devices/audio/midi_arch.jd

page.title=MIDI Architecture
@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>
This article describes the generic MIDI architecture, independent of
any platform implementation, API, or platform-specific features.
</p>

<h2 id="keyConcepts">Key concepts</h2>

<h3 id="events">Events</h3>

<p>
The MIDI protocol is designed for event-based communication.
An <a href="https://en.wikipedia.org/wiki/Event_(computing)">event</a>
is an indication that something happened or will happen at a specified
time. MIDI events are represented by <em>messages</em>, atomic
bundles of information.
</p>

<h3 id="transport">Transport</h3>

<p>
MIDI messages are encoded and delivered via a
<a href="https://en.wikipedia.org/wiki/Transport_layer">transport layer</a>,
abbreviated <em>transport</em>, which sends the raw MIDI data
to the recipient who then decodes the data into messages.
</p>

<p>
Hardware-based MIDI transports include:
</p>
<ul>
<li>MIDI 1.0 current loop with
<a href="https://en.wikipedia.org/wiki/DIN_connector">5-pin DIN</a> connector</li>
<li>USB</li>
<li>Bluetooth Low Energy (BLE)</li>
</ul>

<h3 id="messageRepresentation">Message representation</h3>

<p>
A MIDI transport specification describes how to convey messages.
Although the packaging of messages is transport-specific at the
lowest level, at a higher level applications can consider a
time-ordered sequence of messages to be a demarcated
<a href="https://en.wikipedia.org/wiki/Bytestream">byte stream</a>.
This is possible because each message contains
enough information to determine the total length of the message,
provided the start of the message boundary is known.
</p>

<p>
Most MIDI messages are short (one to three bytes), yet there is the
capability for longer messages via <em>SysEx</em>.
</p>

<h3 id="timestamps">Timestamps</h3>

<p>
A <a href="https://en.wikipedia.org/wiki/Timestamp">timestamp</a>
is an optional label attached to a message at origination or upon receipt,
depending on the transport.  The timestamp is expressed in time units
such as seconds or
<a href="https://en.wikipedia.org/wiki/Jiffy_(time)">ticks</a>.
</p>

<p>
In the absence of an explicit timestamp, the system must substitute
the timestamp of the immediately preceding message or the current
time.  The accuracy of these timestamps, whether explicit or implicit,
is an important aspect of the reliability of a MIDI-based system.
</p>

<p>
Timestamps are not part of the MIDI 1.0 protocol. They are often added
as part of a platform-specific API.  The BLE transport has timestamps
to indicate the timing of the multiple individual messages sent within
one BLE packet.
</p>

<h3 id="devices">Devices</h3>

<p>
A <a href="https://en.wikipedia.org/wiki/Peripheral">peripheral</a>
provides input/output (I/O) capability for a computer.  The terms
<em>MIDI peripheral</em> and <em>MIDI device</em> commonly
refer to any hardware or software module that supports the MIDI protocol.
Within this document, <em>MIDI peripheral</em> refers to the
physical entity and <em>MIDI device</em> describes the module that
actually implements MIDI.
</p>

<h3 id="ports">Ports</h3>

<p>
A <a href="https://en.wikipedia.org/wiki/Computer_port_(hardware)">port</a>
is an interface point between computers and peripherals.
</p>

<p>
MIDI 1.0 uses a female 5-pin DIN socket as the port.
Each port is either <em>OUT</em> (source of MIDI data), <em>IN</em> (sink for MIDI data),
or <em>THRU</em> (meaning an <em>IN</em> which is directly routed to an <em>OUT</em>).
</p>

<p>
Other transports such as USB and BLE extend the
<a href="https://en.wikipedia.org/wiki/Computer_port_(software)">port concept</a>.
</p>

<p>
A MIDI device has at least one <em>OUT</em> port, <em>IN</em> port, or both.
</p>

<p>
The MIDI device supplies stream(s) of messages originating at each <em>OUT</em> port,
and receives stream(s) of messages arriving at each <em>IN</em> port.
The terms <em>IN</em> and <em>OUT</em> are of course relative to one port;
from the perspective of the other port the reverse term applies.
</p>

<h3 id="connection">Connection</h3>

<p>
In the MIDI 1.0 transport, an <em>OUT</em> port connects to at most
one <em>IN</em> or <em>THRU</em> port due to the nature of the current loop.
In USB and BLE transports, the same is true at the lowest layer, though
an implementation may re-condition the message stream so that it can
be broadcast to multiple <em>IN</em> ports.
</p>

<h3 id="cable">Cables</h3>

<p>
A MIDI 1.0 <a href="https://en.wikipedia.org/wiki/Cable">cable</a> is the
physical bundle of wires that connects an <em>OUT</em> port to an <em>IN</em> or <em>THRU</em> port.
The cable carries data only.
</p>

<p class="note">
<strong>Note:</strong>
There are non-standard modifications to MIDI that supply power over the
two unused pins.  This is called <em>phantom power</em>.
</p>

<p>
A <a href="https://en.wikipedia.org/wiki/USB#Cabling">USB cable</a>
is similar, except there is a wide variety of connector types,
and the <em>IN</em>/<em>OUT</em>/<em>THRU</em> concept is replaced by the host/peripheral role.
</p>

<p>
When operating in USB host mode, the host device supplies power to the
MIDI peripheral.  Most small MIDI peripherals take one USB unit load (100
mA) or less.  However some larger peripherals, or peripherals with audio
output or lights, require more power than the host device can supply.
If you experience problems, try another MIDI peripheral or a powered
USB hub.
</p>

<h3 id="channel">Channel</h3>

<p>
Each MIDI message stream in multiplexed among 16 <em>channels</em>.
Most messages are directed at a specific channel,
but there are message types that aren't channel-specific.
Conventionally the channels are numbered one to 16, though
represented by channel values of zero to 15.
</p>

<p>
If the application needs more than 16 channels or a higher throughput
than one message stream can support, then multiple ports
must be used.
</p>

<p>
In MIDI 1.0, this is accomplished by multiple cables connecting pairs of ports.
</p>

<p>
In the MIDI over USB transport, a single USB endpoint can support multiple
ports, each identified by a <em>cable number</em> [sic].
According to the USB MIDI specification,
the <em>cable number</em> identifies the virtual port within the endpoint.
</p>

<p class="note">
<strong>Note:</strong>
<em>port number</em> would have been a more accurate term,
given that it identifies a port.
</p>

<p>
Thus a single USB physical cable can carry more than one set of 16 channels.
</p>

<h2 id="platformImplementation">Platform implementation</h2>

<p>
As noted in the introduction, these generic MIDI concepts apply to all
implementations.  For the interpretation of the concepts on the Android
platform, see the
<a href="http://developer.android.com/reference/android/media/midi/package-summary.html">
Android MIDI User Guide for <code>android.media.midi</code></a>.
</p>