The MIDI Standard 1 Overview What is MIDI? Brief history of - - PowerPoint PPT Presentation

The MIDI Standard

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Overview

• What is MIDI?
• Brief history of MIDI
• How does MIDI work?
• Short Example
• MIDI Files and Connections
• Timing Issues
• Advantages and Disadvantages

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What is MIDI?

• MIDI stands for Musical Instrument Digital

Interface

• Standard that specifies the hardware interface

and the data format of electronic instruments and audio systems

• Intended to connect to musical instruments,

computers, and related audio devices

• Small file size, large range of instruments, and

easy to modify

• MIDI is like sheet music, and sound cards are

the instrument and musician

3 https://store.djtechtools.com/products/midi-fighter-64

Why does MIDI exist?

• Before MIDI (early 1980s) there was no standard for communication between

electronic musical instruments

○ different manufacturers means different standards ○ possibly trying to link analog and digital devices to each other ○ limiting musicians ability to perform with many different machines

• Roland, a Japanese musical instrument company, proposed making a standard
• Wanted it to be simple and small
• The music community, specifically Roland, Oberheim Electronics, Sequential

Circuits, Yamaha, Korg and Kawai, then created MIDI

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Basic Vocabulary

• Channel: synonymous to slave select

○ Each MIDI device usually has several channels ○ Think of a channel as an individual capable of playing many instruments and switching between them

• Program/Patch: the “instrument” that the channel will be imitating (e.g. electric

guitar, drum, bass, etc)

• MIDI Instrument: MIDI defines this as any MIDI device

○ Include sequencer, synthesizer, drum machine, etc.

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MIDI Communication Protocol

• MIDI messages are sent over asynchronous serial at 31250 baud
• Start bit, 8 data bits, stop bit

○ 10 bits over a period of 320 mircoseconds per serial byte

• The messages are broken up into Status bytes and 0 - 2 Data bytes
• Status byte is sent first, followed by each data byte

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From MIDI 1.0 Specification

MIDI Communication Protocol

• Upper 4 bits of the Status describe the command (MSB is always 1)
• Lower 4 bits are the channel number
• The structure of the data bytes is dependent on the command
• Instructions can be sent in real-time or stored in a MIDI file (depending on

device)

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http://properlydone.org/MidiTheory/midi/midi_messages.htm

Some Important MIDI messages

• NoteOn - 0x9M 0xNN 0xNN

○ Plays a note with specific pitch and attack (volume)

• NoteOff - 0x8M 0xNN 0xNN

○ Turns off a note on a channel

• ControlChange - 0xCM 0xNN

○ Knob, switch, pedal, etc. ○ Generated when state of a controller changes

• ProgramChange - 0xBM 0xNN

○ Used to switch program on a specified channel

• System Exclusive Messages

○ Unique to specific MIDI devices (determined by the manufacturer)

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M - channel select bits N - depends on input

MIDI Note Range

• Zero-indexed from C-1 (note 0) to

G9 (note 127)

• This goes beyond the range of an

88-key piano

• C-1 is approximately 8 Hz -- below

the human hearing range

• G9 is approximately 12.5 KHz --

within the typical human hearing range

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How does a MIDI synthesizer work?

• When a key is pressed, a “NoteOn” message is generated based on the key
• That message is sent over serial to a microcontroller
• The microcontroller reads the message and play audio from a stored bank of

samples

• When the key is released, a “NoteOff” message is sent in the same way
• Pedals, pitch wheels, knobs, and other devices can be used to send

“ControlChange” messages that can modify the sound, select what instrument is being played, etc.

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Example MIDI Sequence

Example from http://www.music-software-development.com/midi-tutorial.html Status Data1 - Pitch Data2 - Attack

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Standard MIDI File (SMF)

• Standardized file formats to save sequences that can then be played back on
• ther MIDI devices
• Header contains information about the file

○ arrangement track count, tempo, and format

• Type 0, 1 and 2 files

○ Type 0: single track of entire performance ○ Type 1: multitrack to be played back simultaneously ○ Type 2 (rare): multiple arrangements, each arrangement in own track to be played sequentially

• Popular way to distribute music in Europe and Japan in the 1990s

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MIDI Connections

• 180° five-pin DIN connector

○ Typically only 3 pins used

• USB connectors also now common
• MIDI in, thru and out

○ In: provides input from MIDI controller ○ Thru: output signal that provides copy of the MIDI in (daisy chaining) ○ Out: provides output to another MIDI device, perhaps a synthesizer

Image sources: https://www.amazon.com/Monoprice-108532-Cable-Plugs-2-Pack/dp/B009G UP5SW, MIDI 1.0 Specification 13

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MIDI 1.0 Specification

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https://whitefiles.org/b1_s/1_free_guides/fg1mt/pgs/h15g.htm

Timing Issues

• 3 Bytes of data takes around 1ms to send
• Sending the same info to all 16 channel can be a delay of 16ms
• Only one note can start or stop per MIDI instruction
• What if you want more than one to start or stop simultaneously?

○ Some MIDI devices use timestamps to specify in advance when notes should stop or start. Using timers and interrupts, these instructions can be executed with significantly less delay!

• Length of cords can cause delay

○ MIDI specifies maximum of 50 ft

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http://expressiveness.org/2012/12/04/midi-jitter

How has it changed over the years?

• General MIDI (1991)

○ Further specifications for MIDI-compatible instruments ○ Must support 16 simultaneous channels ○ Must support multiple simultaneous notes on each channel ○ Defines the instrument mapped to each of the 128 possible patch numbers ○ Also General MIDI Level 2 added a couple features

• Manufacturers have created their own specific supersets of MIDI

○ Most just add support for their own patches ○ Roland GS (1991) ○ Yamaha XG (1994)

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MIDI Applications

• Primary application is based on audio and audio equipment

○ Digital synthesizers and sequencers ○ Digital Audio Workstations ○ Effects units

• Lighting systems
• Video game music
• Early computer audio

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https://blog.etcconnect.com/2017/03/midi-show-control-midi-note/

Advantages/Disadvantages

Advantages:

• File sizes are much smaller than audio files

○ Only contain instructions on how to play audio

• Multiple devices can be linked together
• MIDI sequences can be edited in ways that audio files cannot, such as changing

pitch of a section Disadvantages:

• MIDI specification only describes how MIDI devices communicate

○

Not what they do: slightly different sounds, different capabilities across devices

• Quality of sound is limited by playback device

○ Most MIDI instruments have a different sound from their real instrument counterpart

• Can’t store vocals - still an audio file

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Conclusion

• MIDI is an excellent tool for communicating with or between musical

instruments

• It has a large set of instructions that give plenty of versatility
• The communication protocol it uses is very simple
• MIDI is very widely-adopted
• On the other hand, vendor-specific supersets of MIDI can get confusing fast
• Sound quality cannot match actual recordings or real instruments

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Further reading

• The MIDI 1.0 Specification:

https://www.midi.org/specifications-old/category/midi-1-0-detailed-specification s

○ Requires a free account to download

• Summary of MIDI Messages:

https://www.midi.org/specifications-old/item/table-1-summary-of-midi-message

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Questions?

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