MIDI

1
MIDI

• Musical Instrument Digital Interface

A digital communication protocol allowing devices
to send and/or respond to instructions
2
Function of MIDI

• The MIDI protocol allows devices to exchange
  various types of instructions
• MIDI data is not an audio signal
• As a comparison
• a tape or CD contains a description of an audio
  signal
• a printed page of music contains instructions
  that a (human) instrumentalist is trained to
  carry out
• a MIDI file contains instructions that a
  (computer) instrumentalist is trained to carry out

3
Function of MIDI

• Thus, a MIDI file is small and easily transmitted
• It contains instructions only, leaving the
  creation of the actual audio signal to the
  receiving device
• Instruction messages are transmitted in the form
  of binary numbers
• Binary numbers may be transmitted by alternating
  voltage between set high and low levels
• high 1
• low 0

4
Connecting MIDI Instruments

• Typically, a MIDI instrument has three MIDI ports
  IN, OUT and THRU

• In a simple configuration, one main instrument
  will act as the master to a number of slave
  instruments.

5
Connecting MIDI Instruments

• The sound of the master may be enhanced by
  doubling its sound with a slave instrument that
  is set to play a different type of sound.

master
slave

• This is accomplished by connecting the MIDI OUT
  Of the master to the MIDI IN of the slave.

6
Connecting MIDI Instruments

• To enhance the sound still further, more slave
  devices may be added by daisy chaining more
  instruments together via the MIDI THRU port.

master
. . .
slave 1
slave 2
slave n

• The MIDI THRU port simply passes on what comes to
  the instrument via the IN port.

7
Connecting MIDI Instruments

• In some situations, daisy chains can lead to
  delays at the end of the chain.
• A tidier configuration is to use a MIDI Thru Box

master
8
Connecting MIDI Instruments

• To play an arrangement, a sequencer can send MIDI
  to a group of instruments.

sequencer
. . .
Channel 1
Channel 2
Channel 16

• MIDI has 16 channels
• Each instrument can be set to play a different
  sound and base channel, with each part
  corresponding to a different channel.

9
Configuring MIDI Instruments

• MIDI came into being at the same time as the
  personal computer. Soon, programmers began
  writing MIDI software.

A MIDI interface translates computer code to MIDI
code
10
MIDI Software

• A software sequencer could have more features
  than a dedicated hardware sequencer, and have
  more memory.
• Editor/Librarian software could allow complex
  synthesizer patches to be created more
  intuitively.
• Patches could be arranged and stored in banks
• Groups of banks could be stored in a computer and
  transferred to the instrument
• Notation software could translate MIDI
  information into notated music, as well as
  transpose, create parts, allow easy creation of
  alternate arrangements, etc.

11
Configuring MIDI Instruments

• The most flexible configuration makes use of a
  MIDI patchbay

With successive IN/OUT port pairs, each device
connected to the patchbay can send and receive
information
Many devices can play on more than one MIDI
channel
MIDI patchbay
A patchbay allows a software synthesizer to
address both port and channel number
Tone modules are synthesizers without keyboards
A master controller keyboard produces no sound,
but only sends MIDI information
Thus, the number of available channels becomes
lt portsgt 16
12
Computer as Sound Generator

• It is also possible to use the computer as a
  sound generating instrument
• A MIDI interface is often combined with a sound
  card that has banks of wavetable sounds
• Apples QuickTime, a free download, comes bundled
  with a set of instrument sounds, which are used
  by the computer to play MIDI files

J.S. Bach, Trio Sonata BWV.525

• The QuickTime instruments also function as an
  internet browser plug-in, allowing web pages to
  play MIDI files on a computer that has the
  plug-in installed.

13
The MIDI Language

• MIDI messages fall into two main categories
• Channel (pertaining to information on a
  particular channel)
• Voice
• Mode
• System (pertaining to the system as a whole)
• Real time
• Common
• System Exclusive (SysEx)

14
Channel Voice Messages

• MIDI is transmitted in 10-bit bytes

1
0
start
stop

• The first and last bits are start and stop bits,
  used to identify the beginnings and ends of bytes

• The actual MIDI information is in the middle
  eight bits

15
Channel Voice Messages

• There are two types of MIDI Channel Voice bytes
  status bytes and data bytes
• A status byte determines the type of message and
  the channel (what? and where?)
• The MSB of a status byte is 1

1nnnnnnn

• A data byte gives a value (how much?)
• The MSB of a data byte is 0

0nnnnnnn

• With seven bits following the MSB, a data bytes
  range of expressible values is 0-127

16
Channel Voice Messages

• A Channel Voice message consists of one status
  byte and at least one data byte
• The status byte is divided into nibbles

1nnn nnnn
message type
channel no. (0-15)

• With three bits to define message types, there
  are eight possible types of messages. Seven are
  implemented.

17
Channel Voice Messages
Note Off
1000nnnn 0nnnnnnn 0nnnnnnn
status
two data bytes
note number
velocity
Middle C MIDI note number 60

• Velocity how quickly a note is released
• A higher number is a quicker release,
  corresponding to a note fading quickly
• A lower number is a slower release, corresponding
  to a note fading slowly

18
Channel Voice Messages
Note On
1001nnnn 0nnnnnnn 0nnnnnnn
status
two data bytes
note number
velocity

• Velocity how quickly a note is depressed
• A higher number results in a louder tone
• A velocity value of 0 is the same as a Note Off.

19
Channel Voice Messages
Polyphonic Key Pressure (Poly Aftertouch)
1010nnnn 0nnnnnnn 0nnnnnnn
status
two data bytes
note number
pressure

• Pressure applied to the keys is mapped to some
  modulation parameter (within the particular
  instrument)
• Thus, each key can generate its own degree of
  modulation
• This is a feature found mainly in higher-end
  instruments (separate sensors must be installed
  for each key)

20
Channel Voice Messages
Channel Key Pressure (Channel Aftertouch)
1101nnnn 0nnnnnnn
status
one data byte
pressure

• Poor persons aftertouch less costly to
  implement, and thus found on more instruments
  than poly aftertouch
• One pressure value is applied to all notes on a
  given channel
• The value is read from the key that is being
  pressed hardest

21
Channel Voice Messages
Program Change
1100nnnn 0nnnnnnn
status
one data byte
program number

• Change to a specified patch number on an
  instrument
• With one data byte, up to 128 patch numbers may
  be specified
• A separate message type (that we will see
  shortly) can allow specification of more than 128
  patches

22
Channel Voice Messages
Pitch Bend
1110nnnn 0nnnnnnn 0nnnnnnn
two data bytes
status
pitch bend value, LSB
pitch bend value, MSB

• Pitch bend has 14 bits of resolution, a value
  range of 0 to 16,383.
• The range of pitch shift is programmable within
  an instrument, typically from one semitone to an
  octave above and below the note being bent.
• 64 values within the range of a semitone would be
  acceptable, but within the range of an octave
  there would be audible discontinuities (zipper
  noise). Hence, two data bytes for better
  resolution.
• Pitch bend is a special class of controller (the
  next message type). It is its own message type
  so that the two data bytes may be combined into
  one value.

23
Channel Voice Messages
Control change
1011nnnn 0nnnnnnn 0nnnnnnn
two data bytes
status
Controller number
Controller value
Controllers modify the sound of a sounding note
The controller number specifies a stream of
information.
Controller information is commonly described
either in terms of what type of device is
producing the stream (pedal, wheel, slider, etc.)
or what type of sound parameter is being
controlled (volume, tremolo, pan position, etc.)
The first data byte defines the control stream
being addressed, the second data byte determines
its value (or position).
24
Channel Voice Messages
Control change
The implementation of controller messages was
meant to be open-ended, to allow the creation of
new types of modulation. Many controller numbers
remain undefined. Controller numbers 120-127 are
reserved for another MIDI message type (our next
topic), leaving 120 definable controllers. The
sound parameter affected by a physical controller
may vary from instrument to instrument, or may be
mappable within a particular instrument.
25
Channel Voice Messages
Control change
Controllers 64-69 are typically ON/OFF
controllers. Value of 0 OFF Value of 127 ON
Controller Number
Sostenuto (damper) pedal 66 Soft pedal 67
26
Channel Voice Messages
Control change
Sometimes more resolution than 0-127 is needed.
27
Channel Voice Messages
Control change
Another example addresses the problem of
instruments that have more than 128 patches. Such
an instrument will have a number of banks, with
each bank containing 128 patches. Controller 0
typically defines bank number. A bank select
message (controller 0) followed by a program
change message allows selection of this many
patches ltnumber of banksgt 128