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An Introduction to MIDI Technology
Written by Scott Wilkie - © 2005, PRIMEDIA Business Magazines & Media Inc. Reprinted with Permission.
These days, MIDI (pronounced MID-ee) is such an important part of making music with electronic instruments that virtually any keyboard, digital piano, or synthesizer on the market is MIDI equipped. To understand why, let's start at the beginning.

What's It All About?
MIDI (short for Musical Instrument Digital Interface) was introduced more than 20 years ago as a method of communication between synthesizers from different manufacturers. Prior to MIDI's inception, it was virtually impossible to integrate several electronic musical instruments into a system where they could work together simply and reliably.

The most common misconception about MIDI is that it contains audio information. In fact, it doesn't carry any data that you can actually hear. Instead, it's a complex computer language used to relay performance information to and from an instrument.

A MIDI cable features a small, round 5-pin connector on either end, and it easily connects one instrument to another. You'll find MIDI In and Out jacks on any MIDI instrument. Often, instruments include a Thru jack as well, which echoes the information received at the In so that it can be passed on to other MIDI devices.

Unlike many computer interfaces, MIDI communicates in only one direction. To send data from one instrument to another, you need to connect the Out from the first instrument to the In of the second instrument. In this configuration, the first instrument is called the master and the second instrument is called the slave. Additional slaves can be added by “daisy-chaining” them to the Thru ports.

You can think of MIDI information as a modern-day, electronic version of the piano roll found on old player pianos. Those rolls had holes punched in a specific order that represented the actual performances of the music. The holes in the roll correlated with specific notes on the piano and “communicated” to the piano which notes to play and how long to hold them.

Similarly, the data that goes through a MIDI cable communicates to a MIDI instrument, telling it not only what notes to play and how long to hold them, but, as you'll see, quite a bit more.

MIDI also spawned a new type of instrument called the sound module. This is a box that contains a music synthesizer but that has no keyboard. The sounds from the module are played by connecting another MIDI instrument and triggering the notes from the other instrument's keyboard. This makes it possible to have many synthesizers in a small amount of space because only one keyboard is needed to control them all. You'll find that many of today's most popular instruments are available as both a keyboard and a sound module.

One Cable, Lots Of Music
If MIDI isn't carrying audio information, how does it work? In order to faithfully transmit every nuance of a performance, MIDI uses many different messages.

When you hit a key on a keyboard, MIDI sends a Note On message with a note number for that specific key. This message also includes a Velocity value, which indicates how hard the key was pressed. When you release the key, a Note Off message is sent, which tells the connected instrument to stop playing that note. As you can imagine, playing a simple two-handed piano passage creates a flurry of MIDI information!

But that's just the beginning. Many of the most important aspects of MIDI are handled by Control Change (CC) messages. One common CC is used for sustain pedal information (also known as Damper Pedal or Hold Pedal). When you press the pedal, MIDI sends an On message for CC 64 (the Control Change number dedicated for sustain pedal information). When you release the pedal, an Off message is sent. The sustain pedal is an example of a switched CC message. It's either On or Off, with no gradients in between.

CC messages can also be continuous, with 128 steps of control (0-127). Volume (CC 7) is an example of a continuous controller that can be smoothly and gradually changed over time. Other commonly used continuous Control Change messages include Modulation (CC 1), often used for vibrato effects; Pan (CC 10), which is the placement of the sound between the left and right speakers; Reverb (CC 91); and Chorus (CC 93), a detuning effect which is used to fatten or enhance a sound.

Two more important MIDI messages are Pitch Bend and Aftertouch, which are crucial for re-creating the performance nuances of other instruments.

Pitch Bend can be used in small amounts, for example, to emulate the subtle finger-bending of a string on a guitar. Or the bend range can be much more dramatic, emulating the sound of a slide trombone. In addition to emulating acoustic instruments, Pitch Bend is commonly used to add expressiveness to synthesizer lead sounds. Different keyboards allow pitch bending in different ways, using levers, wheels, or even pedals. Unlike other MIDI controllers, Pitch Bend is centered at a zero value and can have negative or positive values. (Most MIDI controllers start at zero and can only increase with positive values.)

Aftertouch (also known as Pressure) is a MIDI message that is created by pressing down on a key after the initial attack. This gives the performer a very natural way to add vibrato or sforzando (volume swell) by simply pressing harder on the keys. Many pianists find this effect to be odd at first. With practice, however, it becomes an expressive technique to add nuance to your performance, because it doesn't require the use of additional pedals, levers, or sliders.

All of these MIDI messages can be separated onto 16 MIDI Channels. By using different channels, it's possible to have many musical parts carried by a single MIDI cable. This works similarly to cable television: the cable connected to your TV actually carries the programs found on all channels, but you “tune in” to one channel at a time to view a specific broadcast. Of course, with only two hands, you may feel that there's no need to worry about separating your musical performance onto different channels. This brings us to one of the most exciting applications of MIDI, known as sequencing.

A sequencer is a multitrack recorder for MIDI information. It allows you to record, overdub, and edit MIDI performances on different channels and play them back simultaneously, triggering sounds from your MIDI instrument. Sequencers are often included in many of today's keyboards, but the most popular way to record MIDI is by using computer-based sequencing software. This is an excerpt from the following article: MIDI 101.

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