ktmidi: Kotlin Multiplatform library for MIDI 1.0 and MIDI 2.0

ktmidi is a Kotlin Multiplatform library for MIDI Access API and MIDI data processing that covers MIDI 1.0 and MIDI 2.0.

Features

It provides various MIDI features, including:

• MIDI 1.0 messages and 2.0 UMPs everywhere.
• MidiAccess : MIDI access abstraction API like what Web MIDI API 1.0 provides.
  • There are actual implementations for some platform specific MIDI API within this library, and you can implement your own backend if you need.
  • Unlike javax.sound.midi API, this API also covers creating virtual ports wherever possible.
    • ktmidi-jvm-desktop module contains ALSA backend as well as RtMidi backend via atsushieno/rtmidi-jna that support it (Unsupported platforms are left unsupported). Note that they are for Kotlin/JVM.
    • ktmidi-native-ext module contains RtMidi native backend for Kotlin-Native. It is built for both static and shared, and known to work as player-sample-native sample app module.
  • For Kotlin/JS, JzzMidiAccess which wraps Jazz-Soft JZZ is included. It should cover both node.js and web browsers.
• MidiMusic and Midi2Music : reflects Standard MIDI File format structure, with reader and writer. (MIDI 2.0 support is not based on standard, as there is nothing like SMF specification for MIDI 2.0 yet.)
  • No strongly-typed message types (something like NoteOnMessage, NoteOffMessage, and so on). There is no point of defining strongly-typed messages for each mere MIDI status byte - you wouldn't need message type abstraction.
  • No worries, there are contants of Int or Byte in MidiChannelStatus, MidiCC, MidiRpn, MidiMetaType etc. so that you don't have to remember the actual constant numbers.
  • MidiMusic.read() reads and MidiMusic.write() writes to SMF (standard MIDI format) files with MIDI messages, with Midi1TrackMerger, Midi2TrackMerger, Midi1TrackSplitter and Midi2TrackSplitter that help you implement sequential event processing for your own MIDI players, or per-track editors if needed.
  • UmpFactory and UmpRetriever provides various accessors to MIDI 2.0 Ump data class, as well as some preliminary MIDI-CI support in CIFactory class.
• MidiPlayer and Midi2Player: provides MIDI player functionality: play/pause/stop and fast-forwarding.
  • Midi messages are sent to its "message listeners". If you don't pass a Midi Access instance or a Midi Output instance, it will do nothing but event dispatching.
  • It is based on customizible scheduler MidiPlayerTimer.
  • Not realtime strict (as on GC-ed language / VM), but would suffice for general usage.

There are three sample project modules for two sample programs:

• player-sample is an example console MIDI player for Kotlin/JVM desktop.
• player-sample-native is almost the same, but for Kotlin/Native desktop.
• input-sample is an example console MIDI input receiver that dumps the MIDI messages it received, for Kotlin/JVM desktop.

Using ktmidi

Here is an example code excerpt to set up platform MIDI device access, load an SMF from some file, and play it:

// for some complicated reason we don't have simple "default" MidiAccess API instance
val access = if(File("/dev/snd/seq").exists()) AlsaMidiAccess() else JvmMidiAccess()
val bytes = Files.readAllBytes(Path.of(fileName)).toList()
val music = MidiMusic()
music.read(bytes)
val player = MidiPlayer(music, access)
player.play()

To use ktmidi, add the following lines in the dependencies section in build.gradle(.kts):

dependencies {
    implementation "dev.atsushieno:ktmidi:+" // replace + with the actual version
}

The actual artifact might be platform dependent like dev.atsushieno:ktmidi-android:+ or dev.atsushieno:ktmidi-js:+, depending on the project targets.

If you want to bring better user experience on desktop (which @atsushieno recommends as javax.sound.midi on Linux is quite featureless), add ktmidi-jvm-desktop too,

dependencies {
    implementation "dev.atsushieno:ktmidi-jvm-desktop:+" // replace + with the actual version
}

... and use AlsaMidiAccess on Linux, or RtMidiAccess elsewhere. I use if (File.exists("/dev/snd/seq")) AlsaMidiAccess() else RtMidiAccess() (or JvmMidiAccess instead of RtMidiAccess) to create best MidiAccess instance.

ktmidi is released at sonatype and hence available at Maven Central.

Platform Access API

For platform MIDI access API, we cover the following APIs:

• AndroidMidiAccess: Android MIDI API (in Kotlin)
• AlsaMidiAccess: ALSA sequencer
• RtMidiAccess: RtMidi (which covers Windows, Mac, Linux, and iOS, but iOS is in general excluded in JVM solution. Also note that rtmidi-jna contains prebuilt binaries only for those x86_64 desktop targets. Due to the jar packaging issue, we cannot provide automatic native library loading solution. You are supposed to set up rtmidi 5.0.x locally.)
• JvmMidiAccess: javax.sound.midi API (with limited feature set).

For dependency resolution reason, ALSA implementation and RtMidi implementation are split from ktmidi-jvm and formed as ktmidi-jvm-desktop.

ktmidi builds for Kotlin/JVM, Kotlin/JS and Kotlin/Native (though I only develop with Kotlin/JVM and Kotlin/JS so far).

The entire API is still subject to change, and it had been actually radically changing when development was most active.

MIDI 2.0 support

ktmidi supports MIDI 2.0. It can be either in our own manner (i.e. it presumes MIDI 2.0 protocol is already established elsewhere), or partially using MIDI-CI protocol. It is up to apps... for example, atsushieno/kmmk sends MICI-CI "Set New Protocol" message to the output devices, which kind of declares that it will be sending MIDI 2.0 UMPs (but without "Test New Protocol", as it requires bidirectional messaging that kmmk does not implement). If you want to follow MIDI-CI system exclusive messages, establish pair of MIDI input and output and handle message exchanges manually.

ktmidi assumes there are various other use-cases without those message exchanges e.g. use of UMPs in MIDI 2.0-only messaging in apps or audio plugins.

Since you can derive from MidiAccess abstract API, you can create your own MIDI access implementation and don't have to wait for platform native API to support MIDI 2.0. Note that MIDI 2.0 support is still an experimental work.

It would be useful for general MIDI 2.0 software tools such as MIDI 2.0 UMP player.

Here is a list of MIDI 2.0 extensibility in this API:

• MidiInput and MidiOutput now has midiProtocol property which can be get and/or set. When MidiCIProtocolType.MIDI2 is specified, then the I/O object is supposed to process UMPs (Universal MIDI Packets).
• Midi2Music is a feature parity with MidiMusic, but all the messages are stored as UMPs. However, since SMF concepts of time calculation (namely delta time quantization / specification) is useful, we optionally blend it into UMPs and their JR Timestamp messages are actually fake - they store delta times just like SMF. Also there are tailored support for meta events equivalent. See docs/design/MidiMusic.md for details.
• Midi2Player is a feature parity with MidiPlayer.
• UmpFactory class contains a bunch of utility functions that are used to construct UMP integer values.
• dev.atsushieno.ktmidi.ci package contains a bunch of utility functions that are used to construct MIDI-CI system exclusive packets.

atsushieno/kmmk supports "MIDI 2.0 mode" which sends MIDI messages in MIDI 2.0 UMPs. There is also an ongoing experimental project to process MIDI 2.0 UMPs in audio plugins on Android.

SMF alternative format

Since there is no comparable standard music file format like SMF for MIDI 2.0, we had to come up with our own. See docs/MIDI2_FORMATS.md for details.

mugene-ng can generate music files based on this format.

Historical background

It started as the Kotlin port of C# managed-midi library. Also it started with partial copy of fluidsynth-midi-service-j project.

However everything in this project went far beyond them and now we are making it usable for MIDI 2.0.

Some of the MIDI 2.0 related bits are ported from cmidi2 library.

Historically ktmidi-jvm-desktop used to reside in its own repository to avoid complicated dependency resolution, so there would be some outdated information that tells it was isolated from this project/repository.

Resources

We use GitHub issues for bug reports etc., and GitHub Discussions boards open to everyone.

For hacking and/or contributing to ktmidi, please have a look at docs/HACKING.md.

For Applications that use ktmidi, check out this Discussions thread.

API documentation is published at: https://atsushieno.github.io/ktmidi/

The documentation can be built using ./gradlew dokkaHtml and it will be generated locally at build/dokka/html.

There are couple of API/implementation design docs:

• docs/design/MidiAccess.md
• docs/design/MidiMusic.md
• docs/design/MidiPlayer.md

You can also find some real-world usage examples:

• kmmk for MidiAccess/MidiOutput
• mugene-ng for SMF/MIDI2 file generator
• augene-ng for SMF/MIDI2 file reader

License

ktmidi is distributed under the MIT license.

jazz-soft/JZZ is included in ktmidi-js package and is distributed under the MIT license.

rtmidi is included in ktmidi-native-ext package and is distributed under the MIT license. (It is also indirectly referenced via rtmidi-jna, which may be a different version.)