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### The `midi` backend
The MIDI backend provides read-write access to the MIDI protocol via virtual ports.
#### Global configuration
| Option | Example value | Default value | Description |
|---------------|-----------------------|-----------------------|-----------------------|
| `name` | `MIDIMonster` | none | MIDI client name |
| `detect` | `on` | `off` | Output channel specifications for any events coming in on configured instances to help with configuration. |
#### Instance configuration
| Option | Example value | Default value | Description |
|---------------|-----------------------|-----------------------|-----------------------|
| `read` | `20:0` | none | MIDI device to connect for input |
| `write` | `DeviceName` | none | MIDI device to connect for output |
| `epn-tx` | `short` | `full` | Configures whether to clear the active parameter number after transmitting an `nrpn` or `rpn` parameter |
MIDI device names may either be `client:port` portnames or prefixes of MIDI device names.
Run `aconnect -i` to list input ports and `aconnect -o` to list output ports.
Each instance also provides a virtual port, so MIDI devices can also be connected with `aconnect <sender> <receiver>`.
#### Channel specification
The MIDI backend supports mapping different MIDI events to MIDIMonster channels. The currently supported event types are
* `cc` - Control Changes
* `note` - Note On/Off messages
* `pressure` - Note pressure/aftertouch messages
* `aftertouch` - Channel-wide aftertouch messages
* `pitch` - Channel pitchbend messages
* `rpn` - Registered parameter numbers (14bit extension)
* `nrpn` - Non-registered parameter numbers (14bit extension)
A MIDIMonster channel is specified using the syntax `channel<channel>.<type><index>`. The shorthand `ch` may be
used instead of the word `channel` (Note that `channel` here refers to the MIDI channel number).
The `pitch` and `aftertouch` events are channel-wide, thus they can be specified as `channel<channel>.<type>`.
MIDI channels range from `0` to `15`. Each MIDI channel consists of 128 notes (numbered `0` through `127`), which
additionally each have a pressure control, 128 CC's (numbered likewise), a channel pressure control (also called
'channel aftertouch') and a pitch control which may all be mapped to individual MIDIMonster channels.
Every channel also provides `rpn` and `nrpn` controls, which are implemented on top of the MIDI protocol, using
the CC controls 101/100/99/98/38/6. Both control types have 14-bit IDs and 14-bit values.
Example mappings:
```
midi1.ch0.note9 > midi2.channel1.cc4
midi1.channel15.pressure1 > midi1.channel0.note0
midi1.ch1.aftertouch > midi2.ch2.cc0
midi1.ch0.pitch > midi2.ch1.pitch
midi1.ch0.nrpn900 > midi2.ch0.rpn1
```
#### Known bugs / problems
Extended parameter numbers (`rpn` and `nrpn` control types) can currently only be transmitted, not properly
received as such. Support for this functionality is planned.
To access MIDI data, the user running MIDIMonster needs read & write access to the ALSA sequencer.
This can usually be done by adding this user to the `audio` system group.
Currently, no Note Off messages are sent (instead, Note On messages with a velocity of 0 are
generated, which amount to the same thing according to the spec). This may be implemented as
a configuration option at a later time.
To see which events your MIDI devices output, ALSA provides the `aseqdump` utility. You can
list all incoming events using `aseqdump -p <portname>`.
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