MIDI

One choice for adding sounds to multimedia applications is the use of digital audio soundfiles This can become very memory intensive, however, for large soundfiles For example, a stereo 16 bit/sample 22 kHz soundfile requires > 5

mbytes per minute of audio May be too much (e.g. for ringtones on a cellphone)

As an alternative to the use of soundfiles, MIDI files have been considered

MIDI

MIDI (Musical Instrument Digital Interface) is a standardized control language and hardware specification allows suitably equipped electronic musical instruments and devices to

communicate real-time and nonreal-time performance and control data

MIDI data is communicated digitally through a production system as a string of MIDI messages

Each message describes an event which occurs during a musical performance

MIDI

The messages are transformed into the sound described by the message/event by a synthesizer.

MIDI can be created using a MIDI-editing program or they can be recorded from a performance on a MIDI-capable device (a MIDI controller) and subsequently edited using the same program.

MIDI data can easily be converted into sample data, but the reverse is not true.

MIDI

MIDI messages are transmitted (in a serial fashion) through a single MIDI line at 31.25 K bits/second MIDI communication is unidirectional No error detection/correction (so max cable length of 50 meters)

Usually, MIDI instruments and devices are linked together in a daisy-chain fashion

The following slide shows an example

MIDI Ports MIDI devices may have up to 3 ports

MIDI IN port to receive data (to be played back/synthesized by the device, for example)

MIDI OUT port for MIDI controllers which generate their own MIDI data

MIDI thru to pass the input data to the next MIDI device in the daisy chain arrangement

MIDI Devices

MIDI controllers generate performance data in MIDI format

MIDI synthesizers generate audio output based on MIDI data A single keyboard might act as both MIDI controller and

MIDI synthesizer A synthesizer can be some hardware (instrument) or a

program (soft synth)

MIDI sequencers can be used to receive, store and edit MIDI data Either hardware device or an application program

MIDI Sequencers

The sequencer software will typically run on a Digital Audio Workstation (DAW) Originally, special purpose microprocessor-based

hardware for capturing, editing, creating music Today, a computer running special-purpose software

with audio hardware interfaces Example sequencing software – Cubase

Examples of hardware-based sequencers are MIDI keyboards and drum machines These also may contain a synthesizer component

MIDI Sequencers

MIDI data can be used to produce musical scores corresponding to the data, so sequencing software is often tightly coupled to musical notation software

Musical Acoustics and Notation

Musical sounds are characterized by pitch (frequency), timbre, loudness

A note also carries start and duration info Two notes which differ in pitch by 2^n, sound alike to

the human ear (except for the higher pitch) An octave is the interval between one musical pitch and

another with half or double its frequency The division into notes of an octave differs from culture

to culture In Western culture, 440 Hz is a reference point and is

called note A The octave between one note A and another (440-880

Hz, e.g.) is divided into 12 notes

Musical Acoustics and Notation

On a keyboard, 8 white notes and 4 black notes Musical notation is written on a musical staff,

with a key signature The timbre of a musical sound is a function of its

overtones The perceived loudness of a musical sound is a

function of the air pressure amplitude The amplitude envelope covers the period of a single

musical note

Musical Acoustics and Notation

Musical Acoustics and Notation

MIDI messages are bytes that are interpreted by the MIDI devices The messages are used to convey a series of instructions to one or all

of the MIDI devices within the system.

The messages can be divided into two types: Channel messages are messages assigned to a specific MIDI channel System messages address all devices in a system, without regard to

channel assignment

Messages are transmitted in 10-bit bytes Each begins with start bit of 0 and ends with a stop bit of 1 The start and stop bits are “stripped out” at the serial port, so we are

left with 8-bit bytes of data

MIDI Messages

MIDI Messages

The following is a typical 3 byte MIDI Note On message: (10010101) (01000000) (01011010)

This message is interpreted as follows. The first byte is the status byte while the second and third are data bytes The most significant bit of the byte determines which type of byte it is The next three bits of the status byte code the Note On message The four least significant bits give the channel to which this message is

directed In the case, channel #5. (Note that we can have up to 16 channels

addressed through a single MIDI cable)

MIDI Channels

MIDI Channel Messages

Whenever a MIDI device is instructed to respond to a specific channel number, it will ignore any message not directed to that channel On the other hand, if a message is transmitted to that channel, the device will

respond to the message (within the device’s capability limits).

The 7 bits (not including the MSB) of the first data byte code the note # that should be turned on - here, it is 64

The 7 bits of the second data byte indicate the attack velocity (volume level of the note) - here, it is 90

MIDI Channel Voice Messages

Channel voice messages are used to transmit real-time performance data throughout a connected MIDI system

There are seven channel voice messages: Note On Note Off Polyphonic Key Pressure Channel Pressure Program Change Control Change Pitch Bend Change

MIDI Channel Voice Messages

A Note On message indicates the beginnning of a MIDI note

The message consists of three bytes of information: MIDI channel number; MIDI note number; Attack velocity value In general, MIDI note 60 is assigned to middle C key, and notes 21-108

correspond to the 88 keys of an extended keyboard controller

The final byte indicates the velocity at which the key was pressed Higher velocities lead to louder notes Not all instruments interpret all attack velocities, and some do not respond

dynamically at all

MIDI Channel Voice Messages

A Note Off message indicates the end of a MIDI note If the instrument being played has a release (or decay) phase, it will

begin that phase when the message is received The release velocity byte indicates the speed at which the key was

released Few instruments generate or respond to release velocity

MIDI Channel Voice Messages

Polyphonic Key Pressure messages are transmitted by instruments that are capable of playing more than one sound at a time (e.g. a chord on a keyboard instrument) The message indicates a pressure message for each key that is depressed Pressure values can commonly be assigned to such parameters as vibrato,

loudness, and pitch

Channel Pressure messages are commonly transmitted by polyphonic instruments that will only respond to a single overall pressure applied to their controllers, regardless of the number of keys being played at any one time

MIDI Channel Voice Messages

The Program Change message changes the program or preset number that is active in a device or instrument Up to 128 presets can be selected by using this message This can be used, for example, to switch between the different sounds

of a synthesizer or to change the rhythm patterns of a drum machine

MIDI Channel Voice Messages

The Control Change message transmits information that relates to real-time control over the performance parameters of a MIDI instrument Control change messages correspond to changes in controllers such as foot

pedals, relative balance of a stereo sound field, etc.

Pitch Bend Change messages are transmitted by an instrument whenever its pitch bend wheel is moved either in the positive (raise pitch) or negative (lower pitch) position from its central (no pitch bend) point

Pitch and Mod. Wheels

MIDI Channel Mode Messages

Controller numbers 121-127 are reserved for Channel Mode messages. These include:

Reset all controllers

Local control

All Notes Off

MIDI Mode messages

General MIDI

The basic instrument sounds that are reproduced by almost any electronic music setup will invariably change from one setup to the next

This lack of conformity has brought about a single set of standardized MIDI patch settings that have come to be known as General MIDI (1991) This standardized series of sound settings has been defined so that common

and popular instrument sounds are mapped to various program change numbers

General MIDI

Other standards followed General MIDI Level 2 (GM2) released in 1999 increases

polyphony to 32 voices, standardizes controller numbers and system messages, etc. while maintaining backwards compatibility with GM

GM2 also incorporates MIDI Tuning Standard (MTS) from 1992 which allows for alternate (non-Western) tunings to be specified

MIDI Time Code (MTC) embeds SMPTE timecode timing information in a sequence of MIDI messages

General MIDI

Program # Instrument Group1-8 Piano9-16 Chromatic Percussion17-24 Organ25-32 Guitar33-40 Bass41-48 Strings49-56 Ensemble57-64 Brass65-72 Reed

General MIDI

Program # Instrument Group73-80 Pipe81-88 Synth Lead89-96 Synth Pad97-104 Synth Effects105-112 Ethnic113-120 Percussive121-128 Sound Effects

Playback of MIDI

Sampled sounds are played back by converting the samples to an analog voltage which can be used to drive speakers Done in CD players, MP3 players and sound cards

Playback of MIDI

For MIDI files, the sound must be built up or synthesized Two methods for synthesizing sounds are frequency

modulation (FM) synthesis and wavetable synthesis FM synthesis modulates sinusoidal frequencies An amplitude envelope function is used Fairly inexpensive Wavetable synthesis makes use of stored sound samples

of real instruments More realistic, but more expensive due to the need for

memory to store the samples

Alternative MIDI Uses

Besides being used for musical performance, MIDI messages can also be used for non-musical apps, as long as sender and receiver agree on the meaning of MIDI messages

Some of the alternative uses of MIDI include (note that timing is important in each)

Show control Theatre lighting Special effects Etc.

MIDI Future

Alternative transportation mechanisms for MIDI are being explored MIDI on ethernet MIDI over RTP/IP MIDI over wireless

HD Protocol under discussion allows Higher speed transport Device discovery and enumeration Increased numbers of channels and controllers New kinds of events Etc.