Midi messages  (Page 4/4)

Musicians use a synthesizer’s pitch wheel introduces to bend (temporarily raise or lower) the pitch of the entire keyboard. In order to provide more frequency resolution, the pitch wheel has a dedicated MIDI message; the two data bytes afford fourteen bits of resolution rather than the seven bits of resolution that are possible with a control change message.

The Pitch Wheel Change message indicates that the pitch wheel has just moved. The status byte of a Pitch Wheel Change message is 1110nnnn (0x En ), where nnnn indicates the channel number. The first data byte following the status byte is the lower seven bits of the overall 14-bit position value, and the second data byte contains the upper seven bits of the position value. The nominal value is 0x2000 when the pitch wheel is centered (data byte #1 = 0x00, data byte #2 = 0x40). Moving the pitch wheel to its lower (leftmost limit) produces a value of 0x0000, while moving it to its rightmost limit produces a value of 0x3FFF (data byte #1 = 0x7F, data byte #2 = 0x7F). Note that the Pitch Wheel Change message simply indicates the position of the pitch wheel; the synthesizer may interpret this position in various ways depending on other settings. For example, my synthesizer defaults to a whole step down when the pitch wheel is moved to its left limit, however, it can be adjusted to shift by an entire octave for the same amount of pitch wheel movement.

The following screencast illustrates the MIDI messages generated by the slider controls and pitch wheel.

Transferring data: system exclusive messages

The System Exclusive ( SysEx ) message provides a mechanism to transfer arbitrary blocks of information between devices. For example, the complete configuration of a synthesizer can be uploaded to a computer to be retrieved at a later time. The term “exclusive” indicates that that information pertains only to a particular vendor’s piece of equipment, and that the organization of the information is vendor-specific.

The transfer process begins with the System Exclusive Start message with status byte 11110000 (0x F0 ). The following data byte is the manufacturer ID. All following bytes must be data bytes in the sense that their most-significant bit is always zero. An arbitrary number of bytes may follow the manufacturer ID. The transfer process ends with the System Exclusive End message with status byte 11110111 (0x F7 ); no data bytes follow this status byte.

Other messages

The messages discussed in this module are representative of what you will encounter when working with synthesizers and standard MIDI files, but are by no means a complete listing of all available messages. Refer to the MIDI Manufacturers Association for full details on the MIDI standard; see the specific tables noted in the last section of this module.

For further study

• MIDI Manufacturers Association (MMA)
• MMA: Tutorial on MIDI and Music Synthesis
• MMA: Table 1: Summary of MIDI messages
• MMA: Table 2: Expanded Messages List (Status Bytes)
• MMA: Table 3: Summary of Control Change Messages (Data Bytes)
• MMA: About General MIDI
• MMA: General MIDI Level 1 Sound Set
• Textbook: Francis Rumsey, MIDI Systems and Control, 2nd ed., Focal Press, 1994. See Tables 3.1 (MIDI messages summarized), 2.2 (MIDI note numbers related to the musical scale), and 2.4 (MIDI controller functions).