MIDI

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MIDI
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Agenda

• Introduction
• MIDI
• MIDI Message

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How music is sent ?

• Two ways to send a piece of music to you
• Record the performance and send you the recording
• Write down the notation and send you the sheet
  music
• Assumption here
• You have the machine capable to playback the
  recording
• You have instruments to play the music notation

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How music is sent ?

• Similar in digitised sound, two ways for
  delivering music
• Send the whole recording
• Send sheet music (music notations)
• Similar assumptions been made

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What is MIDI?

• MIDI Musical Instruments Digital Interface
• Originally, MIDI was devised as standard protocol
  for communicating between electronic instruments.
• MIDI allowed instruments to be controlled
  automatically by devices (Sequences) that could
  be programmed to send out sequences of MIDI
  instructions.

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What is MIDI?

• Later, computer based program sequencer was
  introduced to provide more convenient and
  flexible means of sequencing
• Software provides editing and compositional
  functions so it needs to store MIDI sequences in
  files
• This led to development of standard file format
  for MIDI files, a way of storing MIDI on disk

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How MIDI is generated?

• Every key on the keyboard of synthesiser has a
  corresponding electronic switch.
• Every key press, its corresponding switch is
  activated and sends a signal to the electronic
  chip inside keyboard.
• The chip then generate a signal to the MIDI
  interface in the keyboard.
• The MIDI interface will convert the signal to
  MIDI message.
• The messages will then send to the MIDI port of
  PC

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How MIDI is generated?

• Playback MIDI files requires an instrument that
  understand MIDI.
• Sound card is an suitable instrument in PC
• Sounds can be synthesised on a sound card or
  store in files in the form of samples to be
  played back in response to MIDI instructions.
• MIDI does not contain any audio data, so the file
  size is smaller than actual digital file

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MIDI Message

• However, MIDI cannot guarantee the same fidelity
• MIDI message is an instruction that controls some
  aspect of the performance of an instrument
• Massage is divided into
• Status byte indicates the type of message
• One or two data bytes giving the values of
  parameters

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MIDI Message

• Note On message telling PC a key was pressed
• Status Byte The note represented by number
• Parameter byte also called MIDI velocity which
  represent how hard/long the key is pressed.

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MIDI Message

• When a Middle C key is pressed
• A Note On message is sent to the PC telling it
  that a key is just pressed. Number 64
• Another message containing number 60 is sent
  telling the PC that Middle C is pressed.
• Final message is sent containing a number from 1
  to 127 (1 being very soft and 127 being very
  loud), which tells your PC how hard the key is
  hit.

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MIDI Message
144 60 64
Midi out
126 60 64
Midi out
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Differences between Digital Audio and MIDI

• Both send signals to PC
• Recording Digital Audio is actually recording
  actual sound
• Cannot change the sound of performance
• Guarantee sound exactly as the original playback
• Recording MIDI is recording the performance
  instructions
• Easy to manipulate
• Easy to fix mistakes in the performance by simply
  change the pitch of the note
• Cant record sound that contains vocal or sound
  effect
• Cant guarantee sound exactly as the original
  playback depend on the playback device
• The concept similar to musician (PC) reading
  sheet music (MIDI data)