mirror of
https://gitlab.freedesktop.org/pipewire/pipewire.git
synced 2025-11-03 09:01:54 -05:00
103 lines
3.2 KiB
Text
103 lines
3.2 KiB
Text
/** \page page_midi MIDI Support
|
|
|
|
This document explains how MIDI is implemented.
|
|
|
|
|
|
# Use Cases
|
|
|
|
## MIDI Devices Are Made Available As Processing Nodes/Ports
|
|
|
|
Applications need to be able to see a port for each stream of a
|
|
MIDI device.
|
|
|
|
## MIDI Devices Can Be Plugged and Unplugged
|
|
|
|
When devices are plugged and unplugged the associated nodes/ports
|
|
need to be created and removed.
|
|
|
|
## Applications Can Connect To MIDI Devices
|
|
|
|
Applications can create ports that can connect to the MIDI ports
|
|
so that data can be provided to or consumed from them.
|
|
|
|
## Some MIDI Devices Are Sinks Or Sources For MIDI Data
|
|
|
|
It should be possible to create a MIDI sink or source that routes the
|
|
MIDI events to specific MIDI ports.
|
|
|
|
One example of such a sink would be in front of a software MIDI
|
|
renderer.
|
|
|
|
An example of a MIDI source would be after a virtual keyboard or
|
|
as a mix from many MIDI input devices.
|
|
|
|
## Applications Should Auto-connect To MIDI Sinks Or Sources
|
|
|
|
An application should be able to be connected to a MIDI sink when
|
|
it wants to play MIDI data.
|
|
|
|
An application should be able to connect to a MIDI source when it
|
|
wants to capture MIDI data.
|
|
|
|
|
|
# Design
|
|
|
|
## SPA
|
|
|
|
MIDI devices/streams are implemented with an \ref spa_node with generic
|
|
control input and output Ports. These ports have a media type of
|
|
`"application/control"` and the data transported over these ports
|
|
are of type \ref spa_pod_sequence with the \ref spa_pod_control type set to
|
|
\ref SPA_CONTROL_Midi.
|
|
|
|
This means that every MIDI event is timestamped with the sample
|
|
offset against the current graph clock cycle to get sample accurate
|
|
midi events that can be aligned with the corresponding sample data.
|
|
|
|
Since the MIDI events are embedded in the generic control stream,
|
|
they can be interleaved with other control message types, such as
|
|
property updates or OSC messages.
|
|
|
|
## The PipeWire Daemon
|
|
|
|
Nothing special is implemented for MIDI. Negotiation of formats
|
|
happens between `"application/control"` media types and buffers are
|
|
negotiated in the same way as any generic format.
|
|
|
|
## The Session Manager
|
|
|
|
The session manager needs to create the MIDI nodes/ports for the available
|
|
devices.
|
|
|
|
This can either be done as a single node with ports per device/stream
|
|
or as separate nodes created by a MIDI device monitor.
|
|
|
|
The session manager needs to be aware of the various MIDI sinks and sources
|
|
in order to route MIDI streams to them from applications that want this.
|
|
|
|
|
|
# Implementation
|
|
|
|
## PipeWire Media Session
|
|
|
|
PipeWire media session uses the \ref SPA_NAME_API_ALSA_SEQ_BRIDGE plugin for
|
|
the MIDI features. This creates a single SPA Node with ports per
|
|
MIDI client/stream.
|
|
|
|
The media session will check the permissions on `/dev/snd/seq` before
|
|
attempting to create this node. It will also use inotify to wait
|
|
until the sequencer device node is accessible.
|
|
|
|
## JACK
|
|
|
|
JACK assumes all `"application/control"` ports are MIDI ports.
|
|
|
|
The control messages are converted to the JACK event format by
|
|
filtering out the \ref SPA_CONTROL_Midi types. On output ports, the JACK
|
|
event stream is converted to control messages in a similar way.
|
|
|
|
There is a 1 to 1 mapping between the JACK events and control
|
|
messages so there is no information loss or need for complicated
|
|
conversions.
|
|
|
|
*/
|