The polyphony of a sound generator refers to its ability to
play more than one note at a time. Polyphony is generally measured or specified
as a number of notes or voices.
A D V E R T I S E M E N T
Most of the early music synthesizers were
monophonic, meaning that they could only play one note at a time. If you pressed
five keys simultaneously on the keyboard of a monophonic synthesizer, you would
only hear one note. Pressing five keys on the keyboard of a synthesizer which
was polyphonic with four voices of polyphony would, in general, produce four
notes. If the keyboard had more voices (many modern sound modules have 16, 24,
or 32 note polyphony), then you would hear all five of the notes.
The different sounds that a synthesizer or sound generator can produce are
sometimes called "patches", "programs", "algorithms", or "timbres". Programmable
synthesizers commonly assign "program numbers" (or patch numbers) to each sound.
For instance, a sound module might use patch number 1 for its acoustic piano
sound, and patch number 36 for its fretless bass sound. The association of all
patch numbers to all sounds is often referred to as a patch map.
Via MIDI, a Program Change message is used to tell a device receiving on a
given Channel to change the instrument sound being used. For example, a
sequencer could set up devices on Channel 4 to play fretless bass sounds by
sending a Program Change message for Channel four with a data byte value of 36
(this is the General MIDI program number for the fretless bass patch).
A synthesizer or sound generator is said to be multitimbral if it is capable
of producing two or more different instrument sounds simultaneously. If a
synthesizer can play five notes simultaneously, and it can produce a piano sound
and an acoustic bass sound at the same time, then it is multitimbral. With
enough notes of polyphony and "parts" (multitimbral) a single synthesizer could
produce the entire sound of a band or orchestra.
Multitimbral operation will generally require the use of a sequencer to send
the various MIDI messages required. For example, a sequencer could send MIDI
messages for a piano part on Channel 1, bass on Channel 2, saxophone on Channel
3, drums on Channel 10, etc. A 16 part multitimbral synthesizer could receive a
different part on each of MIDI's 16 logical channels.
The polyphony of a multitimbral synthesizer is usually allocated dynamically
among the different parts (timbres) being used. At any given instant five voices
might be needed for the piano part, two voices for the bass, one for the
saxophone, plus 6 voices for the drums. Note that some sounds on some
synthesizers actually utilize more than one "voice", so the number of notes
which may be produced simultaneously may be less than the stated polyphony of
the synthesizer, depending on which sounds are being utilized.