MIDI Synthesisers
The polyphony of a sound generator refers to its ability to play more than one note at a time.
MIDI Synth Polyphony
Polyphony is generally measured or specified as a number of notes or voices.
Most of the early music synthesisers 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 synthesiser, you would only hear one note. Pressing five keys on the keyboard of a polyphonic synthesiser which had only 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.
Sounds
The different sounds that a synthesizer or sound generator can produce are sometimes called "patches", "programs", "algorithms", or "timbres".
Programmable synthesisers 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 receiving device 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).
Multitimbral Mode
A synthesiser 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 synthesiser 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 synthesiser could receive a different part on each of MIDI's 16 logical channels.
The polyphony of a multitimbral synthesiser 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 synthesisers actually utilise more than one "voice", so the number of notes which may be produced simultaneously may be less than the stated polyphony of the synthesisers, depending on which sounds are being utilised.
Daisy Chains
Information received on the MIDI IN connector of a MIDI device is transmitted back out (repeated) at the devices' MIDI THRU connector.
Several MIDI sound modules can be daisy-chained by connecting the THRU output of one device to the IN connector of the next device downstream in the chain.
Figure 2 shows a more elaborate MIDI system. In this case, a MIDI keyboard is used as an input device to a MIDI sequencer, and there are several sound modules connected to the sequencer's MIDI OUT port.
You might utilize a system like this to write a piece of music consisting of several different parts, where each part is written for a different instrument. You might play the individual parts on the keyboard one at a time into a sequencer. The sequencer would then play the parts back together through the sound modules. Each part would be played on a different MIDI Channel, and the sound modules would be set to receive different channels.
Example:
Sound module number 1 might be set to play the part received on Channel 1 using a piano sound.
Module 2 plays the information received on Channel 5 using an acoustic bass sound, and the drum machine plays the percussion part received on MIDI Channel 10.
In this example, a different sound module is used to play each part. However, sound modules which are "multitimbral" are capable of playing several different parts simultaneously.
A single multitimbral sound module might be configured to receive the piano part on Channel 1, the bass part on Channel 5, and the drum part on Channel 10, and would play all three parts simultaneously.
In this system, the PC is equipped with an internal MIDI interface card which sends MIDI data to an external multitimbral MIDI synthesisers module.
Application software, such as Multimedia presentation packages, educational software, or games, sends MIDI data to the MIDI interface card in parallel form over the PC bus.
The MIDI interface converts this information into serial MIDI data which is sent to the sound module. Since this is a multitimbral module, it can play many different musical parts, such as piano, bass and drums, at the same time.
Sophisticated MIDI sequencer software packages such as Cubase are also available for the PC. With this software running, a user could connect a MIDI keyboard controller to the MIDI IN port of the MIDI interface card, and have the same music composition capabilities discussed in the last two paragraphs.
Soundcards
Until recently, most PC sound cards included FM synthesisers with limited capabilities and marginal sound quality.
With these systems, an external wavetable synthesiser module might be added to get better sound quality. Recently, more advanced sound cards have been appearing which include high quality wavetable music synthesisers on-board, or as a daughter-card option.
