Android MIDI User Guide

# Overview

This document describes how to use the Android MIDI API in Java.

The Android MIDI package allows users to:

- Connect a MIDI keyboard to Android to play a synthesizer or drive music apps.
- Connect alternative MIDI controllers to Android.
- Drive external MIDI synths from Android.
- Drive external peripherals, lights, show control, etc from Android.
- Generate music dynamically from games or music creation apps.
- Generate MIDI messages in one app and send them to a second app.
- Use an Android device running in peripheral mode as a multitouch controller connected to a laptop.

## The API features include:

- Enumeration of currently available devices. Information includes name, vendor, capabilities, etc.
- Provide notification when MIDI devices are plugged in or unplugged.
- Support efficient transmission of single or multiple short 1-3 byte MIDI messages.
- Support transmission of arbitrary length data for SysEx, etc.
- Timestamps to avoid jitter.
- Support creation of virtual MIDI devices that can be connected to other devices. An example might be a synthesizer app that can be controlled by a composing app.
- Support direct connection or “patching” of devices for lower latency.

## Transports Supported

The API is “transport agnostic”. But there are several transports currently supported:

- USB
- software routing
- BTLE

# Android MIDI Terminology

## Terminology

A Device is a MIDI capable object that has zero or more InputPorts and OutputPorts.

An InputPort has 16 channels and can receive MIDI messages from an OutputPort or an app.

An OutputPort has 16 channels and can send MIDI messages to an InputPort or an app.

MidiService is a centralized process that keeps track of all devices and brokers communication between them.

MidiManager is a class that the application or a device manager calls to communicate with the MidiService.

# Writing a MIDI Application

## Declare Feature in Manifest

An app that requires the MIDI API should declare that in the AndroidManifest.xml file. Then the app will not appear in the Play Store for old devices that do not support the MIDI API.

    <uses-feature android:name="android.software.midi" android:required="true"/>

## Check for Feature Support

An app can also check at run-time whether the MIDI feature is supported on a platform. This is particularly useful during development when you install apps directly on a device.

    if (context.getPackageManager().hasSystemFeature(PackageManager.FEATURE_MIDI)) {
        // do MIDI stuff
    }

## The MidiManager

The primary class for accessing the MIDI package is through the MidiManager.

    MidiManager m = (MidiManager)context.getSystemService(Context.MIDI_SERVICE);

## Get List of Already Plugged In Entities

When an app starts, it can get a list of all the available MIDI devices. This information can be presented to a user, allowing them to choose a device.

    MidiDeviceInfo[] infos = m.getDevices();

## Notification of MIDI Devices HotPlug Events

The application can request notification when, for example, keyboards are plugged in or unplugged.

    m.registerDeviceCallback(new MidiManager.DeviceCallback() {
        public void onDeviceAdded( MidiDeviceInfo info ) {
          ...
        }
        public void onDeviceRemoved( MidiDeviceInfo info ) {
          ...
        }
      });

## Device and Port Information

You can query the number of input and output ports.

    int numInputs = info.getInputPortCount();
    int numOutputs = info.getOutputPortCount();

Note that “input” and “output” are from the standpoint of the device. So a synthesizer will have an “input” port that receives messages. A keyboard will have an “output” port that sends messages.

The MidiDeviceInfo has a bundle of properties.

    Bundle properties = info.getProperties();
    String manufacturer = properties
          .getString(MidiDeviceInfo.PROPERTY_MANUFACTURER);

Other properties include PROPERTY_PRODUCT, PROPERTY_NAME, PROPERTY_SERIAL_NUMBER

You can get the names and types of the ports from a PortInfo object. The type will be either TYPE_INPUT or TYPE_OUTPUT.

    MidiDeviceInfo.PortInfo[] portInfos = info.getPorts();
    String portName = portInfos[0].getName();
    if (portInfos[0].getType() == MidiDeviceInfo.PortInfo.TYPE_INPUT) {
        ...
    }

## Open a MIDI Device

To access a MIDI device you need to open it first. The open is asynchronous so you need to provide a callback for completion. You can specify an optional Handler if you want the callback to occur on a specific Thread.

    m.openDevice(info, new MidiManager.OnDeviceOpenedListener() {
        @Override
        public void onDeviceOpened(MidiDevice device) {
            if (device == null) {
                Log.e(TAG, "could not open device " + info);
            } else {
                ...
            }, new Handler(Looper.getMainLooper())
        );

## Open a MIDI Input Port

If you want to send a message to a MIDI Device then you need to open an “input” port with exclusive access.

    MidiInputPort inputPort = device.openInputPort(index);

## Send a NoteOn

MIDI messages are sent as byte arrays. Here we encode a NoteOn message.

    byte[] buffer = new byte[32];
    int numBytes = 0;
    int channel = 3; // MIDI channels 1-16 are encoded as 0-15.
    buffer[numBytes++] = (byte)(0x90 + (channel - 1)); // note on
    buffer[numBytes++] = (byte)60; // pitch is middle C
    buffer[numBytes++] = (byte)127; // max velocity
    int offset = 0;
    // post is non-blocking
    inputPort.send(buffer, offset, numBytes);

Sometimes it is convenient to send MIDI messages with a timestamp. By scheduling events in the future we can mask scheduling jitter. Android MIDI timestamps are based on the monotonic nanosecond system timer. This is consistent with the other audio and input timers.

Here we send a message with a timestamp 2 seconds in the future.

    final long NANOS_PER_SECOND = 1000000000L;
    long now = System.nanoTime();
    long future = now + (2 * NANOS_PER_SECOND);
    inputPort.send(buffer, offset, numBytes, future);

If you want to cancel events that you have scheduled in the future then call flush().

    inputPort.flush(); // discard events

If there were any MIDI NoteOff message left in the buffer then they will be discarded and you may get stuck notes. So we recommend sending “all notes off” after doing a flush.

## Receive a Note

To receive MIDI data from a device you need to extend MidiReceiver. Then connect your receiver to an output port of the device.

    class MyReceiver extends MidiReceiver {
        public void onSend(byte[] data, int offset,
                int count, long timestamp) throws IOException {
            // parse MIDI or whatever
        }
    }
    MidiOutputPort outputPort = device.openOutputPort(index);
    outputPort.connect(new MyReceiver());

The data that arrives is not validated or aligned in any particular way. It is raw MIDI data and can contain multiple messages or partial messages. It might contain System Real-Time messages, which can be interleaved inside other messages.

# Creating a MIDI Virtual Device Service

An app can provide a MIDI Service that can be used by other apps. For example, an app can provide a custom synthesizer that other apps can send messages to. The service must be guarded with permission "android.permission.BIND_MIDI_DEVICE_SERVICE".

## Manifest Files

An app declares that it will function as a MIDI server in the AndroidManifest.xml file.

    <service android:name="MySynthDeviceService"
      android:permission="android.permission.BIND_MIDI_DEVICE_SERVICE">
      <intent-filter>
        <action android:name="android.media.midi.MidiDeviceService" />
      </intent-filter>
      <meta-data android:name="android.media.midi.MidiDeviceService"
          android:resource="@xml/synth_device_info" />
    </service>

The details of the resource in this example is stored in “res/xml/synth_device_info.xml”. The port names that you declare in this file will be available from PortInfo.getName().

    <devices>
        <device manufacturer="MyCompany" product="MidiSynthBasic">
            <input-port name="input" />
        </device>
    </devices>

## Extend MidiDeviceService

You then define your server by extending android.media.midi.MidiDeviceService. Let‘s assume you have a MySynthEngine class that extends MidiReceiver.

    import android.media.midi.MidiDeviceService;
    import android.media.midi.MidiDeviceStatus;
    import android.media.midi.MidiReceiver;

    public class MidiSynthDeviceService extends MidiDeviceService {
        private static final String TAG = "MidiSynthDeviceService";
        private MySynthEngine mSynthEngine = new MySynthEngine();
        private boolean synthStarted = false;

        @Override
        public void onCreate() {
            super.onCreate();
        }

        @Override
        public void onDestroy() {
            mSynthEngine.stop();
            super.onDestroy();
        }

        @Override
        // Declare the receivers associated with your input ports.
        public MidiReceiver[] onGetInputPortReceivers() {
            return new MidiReceiver[] { mSynthEngine };
        }

        /**
         * This will get called when clients connect or disconnect.
         * You can use it to turn on your synth only when needed.
         */
        @Override
        public void onDeviceStatusChanged(MidiDeviceStatus status) {
            if (status.isInputPortOpen(0) && !synthStarted) {
                mSynthEngine.start();
                synthStarted = true;
            } else if (!status.isInputPortOpen(0) && synthStarted){
                mSynthEngine.stop();
                synthStarted = false;
            }
        }
    }

# Using MIDI Over Bluetooth LE

MIDI devices can be connected to Android using Bluetooth LE.

Before using the device, the app must scan for available BTLE devices and then allow the user to connect. An example program will be provided so look for it on the Android developer website.

## Request Location Permission for BTLE

Applications that scan for Bluetooth devices must request permission in the manifest file. This LOCATION permission is required because it may be possible to guess the location of an Android device by seeing which BTLE devices are nearby.

    <uses-permission android:name="android.permission.BLUETOOTH"/>
    <uses-permission android:name="android.permission.BLUETOOTH_ADMIN"/>
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION"/>

Apps must also request location permission from the user at run-time. See the documentation for `Activity.requestPermissions()` for details and an example.

## Scan for MIDI Devices

The app will only want to see MIDI devices and not mice or other non-MIDI devices. So construct a ScanFilter using the UUID for standard MIDI over BTLE.

    MIDI over BTLE UUID = "03B80E5A-EDE8-4B33-A751-6CE34EC4C700"

## Open a MIDI Bluetooth Device

See the documentation for `android.bluetooth.le.BluetoothLeScanner.startScan()` method for details. When the user selects a MIDI/BTLE device then you can open it using the MidiManager.

    m.openBluetoothDevice(bluetoothDevice, callback, handler);

Once the MIDI/BTLE device has been opened by one app then it will also become available to other apps using the MIDI device discovery calls described above.