An Introduction to MIDI

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Contents

Sequencer Editing Features Editing features Quantizing: keeping time Conductor tracks and tempo maps Synchronizing with external equipment Combinging MIDI with digital audio Introducing standard MIDI files

Introduction

In 1982 the largest companies in the electronic music industry overcame their normally secretive and competitive urges and agreed to cooperate. The result of their collaboration was not a hot new musical instrument, but a 13-page document that has literally changed the way the world makes music.

That document described the MIDI specification. MIDI was developed to enable musicians to connect electronic instruments to each other and to computers. The MIDI specification spells out the types of wires and connectors that unite musical instruments, as well as the commands and codes that MIDI-equipped instruments transmit and respond to. Generally, any piece of equipment with MIDI -- whether a musical instrument or a computer -- can talk with any other piece of MIDI gear.

On a basic level, MIDI lets you create a network of two or more instruments that you can play from just one instrument. Musicians often use this technique, called layering, to play multiple instruments simultaneously to obtain a richer sound.

On a somewhat more advanced level, MIDI lets you connect one or more instruments to a computer to record and play back music and add accompaniments. This aspect of MIDI has helped create a new phenomenon -- the home recording studio.

And at its most advanced level, MIDI lets you combine a computer-controlled network of instruments with audio equipment and even stage lighting to automate an entire performance environment.

MIDI Basics

MIDI data can travel in two directions at the same time -- from an instrument to a computer and from a computer to an instrument. To accommodate this two-way traffic, every MIDI device has two connectors -- MIDI In and MIDI Out. Some devices have another connector called MIDI Thru, which can be used for chaining MIDI devices together.

How MIDI interfaces work

Peer behind the Mac and you'll notice there are no such connectors. Unlike some personal computers, Macs don't come equipped for MIDI but need a separate piece of hardware called a MIDI interface, which connects to the Mac's modem or printer port and provides MIDI In and MIDI Out connectors. Several MIDI interfaces are available for the Mac, ranging from Apple's $99 Apple MIDI Interface to Opcode's Studio 5 and Mark of the Unicorn's MIDI Time Piece series. The high-end MIDI interfaces provide more MIDI In and MIDI Out connectors, enabling you to create a larger MIDI network. They also provide sync features that enable the Mac's MIDI playback to be synchronized with an external device such as an audio tape recorder. More about syncing later.

Understanding MIDI data

The illustration at the top of this page shows three different ways to connect MIDI instruments with an interface to relay MIDI data (also called messages or events) between instruments and a Mac.

What kind of data travels via MIDI? First and foremost, note data. When you play a MIDI instrument's keyboard, it tells the Mac which keys were pressed and for how long. Velocity-sensitive keyboards also note how hard each key was pressed, letting the Mac capture the varied dynamics of your performance. Note data is by no means the only kind of information that can travel on MIDI cables. Here are some MIDI messages that instead of playing notes play other roles in the performance.

Program changes instruct an instrument to switch sounds -- from piano to strings, for example.

Continuous data generally modifies the way a sound is played. For example, many instruments have pitch bend wheels or levers that let you slide between pitches the way guitar players do when they bend a string. Another kind of continuous data is aftertouch, which describes how hard a note key is being held down. By pressing harder on a key after you press it, you may add vibrato or cause a string sound to get progressively louder. Not all keyboards send aftertouch, but those that do allow for a greater range of expression.

Clock or sync data carries information about the timing of a MIDI performance. It's often used to synchronize a network of MIDI instruments to an audio tape recording.

System-exclusive data includes information pertinent to a specific model of MIDI instrument, such as the contents of its internal memory, or the MIDI-channel assignments of its sounds. By transferring system-exclusive data to the Mac, you can store and alter an instrument's sounds, and then transfer the data back to the MIDI instrument.

Understanding MIDI channels

MIDI instruments can receive or transmit data on any of 16 independent channels -- electronic mailing addresses that accompany MIDI data and specify its destination. Not only can you specify the channel MIDI instruments use to transmit data, you can also configure them to respond to data sent on all MIDI channels (omni mode) or only to certain ones (poly mode). This ability to channel MIDI data is important because many MIDI setups comprise more than one instrument, some of which may be multitimbral -- capable of simultaneously producing different types of sounds, such as those of a drum set and a horn section. If you couldn't assign certain MIDI data to certain channels, there'd be nothing to stop one instrument from playing another's part.

Surveying MIDI Software

Without a computer, MIDI data plays a valuable but limited role: It lets you play numerous instruments using just one controller. It was for this humble role that MIDI was originally created.

How MIDI sequencers works

But MIDI data becomes much more useful when it's combined with a computer and software that can store and manipulate it. The most popular kind of MIDI software is the sequencer, a kind of tapeless tape deck that lets you build your own arrangements by recording parts one track at a time. You may start with a drum or bass track to establish a rhythm, and perhaps specify that it loop, or repeat, continuously. Next, you may add a guitar melody, and then some strings to sweeten things up. During playback, you route the tracks to the appropriate instruments or to the appropriate sounds within a multitimbral instrument by specifying a different playback channel for each one.

Sequencers versus audio recording software

On the surface, a sequencer seems similar to a multitrack tape recorder or to digital audio-recording software such as Macromedia's Deck II. But a sequencer doesn't store sound; instead, it stores the sequence of MIDI data that describes what you played. MIDI's storage technique has a few significant pluses. First, MIDI data requires far less disk space than digital audio data does. A ten-minute, CD-quality stereo audio recording requires 100MB of disk space; a ten-minute MIDI sequence may use 30K or so. It's MIDI's bird-like appetite for disk space that makes MIDI movies so efficient in QuickTime 2.x. Also, because the MIDI data in a sequence isn't tied to a particular sound, you can change an instrument's settings before or during playback to hear how that electric guitar part sounds when played by an acoustic guitar or maybe an oboe. You can also work up an arrangement using an economical home system and then take your disk into a recording studio and play the sequence using state-of-the-art gear.
And because you're working with MIDI data, you can continue adding tracks without compromising the sound quality. With analog audio recording, each time you bounce two or more tracks to a single track to free up a track for recording, the sound quality of the older tracks suffers. With a sequencer, the tracks exist in the Mac's memory, not on audio tape. So you can add as many tracks as you have memory for, and every playback is an original performance.

MIDI Vanilli?

Perhaps best of all (at least for those of us who can't practice eight hours a day), you can use a sequencer's extensive editing features to correct misplayed notes or to add more dynamic expression. You can cut and paste sections of a recording, for example, to remove extra verses or repeat a part. And with a sequencer's step recording mode, you can manually enter difficult parts one note at a time, or slow down the tempo and record them at a more leisurely pace. Is it cheating? Some may say so, but it lets you make better music, and the results go a long way toward soothing your guilt.

Sequencer Editing Features
Here's a closer look at the kinds of features you can find in Macintosh sequencers.

Editing features

For correcting or inserting notes in existing tracks, three basic schemes exist. Graphic editing displays a track's contents on a music staff-like grid, except that notes are shown as horizontal bars, with longer bars representing longer notes. Graphic editing lets you select and drag notes from one position to another using the mouse. Because a graphic editing display resembles a player piano roll, it's often called a piano roll display. Some programs, including Mark of the Unicorn's Performer and Opcode's Studio Vision Pro, can also display a sequence in standard music notation. Event list editing displays a track's contents as a table of MIDI data. It doesn't give you the click-and-drag convenience of graphic editing, but it allows for greater precision, because you can type and edit the exact values that describe individual notes or other MIDI data. Better sequencers provide both types of editing windows.

Quantizing: keeping time

For tweaking the timing of notes, sequencers provide quantizing features, which cause the program to move notes to the nearest note value you specify. If used excessively, however, quantizing can give sequenced music an overly mechanized feel; after all, no one plays every note exactly on time. To eliminate this undesirable side effect, most sequencers let you specify a margin within which notes aren't quantized, and thus you can neaten up your playing without making it sound robotic. Some sequencers also provide a humanize option, which does the opposite of quantizing: It nudges notes off their exact beat values to improve the feel of a passage that was overly quantized or entered using a step-recording mode.

Conductor tracks and tempo maps

Many pieces of music don't have the same tempo throughout. To accommodate such pieces, sequencers provide a special track, often called a conductor track, that stores tempo information. Using the conductor track, you can create a tempo map that describes the tempo changes in the piece. With many sequencers you can specify the tempo by tapping a key on a MIDI keyboard.

Synchronizing with external equipment

If you combine a sequencer with external equipment, such as a multitrack audio tape recorder or a high-end video recorder, you'll need a sequencer that can be locked to synchronization codes sent by that external source. By recording a sync track on a tape recorder and feeding that track into a sync-supporting MIDI interface, you keep the sequencer and recorder in exact synchronization. You can use sync to add sequenced electronic music to an acoustic recording, or to create a multitrack audio recording using a single MIDI instrument to record one track at a time, synchronizing the sequencer's playback with the tracks you've already recorded on tape. Sync features are commonly used in TV and movie soundtrack production, in which MIDI sequences of music or even sound effects are synchronized to visual action. In these cases, a sequencer is synchronized to a film editor or videotape recorder using the industry standard SMPTE time code.

Combinging MIDI with digital audio

At the leading edge of the sequencer world, you can find sequencers that can combine MIDI data and digitally recorded audio in the same file, enabling you to add vocals or acoustic instrument recordings to a sequence. It's the best of both worlds. Sequencers that support hard disk digital audio recording include Mark of the Unicorn's Digital Performer, Opcode's Studio Vision Pro and Studio Vision AV, and Steinberg's Cubase Audio. To record audio with these products, you need appropriate digital audio hardware, such as Digidesign's Audiomedia II. Some sequencer-recorder programs, including Opcode's Studio Vision AV, require no additional hardware when used with an AV or Power Mac.

Introducing standard MIDI files

If you're a pro, you may end up using more than one sequencer. Fortunately, virtually all support a standard file format for exchanging sequences. This format is called, amazingly enough, the standard MIDI file format, and it enables sequencers -- even ones running on different computers -- to exchange recordings.
If you're familiar with Mac graphics concepts, think of standard MIDI files as the PICT file of the MIDI world--a file format that all programs can understand. When you want to move a MIDI sequence from one program to another -- or if you want to convert a sequence into a QuickTime music movie -- you save the sequence as a standard MIDI file, usually by using a Save As or Export command.