Center for New Music and Audio Technologies

The MIDI Method

The MIDI specification implements a simple protocol for the transmission and receipt of electronic signals. Messages are sent only if changes in the input device, e.g, a keyboard, take place. This makes the MIDI protocol a so-called event driven system (see It is important to notice that MIDI transmits commands, but it does not transmit any audio signal. MIDI information is transmitted in ``MIDI messages,'' which can be thought of as instructions which tell a music synthesizer how to play a piece of music. The synthesizer receiving the MIDI data must generate the actual sounds.

Figure 1.1: MIDI messages are sent only if changes in the input device take place

MIDI operates at 31.25 kBaud $(\pm1{\%})$ using an asynchronous serial data byte comprising 1 start bit, 8 databits (LSB first) and 1 stop bit. This makes a total of 10 bits per serial byte with a period of 320$\mu$s. Electrically, the MIDI standard is specified as a 1.5mA current loop, with logic 0 is current on. MIDI transmitter and MIDI receiver are galvanically isolated in order to avoid grounding loops and subsequent data errors [1].

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