Electronic musical instruments had been around for decades, but were unable to "talk" to each other until 1982 and the introduction of the Musical Instrument Digital Interface (MIDI) specification. Many musicians used two, three, or more synthesizers in their setups, each with a distinctive palette of sounds, to provide the widest possible range of voices. The problem was that the musicians couldn't combine sounds from different synths and control them from a single keyboard, because of differences in the electronic commands to which each synth responded. This ultimately led to a proposal for a common set of commands to which all digital musical instruments could respond.

In short, MIDI is not a sound source, but a set of digital commands--or, in the language of MIDI, messages--that can control a sound source. MIDI doesn't even refer to notes by their proper names; for example, middle C is note number 60. When you use the recording feature included in most digital pianos, what you're actually recording is a sequence of digital messages; hence the term sequencer for a MIDI recorder (some upper-end models now allow both MIDI and audio recording). These messages form a datastream that represents the musical actions you took. Some of the most common messages are listed in the table below.

Basic MIDI Messages
Message Action
Note On EventThe number of the note played and the key velocity (i.e., how fast the key went down)
Note Off EventThe number of the note released and the key's release velocity
Control ChangeWhen the position of a control such as a pedal is changed, a message indicates the number assigned to that control and a value representing its new position
Control ChangeWhen the position of a control such as a pedal is changed, a message indicates the number assigned to that control and a value representing its new position
Program ChangeWhen a new voice is selected, a message indicates the "patch" number of the new voice (the term patch goes back to the early days of the synthesizer, when different electronic elements were literally wired to each other with "patch cords")

There are many more message types, but this should give you an idea of how MIDI "thinks." Nothing is a sound--everything is a number. When recording or playing back a sequence of MIDI messages, timing--just as in a piece of music--is obviously a critical element, so MIDI uses a "synchronization clock" to control the timing of each message. MIDI can also direct different streams of messages to different channels. Each channel can be assigned to communicate with different devices; for instance, your computer and another keyboard.

While the MIDI specification of 1982 standardized commands for events such as note on, note off, control change, and program change, it didn't include a message type for instrumental voice. It was still necessary to manually set the voice that would play on each synth because there was no consistency between instruments from different manufacturers, or sometimes even within a single manufacturer's product line, as to which command would produce which voice. This changed with the adoption in 1991 of the General MIDI (GM) standard, updated in 1999 to General MIDI 2 (GM2).

Product specifications now frequently state that an instrument is General MIDI, or GM, compatible. Like MIDI, General MIDI specifies not a sound source but a standardized numbering scheme. Any digital instrument "thinks" of the different voices it produces not as Piano or Violin or Harpsichord, but as Program Change numbers. General MIDI established a fixed list of Program Change Numbers for 128 "melodic instruments" and 1 "drum kit." GM2 later expanded this to 256 melodic voices and 9 drum kits. So all GM-compatible instruments use the same numbers to represent a given voice: Acoustic Grand Piano is always Program Change Number 1, Violin is always 41, and Harpsichord is always 7. A standardized numbering scheme of 256 melodic instrumental voices seems big enough to cover everything under the sun with room to spare, until you notice that some MIDI voices are actually combinations of instruments. For instance, Program Change Numbers 49 and 50 are String Ensemble 1 and 2, representing different combinations of string instruments playing in ensemble. Also, there are many Ethnic instruments (voices 105 through 112), and several Sound Effects, from chirping birds to gunshots. If this has you feeling that perhaps 256 wasn't an unreasonably high number of voices after all, consider that many higher-end digital pianos have more than 500 voices, and some more than a thousand. This means that when you record using voices from the far end of the list on one manufacturer's "flagship" model, then play the recording back on someone else's top-of-the-line model, voice consistency once again flies out the window. Perhaps the most important thing to remember is that the GM standard doesn't specify the technology used to create the listed voices. One hint of the degree of variation possible under this system is the fact that your current cell phone is probably GM compatible.

In the 1990s, two proprietary extensions to the General MIDI standard were made, by Roland and Yamaha. Roland's GS extension was largely incorporated into the GM2 standard. Yamaha's XG extension defines far more voices than the other schemes, but hasn't been as widely adopted as General MIDI.


FALL 2009 -- page 117

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