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102 lines
3.2 KiB
Text
102 lines
3.2 KiB
Text
/** \page page_midi MIDI Support
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This document explains how MIDI is implemented.
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# Use cases
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### MIDI 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 a
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MIDI device.
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### MIDI 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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### Applications can connect to MIDI devices
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Applications can create ports that can connect to the MIDI ports
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so that data can be provided to or consumed from them.
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### Some MIDI devices are sinks or sources for midi data
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It should be possible to create a MIDI sink or source that routes the
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midi events to specific midi ports.
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One example of such a sink would be in front of a software midi
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renderer.
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An example of a MIDI source would be after a virtual keyboard or
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as a mix from many midi input devices.
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### Applications should autoconnect to MIDI sinks or sources
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An application should be able to be connected to a MIDI sink when
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it wants to play midi data.
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An application should be able to connect to a MIDI source when it
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wants to capture midi data.
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# Design
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## SPA
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MIDI devices/streams are implemented with an \ref spa_node with generic
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control input and output Ports. These ports have a media type of
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`"application/control"` and the data transported over these ports
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are of type \ref spa_pod_sequence with the \ref spa_pod_control type set to
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\ref SPA_CONTROL_Midi.
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This means that every midi event is timestamped with the sample
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offset against the current graph clock cycle to get sample accurate
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midi events that can be aligned with the corresponding sample data.
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Since the MIDI events are embedded in the generic control stream,
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they can be interleaved with other control message types, such as
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property updates or OSC messages.
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## The PipeWire daemon
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Nothing special is implemented for MIDI. Negotiation of formats
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happens between `"application/control"` media types and buffers are
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negotiated in the same way as any generic format.
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## The session manager
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The session manager needs to create the MIDI nodes/ports for the available
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devices.
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This can either be done as a single node with ports per device/stream
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or as separate nodes created by a MIDI device monitor.
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The session manager needs to be aware of the various MIDI sinks and sources
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in order to route MIDI streams to them from applications that want this.
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# Implementation
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## pipewire-media-session
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PipeWire media session uses the \ref SPA_NAME_API_ALSA_SEQ_BRIDGE plugin for
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the midi features. This creates a single SPA Node with ports per
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MIDI client/stream.
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The media session will check the permissions on `/dev/snd/seq` before
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attempting to create this node. It will also use inotify to wait
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until the sequencer device node is accessible.
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## JACK
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JACK assumes all `"application/control"` ports are midi ports.
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The control messages are converted to the JACK event format by
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filtering out the \ref SPA_CONTROL_Midi types. On output ports, the JACK
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event stream is converted to control messages in a similar way.
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There is a 1 to 1 mapping between the JACK events and control
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messages so there is no information loss or need for complicated
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conversions.
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*/
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