page

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Peter Hutterer 2021-08-02 12:17:05 +10:00 • committed by Wim Taymans
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doc/pipewire-midi.dox
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@ -4,22 +4,22 @@ This document explains how MIDI is implemented.
# Use cases # Use cases
## MIDI devices are made available as processing nodes/ports ### MIDI devices are made available as processing nodes/ports
Applications need to be able to see a port for each stream of a Applications need to be able to see a port for each stream of a
MIDI device. MIDI device.
## MIDI devices can be plugged and unplugged ### MIDI devices can be plugged and unplugged
When devices are plugged and unplugged the associated nodes/ports When devices are plugged and unplugged the associated nodes/ports
need to be created and removed. need to be created and removed.
## Applications can connect to MIDI devices ### Applications can connect to MIDI devices
Applications can create ports that can connect to the MIDI ports Applications can create ports that can connect to the MIDI ports
so that data can be provided to or consumed from them. so that data can be provided to or consumed from them.
## Some MIDI devices are sinks or sources for midi data ### Some MIDI devices are sinks or sources for midi data
It should be possible to create a MIDI sink or source that routes the It should be possible to create a MIDI sink or source that routes the
midi events to specific midi ports. midi events to specific midi ports.
@ -30,7 +30,7 @@ renderer.
An example of a MIDI source would be after a virtual keyboard or An example of a MIDI source would be after a virtual keyboard or
as a mix from many midi input devices. as a mix from many midi input devices.
## Applications should autoconnect to MIDI sinks or sources ### Applications should autoconnect to MIDI sinks or sources
An application should be able to be connected to a MIDI sink when An application should be able to be connected to a MIDI sink when
it wants to play midi data. it wants to play midi data.
@ -42,11 +42,11 @@ wants to capture midi data.
## SPA ## SPA
MIDI devices/streams are implemented with an SPA Node with generic MIDI devices/streams are implemented with an \ref spa_node with generic
control input and output Ports. These ports have a media type of control input and output Ports. These ports have a media type of
"application/control" and the data transported over these ports `"application/control"` and the data transported over these ports
are of type spa_pod_sequence with the spa_pod_control type set to are of type \ref spa_pod_sequence with the \ref spa_pod_control type set to
SPA_CONTROL_Midi. \ref SPA_CONTROL_Midi.
This means that every midi event is timestamped with the sample This means that every midi event is timestamped with the sample
offset against the current graph clock cycle to get sample accurate offset against the current graph clock cycle to get sample accurate
@ -59,7 +59,7 @@ property updates or OSC messages.
## The PipeWire daemon ## The PipeWire daemon
Nothing special is implemented for MIDI. Negotiation of formats Nothing special is implemented for MIDI. Negotiation of formats
happens between "application/control" media types and buffers are happens between `"application/control"` media types and buffers are
negotiated in the same way as any generic format. negotiated in the same way as any generic format.
## The session manager ## The session manager
@ -78,20 +78,20 @@ in order to route MIDI streams to them from applications that want this.
## pipewire-media-session ## pipewire-media-session
PipeWire media session uses the SPA_NAME_API_ALSA_SEQ_BRIDGE plugin PipeWire media session uses the \ref SPA_NAME_API_ALSA_SEQ_BRIDGE plugin for
for the midi features. This creates a single SPA Node with ports per the midi features. This creates a single SPA Node with ports per
MIDI client/stream. MIDI client/stream.
The media session will check the permissions on /dev/snd/seq before The media session will check the permissions on `/dev/snd/seq` before
attempting to create this node. It will also use inotify to wait attempting to create this node. It will also use inotify to wait
until the sequencer device node is accessible. until the sequencer device node is accessible.
## JACK ## JACK
JACK assumes all "application/control" ports are midi ports. JACK assumes all `"application/control"` ports are midi ports.
The control messages are converted to the JACK event format by The control messages are converted to the JACK event format by
filtering out the SPA_CONTROL_Midi types. On output ports, the JACK filtering out the \ref SPA_CONTROL_Midi types. On output ports, the JACK
event stream is converted to control messages in a similar way. event stream is converted to control messages in a similar way.
There is a 1 to 1 mapping between the JACK events and control There is a 1 to 1 mapping between the JACK events and control