Nuts and Volts - February 2009

able (see Resources). Microchip also provides comprehensive datasheets – the PIC18F2550 document runs to 430 pages — so I had a lot of reading to do. A few months later (after sketching out some different approaches), I started serious work on the design.

Referring to Figure ¹, the PIC18F2550 is the heart of mistralXG. It manages data flowing between the MIDI ports and the PC via its serial and USB ports, and provides logic and control for the MIDI switches and the user interface (two pushbuttons and an LCD display).

The code offers the following features:

• Selecting which MIDI stream is sent to the synthesizer

• Running Status control for MIDI OUT

• MIDI data filtering

• MIDI OUT and MIDI THRU control

• Adjustment of LCD brightness

• Monitoring of MIDI IN and MIDI OUT activity

• Maintaining and displaying error information

Technology Overview

Let’s take a brief look at some of the technologies that are used in the design. If you really want to know how mistralXG works, there’s no substitute for diving in and reading up on this stuff. There are also copious comments in the source code, which is available on the Nuts & Volts website at www.nutsvolts.com.

MIDI: The Musical Instrument Digital Interface

MIDI is a serial protocol for passing messages between controllers and music synthesizers. Commands such as “Note on,” “Bend pitch,” “Note off,” and “Change instrument” control the sounds a synthesizer makes. Sixteen channels (1-16) — each mapped to a single instrument — are defined for a single MIDI connection, but MIDI does not define which instrument each channel represents. MIDI composers have to decide the instrument-to channel mapping for each tune.

A command might say, “Play Middle C on channel 5, medium loud.” Three bytes are required to construct this command. These are (in hexadecimal) 0x94, 0x3C, and 0x40. The first four bits of 0x94 (the “ 9”) indicate this command is “Note on.” The “ 4” in 0x94 says that the note should be played on channel 5 (counting starts at 0 for channel 1). Byte 2 selects note number 60 (0x3C) which is middle C. Notes 0-127 are defined (from low to high pitch). The last byte gives the volume, with 0 being silent and 127 (0x7F) being loudest.

Instruments are numbered 1-128 (0-127) and mappings are again arbitrary. The General MIDI specification, however, defines a mapping most synthesizers can be set to match. When playing a MIDI file, you have to make sure that your synth uses the correct mapping to prevent notes meant for one instrument being played by another — perhaps even the drum kit! One synthesizer’s piano

may sound different from another’s, but at least they both generate the right type of sound for the music.

MIDI hardware recognizes command bytes because their most significant bit (MSB) is set to 1. Command bytes are followed by 0, 1, or 2 data bytes which have their MSB=0. Details of the commands and how to interpret them can be found in the Resource links.

MIDI is real-time

MIDI data are transmitted in real time. As soon as a synthesizer receives a completed “Note on” message for a particular note and channel, it starts to play the note. It will continue to sound until a “Stop note” command for the same note and channel is received. So, a synthesizer connected to a MIDI keyboard will reproduce the exact timing of the notes as they are played by the musician, with a slight — usually indiscernible — delay to allow for constructing, transmitting,and receiving the MIDI messages. If multiple MIDI devices are chained together, this delay can become significant and must be kept in mind when setting up complex MIDI systems.

MIDI was launched about 25 years ago, and the specification has changed little since then. The data or bit rate of 31. 25 kHz seems very slow by today’s standards, but MIDI’s wide adoption within the electronic music industry has ensured its survival, and it is likely to be around for many years to come.