src/devices/audio/midi.jd

page.title=MIDI
@jd:body

<!--
    Copyright 2015 The Android Open Source Project

    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
    You may obtain a copy of the License at

        http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
-->

<div id="qv-wrapper">
  <div id="qv">
    <h2>In this document</h2>
    <ol id="auto-toc">
    </ol>
  </div>
</div>

<p>
<a href="http://en.wikipedia.org/wiki/MIDI">MIDI</a> (Musical Instrument Digital Interface)
is a standard protocol for inter-connecting computers with musical instruments, stage lighting,
and other time-oriented media.
</p>

<p>
Strictly speaking, MIDI is unrelated to audio. But since MIDI is commonly used with
music, this article is placed in the audio section.
</p>

<h2 id="transports">Transports</h2>

<p>
The physical <a href="http://en.wikipedia.org/wiki/Transport_layer">transport layer</a>
specified in original MIDI 1.0 is a current loop with
<a href="http://en.wikipedia.org/wiki/DIN_connector">5-pin DIN</a> connector.
</p>

<p>
Since MIDI 1.0, additional transports have been defined, including MIDI over USB
and a proposed draft for MIDI over
<a href="http://en.wikipedia.org/wiki/Bluetooth_low_energy">Bluetooth Low Energy</a> (BLE).
</p>

<h2 id="for-android">MIDI for Android</h2>

<p>
Android 3.1 and later support
<a href="http://en.wikipedia.org/wiki/USB_On-The-Go">USB On-The-Go</a>,
which permits an Android device to act as USB host to drive USB
peripherals. The USB host mode APIs introduced in Android 3.1 permit
developers to implement MIDI over USB at the application level, but until
recently there have been no built-in platform APIs for MIDI.
</p>

<p>
Beginning with the Android 6.0 (Marshmallow) release, device makers can enable optional MIDI support in the platform.
Android directly supports USB, draft BLE, and virtual (inter-app) transports.
Android indirectly supports MIDI 1.0 via an external adapter.
</p>

<p>
For details on application programming with the new MIDI APIs, see the
<a href="https://developer.android.com/reference/android/media/midi/package-summary.html"><code>android.media.midi</code></a>
package.
</p>

<p>
The remainder of this article discusses how an Android device maker can
enable MIDI support in the platform.
</p>

<h2 id="transport">Enabling transports</h2>

<p>
The implementation depends on ALSA for USB host mode and USB peripheral mode transports.
ALSA is not used for the BLE and virtual transports.
</p>

<h3 id="usb-host">USB host mode</h3>

<p>
To enable MIDI for USB host mode, first support USB host mode in general, and
then enable <code>CONFIG_SND_RAWMIDI</code> and <code>CONFIG_SND_USB_MIDI</code> in your kernel configuration.
See <a href="{@docRoot}devices/tech/config/kernel.html">Android Kernel Configuration.</a>
</p>

<p>
The MIDI over USB transport is formally defined by the
<a href="http://www.usb.org/developers/docs/devclass_docs/midi10.pdf">
Universal Serial Bus Device Class Definition for MIDI Devices Release 1.0 Nov 1, 1999</a>
standard published by the
<a href="http://www.usb.org/">USB Implementers Forum, Inc</a>.
</p>

<h3 id="usb-peripheral">USB peripheral mode</h3>

<p>
To enable MIDI for USB peripheral mode, you may need to apply patches
to your Linux kernel to integrate the
<code>drivers/usb/gadget/f_midi.c</code> into the USB gadget
driver. As of this writing, these patches are available for Linux kernel version
3.10. These patches have not yet been updated for
<a href="http://en.wikipedia.org/wiki/Configfs">ConfigFs</a>
(a new architecture
for USB gadget drivers), nor are they merged at upstream
<a href="http://kernel.org">kernel.org</a>.
</p>

<p>
The patches are shown in commit order for the kernel tree at project <code>kernel/common</code>
branch <code>android-3.10</code>:
</p>
<ol>
<li><a href="https://android-review.googlesource.com/#/c/127450/">https://android-review.googlesource.com/#/c/127450/</a></li>
<li><a href="https://android-review.googlesource.com/#/c/127452/">https://android-review.googlesource.com/#/c/127452/</a></li>
<li><a href="https://android-review.googlesource.com/#/c/143714/">https://android-review.googlesource.com/#/c/143714/</a></li>
</ol>

<p>
In addition, the end user must also check the box for MIDI
in the <em>Settings / Developer options / Networking / Select USB Configuration</em> dialog,
or by pulling down from the top of screen while attached
to the USB host, selecting entry "USB for ...", and then choosing <strong>MIDI</strong>.
</p>

<h3 id="ble">BLE</h3>

<p>
MIDI over BLE is always enabled, provided the device supports BLE.
As this transport is in draft status, it is subject to change.
</p>

<p>
The following changes should be applied to platform branch <code>android-6.0.0_r1</code>
to improve stability of MIDI over BLE:
</p>
<ul>
<li><a href="https://android-review.googlesource.com/#/c/179844/">179844</a></li>
<li><a href="https://android-review.googlesource.com/#/c/179845/">179845</a></li>
<li><a href="https://android-review.googlesource.com/#/c/179846/">179846</a></li>
<li><a href="https://android-review.googlesource.com/#/c/179847/">179847</a></li>
<li><a href="https://android-review.googlesource.com/#/c/179848/">179848</a></li>
</ul>

<h3 id="virtual">Virtual (inter-app)</h3>

<p>
The virtual (inter-app) transport is always enabled.
</p>

<h2 id="claim-feature">Claiming the feature</h2>

<p>
Applications can screen for the presence of MIDI support using the
<code>android.software.midi</code> feature.
</p>

<p>
To claim MIDI support, add this line to your <code>device.mk</code>:
</p>
<pre>
PRODUCT_COPY_FILES += \
frameworks/native/data/etc/android.software.midi.xml:system/etc/permissions/android.software.midi.xml
</pre>

<p>
See the
<a href="{@docRoot}compatibility/android-cdd.pdf">Android Compatibility Definition Document (CDD)</a>
for information
on requirements to claim the feature.
</p>

<h2 id="hostDebugging">Debugging while in host mode</h2>

<p>
While in USB host mode, Android Debug Bridge (adb) debugging over USB is unavailable.
See section <a href="http://developer.android.com/tools/help/adb.html#wireless">Wireless usage</a>
of
<a href="http://developer.android.com/tools/help/adb.html">Android Debug Bridge</a>
for an alternative.
</p>