WebMidi for Elm

Web MIDI API for Elm language

Web MIDI API on W3C

Don't look back, the lemmings are gaining on you.

Implementation details

Obtaining Access to MIDI Devices implemented as Task to bind it nicely to promises returned by requestMIDIAccess(), open() and close() methods.

The onstatechange of MIDIAccess object fire task provided to WebMidi.requestMIDIAccess in parameters hash.

All WebMidi.enableInput events from input deviced multiplexed on two signals - WebMidi.channel and WebMidi.system.

WebMidi.enableOutput handle output device and bind it with ChannelMessage and SystemMessage mailboxes .

Channel messages coreponds to Note events and have arout 20ms jitter time until become sensible to processing delay. System messages can be handled in 1s message loop.

There is possibility to butch MIDI events out to the system by declaring mailbox as List of ChannelMessages. Then you can select all events for next tick and send them in one burst.

Examples

Midi API examples mixed up with code from The Haskell School of Expression chapters on computer music.

List MIDI Ports

Demonstrate how to request access to MIDI system.

  WebMidi.requestMIDIAccess defaultSettings

DEMO

Play a Note

• Access MIDI sysbsystem
• Open Synch input port and associate it to output port via mailbox
• Send Event with encoded note to the mailbox

  synch = "Synth input port (16600:0)"

  WebMidi.requestMIDIAccess defaultSettings
           `andThen` \midi -> WebMidi.open (withDefault "none"
                                            (selectInstrument synch midi.outputs)) midiOut.signal
           `andThen` \p -> Signal.send midiOut.address (encodeChannelEvent c4on 0)

DEMO

Listent to input events from keyboard

• Open input port by name. Second argument is WebMidi.channel which is multiplexed input port for all instruments.
• Listen on all input events: channel and system. Those from system comming on predefined WebMidi.system port.

  keyboard = "Virtual Keyboard"

  WebMidi.requestMIDIAccess defaultSettings
           `andThen` \midi ->
             WebMidi.open (withDefault "none" (selectInstrument keyboard midi.inputs)) WebMidi.channel


  main =
    Signal.map show (Signal.map2 (,) WebMidi.channel WebMidi.system)

DEMO

Perform music

• Eccess MIDI out port as in Ex. 2

• Chaine WebMidi.jiffy task which fetch current performance.now() time from browser.

• Use jiffy as time reference to serialize track of MIDI events via task sequencer

• Events track extracted from MIDI File structure which resemble MIDI Type 1 file with stream per instrument track list.

• Compose simple tune

cMaj = [c,e',g] |> List.map (\n -> n 4 hn)

cMajArp = Music.line  cMaj
cMajChd = Music.chord cMaj

tune : Music
tune = (Music.repeatM 3 cMajArp) :+: cMajChd

• Convert it to performance

ctx : Context
ctx = Context 0 AcousticGrandPiano 3 0

performance : Performance
performance = performM ctx tune

DEMO

Configuring MIDI Synch on Linux

• Start JACK

qjackctl &

• Add Virtual Keyboard

vkeybd &

• Chose Synthetizer

zynaddsubfx &

qsynth &

timidity -iA

Install timidity on debian (it sucks big time)

Add deb http://www.fbriere.net/debian stable misc >> /etc/apt/sources.list

sudo apt-get update sudo apt-get install timidity timidity-interfaces-extra timidity-daemon sudo apt-get install eawpatches

Configure /etc/timidity/timidity.cfg add source /etc/timidity/eawpatches.cfg

Link audio inputs and MIDI instruments in qjackl UI.

Using instruments

Piano interface. Under test! DEMO