There are many devices currently on the market with MIDI interfaces. MIDI is an enabling local area network standard used for lighting controllers, mixing consoles, tape recorders, reverberators, synthesizers and delay units.
A device control setting may be downloaded using a stored snapshot. Upon receipt of a Program Change message, the device will return to the stored setting. Control Change messages may alter device settings in real-time.
Control Change messages indicate the occurrence of additional controller events. There are up to 64 possible controllers for each one of 16 channels. Therefore we may directly address 64*16 =1024 controllers. The channel number with controller address is a kind of Medium Access Control (a network layer sublayer called MAC).
Each controller position may be assigned a number between 0-127. Some controllers permit the sending of an optional data byte which subdivides each of the controller positions into 128 finer parts. This is done by sending an additional Control Change message with the controller number offset by 32. This is a different method of obtaining additional resolution then the method used by the Pitch Bender message. Figure 1.1.4-1 shows the MIDI Channel Voice message control change status word as a function of the channel number.
Figure 1.1.4-1 MIDI Channel Voice Message Control Change Status Word
The least significant four bits of a status word are used to signify the channel number. Channel_number = status_word - 175.
The control status must be followed by two bytes, called the controller number and the controller position (these vary from 0-127). The MIDI controller numbers are shown as a function of their respective controller in Figure 1.1.4-2.
Figure 1.1.4-2 MIDI Channel Voice Message Controller Numbers
Values of 0 and 127 are off and on for binary controllers. These values represent the minimum and maximum for continuous controllers. The MIDI Channel Mode messages are in the same format at the controller messages. This means that they must consist of a Control Change status word followed by two data words. The second data word is ignored in four out of the six MIDI Channel Mode messages. The Channel Mode messages are summarized in Figure 1.1.4-3.
Figure 1.1.4-3 MIDI Channel Voice Message Controller Mode Messages
Also known as MIDI Channel Mode Messages or simply as MIDI modes. Some manufacturers have used the MIDI mode term incorrectly.
Generally, two MIDI mode terms are used to define the synthesizers' state, for example: Omni Off/Mono. Each MIDI mode state is a function of the channel assignment (Omni On/Omni Off) and the voice assignment (poly or mono). If an instruments voices are assigned to channels polyphonically then more than one voice will be available per channel. For example, a synthesizers flute sound and piano sound may be set to respond to a single Note On MIDI event. Some synthesizers to not have this ability.
With Omni On, an instrument responds to messages on every channel, but will transmit only on one channel. Omni off gives a synthesizer a specific channel for the transmission and reception of messages. Omni On/Poly is sometimes called mode 1. An instrument in this mode will respond to any messages on any channel, if able. The Poly part of the mode indicates that the notes will be played polyphonically. This used to be the default mode for synthesizers [Penfold 1991].
Channel Mode messages (MIDI Controller number 121-127) alter synthesizer response to MIDI data. Number 121 resets all controllers. Controller number 122 may be used to enable or disable local control. Local Control Off will isolate a keyboard from the synthesizer. Controller numbers 124-127 select between Omni Mode On or Off. They also select between Mono Mode or Poly Mode. With Omni Mode On, the synthesizer responds to all channels. When off, the synthesizer responds to only one channel. When Poly Mode is activated, Note On messages are played polyphonically. Multiple Note On messages cause each note to be assigned to its own voice (subject to the number of voices available on the synthesizer). In Mono mode, 1 voice is assigned per MIDI channel and only one note is played on a channel at a time.
Typically, synthesizers default to Omni On/Poly Mode so the they will play notes received on any MIDI channel, polyphonically. Figure 1.1.4-4 MIDI Mode Combinations shows all the mode combinations and their effects.
Figure 1.1.4-4 MIDI Mode Combinations
The Basic Channel of a instrument is set by panel controls, hard wiring or System Exclusive messages. Mode messages are transmitted on the Basic Channel.
Mono mode is useful for pitch bending, transposition effects and portamento at specific rates between two notes. Portamento is a gradual change between pitches.
In Omni Off/Poly Mode, synthesizers receive a single channel and play polyphonically. This is used when several synthesizers are daisy-chained. According to Penfold, Mode 3 (Omni Off/Poly) is the most general mode [Penfold 1991]
MIDI Channel Mode messages permit sophisticated voice assignment, however, this ability is uncommon.
The General MIDI Specification modifies the controller numbers shown in Figure 1.1.4-2 by adding the following information already given:
Figure 1.1.4-5 General MIDI Minimum Sound Module Specifications
On Power-up, the default is to be a Pitch Bend Amount = 0, Pitch Bend Sensitivity = ± 2 semitones, Volume = 90 and all other controllers = reset.
A semitone is one-half of a whole tone. This is the smallest interval in traditional Western music. Smaller intervals are called microtones. In typical keyboard instruments, we assume that a system of equal temperament is used (i.e., the ratios for all semitones are the same). The general MIDI specification makes no statement about the type of temperament used. If there are 1200 cents to the octave, then a semitone represents 90 cents or 100 cents for the Pythagorean scale or equal temperament system. Just intonation provides for a semitone of 112 and 92 cents for the intervals between C-Db and C-C#. The General MIDI specification and the MIDI Specification 1.0 do not address issues of temperament. The questions of temperament and MIDI Specification remain open.
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