MIDI - Musical Instrument Digital Interface

Also See: http://midipads.homestead.com/

Most sounds used on computers are the result of converting  sound pulses ("waves") into digital format. The process is called Analogue to Digital conversion, or "AtoD conversion".

Analogue to Digital Conversion changes sound 'waves' into digital 'bytes'.
Analogue Waves	01001010  10010010  01011001   10011011  10001010  11101001 ....... Digital Bytes

MP3 and ".WAV" files are produced by using AtoD conversion processes. The sound existed originally and was converted into a format for storage and reproduction on a computer. MIDI files are different.

A MIDI file never really existed as a 'true' sound. MIDI is a set of definitions that describe the sound, rather than a recording of it. MIDI files contain information that represent sounds as a set of 'descriptions', or "messages". MIDI devices convert these 'messages' into sound.

The MIDI specification originated in the early eighties and is now well established. Many electronic musical instruments have built-in MIDI 'synthesizers' that can read and write MIDI instructions. Most computer "sound cards" are able to make use of the MIDI standard.

MIDI Terms:

Channel: A channel is a number from 0-15 which corresponds to channels 1-16.
Pitch: A pitch is a number from 0-127 and corresponds to a note on the instrument.
Velocity: A velocity is a number from 0-127 and corresponds to how fast the key (or string) is pressed or released (most terminology is in reference to keyboards). 0 means is released. For simplicity you can think of "velocity" as meaning "volume", but keep in mind that it is not quite that simple.
Pressure: A pressure is a number from 0-127 and corresponds to the characteristics of how the key is hit.
Program: A program is a number from 0-127 and corresponds to the instrument to use. The term "program" can also be thought of as meaning "voice", or "instrument". See below:

Note: The term "program" is also used to mean "voice", or "instrument".

NOTE: The following notes assume you are using MSW Logo as your control language.

Structure of a MIDI Message -

MIDI messages are made up of a "Command Code" and two 'bytes'. (A byte is just a computer 'word'. A typical 8-bit byte would be: 10011010.) The first byte is called the Status Byte and the second is called the Data Byte.

A typical MIDI message is of the form:

MIDI_Output <CommandCode + Channel> <Status Byte> <Data Byte>

In MSWLogo this would be along the lines of: MIDIMESSAGE (LIST 144+3 50 100) An alternative to the 'list' form of the command is MIDIMESSAGE [147 50 100] The 'list' form is used to clarify the Command Code.

Changing Instruments (or "programs", or "voices") -

To change to a different instrument you must issue a "Program Change" command using:

MIDIMESSAGE (LIST 192+3 <program code*>)

NOTE: The "program code" is the decimal number representing the 'instrument, or 'voice' as in Table 1 above.

Sample MSW Logo MIDI Code -

to sample1
   ;
   ; Sample code to demonstrate simple MIDI commands.
   ; This code will set up channel 3 to play the Sitar (Ethnic 104)
   ; at a 'pitch' of 50 and a velocity of 100.
   ; NOTE: In MIDI terms the word "velocity" refers to how fast a
   ; keyboard key is hit. You can think of it as meaning "volume".
   ;
   ; NOTE: You must begin your MIDI session by turning
   ; on MIDI functionality using: print midiopen
   ; You must turn MIDI off again when you have finished
   ; with the command: midiclose
   ;
   print midiopen
   midimessage (list 192+3 104)
   midimessage (list 144+3 50 100)
   ; Don't forget to issue a midiclose command when you have finished.
end

More Sample MSW Logo MIDI Files -
midi1.LGO - Basics of using MIDI with buttons in MSW Logo
midi2.LGO - Controlling one channel.
midi3.LGO - Template code for a single channel MIDI system
midi4.LGO - A four channel MIDI system

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