In addition to physical connections, the MIDI specification defines standard messages that MIDI devices use to communicate with each other. These messages identify the events to define and reproduce music with one or more MIDI devices. The message content defines events such as striking a note or changing an instrument from a flute to an oboe.
The set of MIDI messages and data values that define and reproduce a song are stored in a file called a MIDI file. Each MIDI file can store up to 16 music channels of information. You build a MIDI file with a sequencer, which captures MIDI messages and stores them in a file.
When you play a MIDI file, the sequencer sends MIDI messages from the file to a synthesizer, which converts messages into sounds of a specific instrument, pitch, and duration. A synthesizer generates music and sound with a DSP or other type of chip by creating and modifying waveforms and sending them out through a sound generator and speakers.
Timbre is the tonal quality that distinguishes instruments from one another.
Some synthesizers synthesize sounds from parameters that define the timbre
of an instrument. Other synthesizers use digitally recorded samples of the original instruments and modify these sounds in memory for volume and pitch changes. Sounds produced synthetically are not as realistic as those produced from the original samples. Multimedia PCs may use either type of synthesizer.
The MIDI message sent to the synthesizer identifies which timbre to use. To find the timbre, look at the patches defined by the MIDI Manufacturers Association (MMA) General MIDI Mode specification (shown later in this chapter).
If a synthesizer is polyphonic, it can play several sounds at once. Polyphony differs slightly from the number of timbres a synthesizer supports. A four-voice synthesizer with six-note polyphony can play six notes simultaneously, but must distribute the sounds among a maximum of four timbres—for instance, a four note piano chord, one note with a flute, and one note with a violin.