Android 6.0 (Marshmallow): How to play midi notes?

Question

I haven’t found any “official” way to control the internal synthesizer from Java code.

Probably the easiest option is to use the Android midi driver for the Sonivox synthesizer.

Get it as an AAR package (unzip the *.zip) and store the *.aar file somewhere in your workspace. The path doesn’t really matter and it doesn’t need to be inside your own app’s folder structure but the “libs” folder inside your project could be a logical place.

With your Android project open in Android Studio:

File -> New -> New Module -> Import .JAR/.AAR Package -> Next -> Find
and select the “MidiDriver-all-release.aar” and change the subproject
name if you want. -> Finish

Wait for Gradle to do it’s magic and then go to your “app” module’s settings (your own app project’s settings) to the “Dependencies” tab and add (with the green “+” sign) the MIDI Driver as a module dependency. Now you have access to the MIDI Driver:

import org.billthefarmer.mididriver.MidiDriver;
   ...
MidiDriver midiDriver = new MidiDriver();

Without having to worry anything about NDK and C++ you have these Java methods available:

// Not really necessary. Receives a callback when/if start() has succeeded.
midiDriver.setOnMidiStartListener(listener);
// Starts the driver.
midiDriver.start();
// Receives the driver's config info.
midiDriver.config();
// Stops the driver.
midiDriver.stop();
// Just calls write().
midiDriver.queueEvent(event);
// Sends a MIDI event to the synthesizer.
midiDriver.write(event);

A very basic “proof of concept” for playing and stopping a note could be something like:

package com.example.miditest;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;
import android.widget.Button;

import org.billthefarmer.mididriver.MidiDriver;

public class MainActivity extends AppCompatActivity implements MidiDriver.OnMidiStartListener,
        View.OnTouchListener {

    private MidiDriver midiDriver;
    private byte[] event;
    private int[] config;
    private Button buttonPlayNote;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        buttonPlayNote = (Button)findViewById(R.id.buttonPlayNote);
        buttonPlayNote.setOnTouchListener(this);

        // Instantiate the driver.
        midiDriver = new MidiDriver();
        // Set the listener.
        midiDriver.setOnMidiStartListener(this);
    }

    @Override
    protected void onResume() {
        super.onResume();
        midiDriver.start();

        // Get the configuration.
        config = midiDriver.config();

        // Print out the details.
        Log.d(this.getClass().getName(), "maxVoices: " + config[0]);
        Log.d(this.getClass().getName(), "numChannels: " + config[1]);
        Log.d(this.getClass().getName(), "sampleRate: " + config[2]);
        Log.d(this.getClass().getName(), "mixBufferSize: " + config[3]);
    }

    @Override
    protected void onPause() {
        super.onPause();
        midiDriver.stop();
    }

    @Override
    public void onMidiStart() {
        Log.d(this.getClass().getName(), "onMidiStart()");
    }

    private void playNote() {

        // Construct a note ON message for the middle C at maximum velocity on channel 1:
        event = new byte[3];
        event[0] = (byte) (0x90 | 0x00);  // 0x90 = note On, 0x00 = channel 1
        event[1] = (byte) 0x3C;  // 0x3C = middle C
        event[2] = (byte) 0x7F;  // 0x7F = the maximum velocity (127)

        // Internally this just calls write() and can be considered obsoleted:
        //midiDriver.queueEvent(event);

        // Send the MIDI event to the synthesizer.
        midiDriver.write(event);

    }

    private void stopNote() {

        // Construct a note OFF message for the middle C at minimum velocity on channel 1:
        event = new byte[3];
        event[0] = (byte) (0x80 | 0x00);  // 0x80 = note Off, 0x00 = channel 1
        event[1] = (byte) 0x3C;  // 0x3C = middle C
        event[2] = (byte) 0x00;  // 0x00 = the minimum velocity (0)

        // Send the MIDI event to the synthesizer.
        midiDriver.write(event);

    }

    @Override
    public boolean onTouch(View v, MotionEvent event) {

        Log.d(this.getClass().getName(), "Motion event: " + event);

        if (v.getId() == R.id.buttonPlayNote) {
            if (event.getAction() == MotionEvent.ACTION_DOWN) {
                Log.d(this.getClass().getName(), "MotionEvent.ACTION_DOWN");
                playNote();
            }
            if (event.getAction() == MotionEvent.ACTION_UP) {
                Log.d(this.getClass().getName(), "MotionEvent.ACTION_UP");
                stopNote();
            }
        }

        return false;
    }
}

The layout file just has one button that plays the predefined note when held down and stops it when released:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.miditest.MainActivity"
    android:orientation="vertical">

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Play a note"
        android:id="@+id/buttonPlayNote" />
</LinearLayout>

It is actually this simple. The code above could well be a starting point for a touch piano app with 128 selectable instruments, very decent latency and a proper “note off” functionality which many apps lack.

As for choosing the instrument: You’ll just need to send a MIDI “program change” message to the channel on which you intend to play to choose one of the 128 sounds in the General MIDI soundset. But that’s related to the details of MIDI and not to the usage of the library.

Likewise you’ll probably want to abstract away the low level details of MIDI so that you can easily play a specific note on a specific channel with a specific instrument at a specific velocity for a specific time and for that you might find some clues from all the open source Java and MIDI related applications and libraries made so far.

This approach doesn’t require Android 6.0 by the way. And at the moment only 4.6 % of devices visiting the Play Store run Android 6.x so there wouldn’t be much audience for your app.

Of course if you want to use the android.media.midi package you could then use the library to implement a android.media.midi.MidiReceiver to receive the MIDI events and play them on the internal synthesizer. Google already has some demo code that plays notes with square and saw waves. Just replace that with the internal synthesizer.

Some other options could be to check out what’s the status with porting FluidSynth to Android. I guess there might be something available.

Edit: Other possibly interesting libraries:

port of Java’s javax.sound.midi package for abstracting the low level MIDI technical details
USB MIDI Driver for connecting to a digital piano/keyboard with a USB MIDI connector
MIDI over Bluetooth LE driver for connecting wirelessly to a digital piano/keyboard that supports MIDI over Bluetooth LE (like e.g. some recent Roland and Dexibell digital pianos)
JFugue Music library port for Android for further abstracting the MIDI details and instead thinking in terms of music theory

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