More on MIDI Technology
Much in the same way that two computers communicate via modems, two synthesizers communicate via MIDI.
The information exchanged between two MIDI devices is musical in nature.

MIDI information tells a synthesizer, in its most basic mode, when to start and stop playing a specific note. Other information shared includes the volume and modulation of the note, if any.
MIDI information can also be more hardware specific. It can tell a synthesizer to change sounds, master volume, modulation devices, and even how to receive information. In more advanced uses, MIDI information can indicate the starting and stopping points of a song or the metric position within a song.

And how is this achieved?
The basis for MIDI communication is the byte. Through a combination of bytes a vast amount of information can be transferred. Each MIDI command has a specific byte sequence. The first byte is the status byte, which tells the MIDI device what function to perform. Encoded in the status byte is the MIDI channel.

MIDI operates on 16 different channels, numbered 0 through 15. MIDI units will accept or ignore a status byte depending on what channel the machine is set to receive. Only the status byte has the MIDI channel number encoded. All other bytes are assumed to be on the channel indicated by the status byte until another status byte is received.

Some of these functions indicated in the status byte are Note On, Note Off, System Exclusive (SysEx), Patch Change, and so on. Depending on the status byte, a number of different byte patterns will follow. The Note On status byte tells the MIDI device to begin sounding a note. Two additional bytes are required, a pitch byte, which tells the MIDI device which note to play, and a velocity byte, which tells the device how loud to play the note. Even though not all MIDI devices recognize the velocity byte, it is still required to complete the Note On transmission.

The command to stop playing a note is not part of the Note On command; instead there is a separate Note Off command. This command also requires two additional bytes with the same functions as the Note On byte. Most people are confused at first by this approach to Note On and Note Off, but after further thought they realize the necessity of the structure.
Another important status byte is the Patch Change byte. This requires only one additional byte: the number corresponding to the program number on the synthesizer. The patch number information is different for each synthesizer, and the standards have been set by the International MIDI Association (IMA). Channel selection is extremely helpful when sending Patch Change commands to a synthesizer.

The SysEx status byte is the most powerful and least understood of all the status bytes because it can instigate a variety of functions. Briefly, the SysEx byte requires at least three additional bytes. The first is a manufacturer's ID number or timing byte, the second is a data format or function byte, and the third is generally an "end of transmission" (EOX) byte.

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