Midi messages  (Page 3/4)

The seven-bit program number can select from among a palette of 128 voices, yet musicians and composers find this value too limiting. Equipment manufacturers can easily provide thousands of voices thanks to the falling cost of solid-state memory.

With these facts in mind, it will be easier to understand why selecting a new voice for a channel can produce up to three distinct MIDI messages! The first two messages are Control Change messages, and the third message is a Program Change message. The status byte of a Control Change message is 1011nnnn (0x Bn ), where nnnn indicates the channel number. The first data byte following the status byte indicates the controller number , and the second data byte indicates the controller value . The Control Change message will be described more fully in the following section. When the Control Change message is used select a voice, the controller number can be either 0x00 or 0x20 to indicate that the second data byte is either the most significant byte or the least significant byte, respectively, of a combined 14-bit bank select value . An example will clarify these concepts.

Suppose that the following three MIDI messages are received by a synthesizer (each message is destined for Channel 1):

0xB0 0x00 0x2F

0xB0 0x20 0x38

0xC0 0x0E

The first message is a Control Change with control number 0x00, so the upper seven bits of the bank select value are 010_1111. The second message is also a Control Change, but the 0x20 control number signifies that the lower seven bits of the bank select value are 011_1000. Thus the first two messages communicate a single 14-bit bank select value of 01_0111_1011_1000. Lastly, the Program Change message indicates that program number 15 is to be used. With all three messages, it is possible to select a unique voice from among 2 raised to the 21st power possible voices (2,097,152 possible voices).

This clever scheme permits newer equipment with more than 128 voices to properly respond to the rich palette of possible voices, while older equipment that only supports the basic 128 voices defined by the General MIDI standard will simply ignore the two bank select messages and respond to the Program Change message. Equipment manufacturers therefore will often define tone variations on a theme defined by the program number. For example, since program number 1 defines “Acoustic Grand Piano,” the bank select technique can be used to define many different types of acoustic grand pianos.

Modifying a sound: control change and pitch wheel change

Musicians need to do more than simply turn a note on and off. To be expressive, a musician needs to be able to modify a sound already in progress. Synthesizers offer various knobs and sliders that can introduce pitch bends and vibrato, for example.

The Control Change message indicates that a control has just been changed. The status byte of a Control Change message is 1011nnnn (0x Bn ), where nnnn indicates the channel number. The first data byte following the status byte indicates the controller number , and the second data byte indicates the controller value . Since the controller number is seven bits, 128 controller types are possible. Typical controllers include modulation wheel (0x01), main volume (0x07), sustain pedal (0x40), and general-purpose controllers (0x50 to 0x53).