Introduction to Computer Music: Volume One

1. How does the MIDI system work? Page 2

Summary of the MIDI hardware specification

The MIDI specification begins with a set of common hardware requirements. Each MIDI-capable instrument is equipped with a transmitter and receiver, though certain peripheral devices, such as signal processors, may have only a receiver or a transmitter. The interface operates at a 31.25 Kbaud asynchronous (serial) transmission rate. Each byte consists of a start bit, eight data bits, and a stop bit, for a total duration of 320 microseconds per byte. While this was adequate for the applications originally envisioned, it has proven to be one of the major stumbling blocks for generating more complex data streams in time-dependent situations. Though several manufacturers originally included alternative parallel interfaces on their instruments for linking their own peripheral devices, the overwhelming success of MIDI has seen the abandonment of almost all other interconnecting formats.

MIDI cables connect instruments by means of MIDI IN, MIDI OUT, and MIDI THRU jacks. The MIDI IN port receives incoming MIDI messages, the MIDI OUT port transmits actions of the keyboard to other keyboards or a computer. The MIDI THRU jack provides a direct copy of data coming into the MIDI IN jack, providing the ability to "daisy chain" several instruments and devices together. Any action on an instrument that corresponds to a particular MIDI code (such as a key depressed, or a program button changed) will normally transmit that action's code to the MIDI OUT, but not to the MIDI THRU. Many more recent instruments have a switch that changes the function of a MIDI THRU port to a MIDI OUT port.

The major potential of MIDI is realized when a network of instruments and other musical devices are connected to a computer by means of a MIDI interface. The primary function of the interface is to match clock speeds between the specified rate from MIDI devices and the computer. An interface's connections may be as simple as one MIDI IN and one MIDI OUT jack with information distributed by means of the daisy chain mentioned above. A simple MIDI network is shown below.

In recent years, interfaces have become more elaborate and now provide more MIDI INs and OUTs for more complex and flexible networks, merging facilities to handle several input devices simultaneously, coded routing of data to specified cables, and generation and/or conversion of timing synchronization codes for use with video and multitrack audio equipment. Some interfaces are also capable of selective filtering and/or remapping of MIDI data.

Pictured below is MOTU's MIDI TImepiece AV which provides 8 MIDI IN's and 8 OUT's, along with timecode and other ports.

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