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Ask a typical Mac user the meaning of the term MIDI, and you'll probably get one of two responses: "You mean those cheesy music files you find on the cheesiest of Web sites?" or "Huh?" MIDI has gotten something of a bad rap, thanks to the Web.

But in fact, it's an important music-making tool. The announcement of GarageBand earlier this year has brought the MIDI discussion to mainstream Mac users -- and for good reason. Thanks to this marvelous application, even people with the barest modicum of musical talent have turned their minds (and ears) to the tuneful potential of the Mac. If you want to gain a better understanding of what makes GarageBand groove, knowing a bit about MIDI helps. We'll provide some background and show you how to use MIDI files to expand GarageBand's repertoire.

What Is MIDI?

MIDI is an acronym that stands for Musical Instrument Digital Interface. Though many people are tempted to describe MIDI as a thing -- a kind of music found on Web sites, for example -- it's actually a communications standard that was developed in the early 1980s by musical-instrument manufacturers. MIDI was created to assist keyboard players who found themselves dashing from one synthesizer to another, playing the variety of sounds necessary to briefly divert the spotlight from the lead guitarist. MIDI cut back on some of the aerobic activity by allowing those keyboard players to control all their instruments -- a keyboard, a woodwind-like wind controller, and a drum pad, for example -- from a single controller. To make that possible, the music manufacturers decreed that MIDI instruments would sport a specific 5-pin connector for moving information from one device to another, and that those devices would communicate using a common data format (delivered in the form of MIDI messages).

There are a variety of MIDI messages. The most common communicates that someone has pressed a particular key on a musical keyboard (called a Note On message), with a certain amount of force (or velocity, in MIDI lingo), and for a specific period of time -- basically replicating what happens when someone strikes a key on a piano. This MIDI message will probably be followed by another that's generated when a musician has stopped playing a note (a Note Off message). Other MIDI messages might tell a drum machine to change the sound it's playing from a thumping bass drum to a sizzling cymbal. Yet another message could instruct a synthesizer to change the key it's playing in (or transpose) from C to G.

Computer manufacturers quickly got hip to MIDI. They understood that it's all well and good for synthesizers to talk to one another, but the real payoff comes when a computer can play, record, and edit MIDI data and control a host of musical instruments. Programs called sequencers, which are essentially MIDI tape recorders, were developed to do exactly that. Early sequencers displayed music not as the kind of notation you'd find in a piece of sheet music, but as a series of events in a long list, or as dots and dashes on a timeline (called piano roll style display).

Some years later, the Standard MIDI File (SMF) format emerged. SMF was devised so that musicians could record MIDI music on one instrument or computer and then play it back on another instrument or computer, and get a file that sounded nearly the same -- containing the same instrument sounds mixed at the same relative volumes.

Playin' in the GarageBand

Beneath its friendly exterior, GarageBand is a MIDI sequencer -- one that also happens to record and play digital audio files, and that carries its own library of sound samples (called Software Instruments) and effects. Plug a MIDI keyboard into your Mac, and, with GarageBand's help, you can record, edit, and play back MIDI data, as well as the sounds included with the program. Unlike most sequencers, however, GarageBand doesn't allow you to import SMFs -- something you might want to do if the grooves and riffs included with GarageBand are too limited for your tastes.

Thankfully, there's a way around this limitation. Follow along and see how to create your own rehearsal backing track by importing the kind of groove any garage band would be proud to play -- the 12-bar blues.

1. Getting the Blues Launch your Web browser, travel to your favorite search engine, and enter MIDI 12-bar blues in the search field. Pick a promising link and look for a blues MIDI file. Now control-click on the link and select the command that downloads the MIDI file to your Mac. The file should end in .mid (Blues_in_C.mid, for example).

2. Making the Blues Download a copy of Bery Rinaldo's free Dent du Midi ( http://homepage.mac.com/beryrinaldo/ddm ) -- a utility that converts SMFs into GarageBand MIDI loops. Launch Dent du Midi and drag the blues file you downloaded into the upper portion of the window. The program will create a folder, extract the individual MIDI tracks that make up the song, and place them in the folder.

Open this folder, double-click on the Report.txt file, and make a note of the song's tempo. When you create a new song in GarageBand, you'll enter this tempo in the BPM (beats per minute) field so the notes will line up with the timeline's grid.

3. Bringing the Blues into GarageBand Launch GarageBand and create a new song. In the New Project window, enter the MIDI file's tempo in the BPM field -- 104, for example. Leave the time signature at 4/4 and, if you know the key the file is in, select that key from the Key pop-up menu. Click on Create to open the new song.

Now select all the .aif files that Dent du Midi created, and drag them into GarageBand's main window to import them. Each file should bear the name of its instrument -- Rhythm Guitar, for example. If the tracks aren't named, you can determine their function by context -- a track with lots of rhythm but no melody is the drum track.

Next, double-click on the New Track entry for the drum track. In the Track Info window that appears, select Drum Kits from the left column and then pick a style, such as Rock Kit, from the right column. Do the same for each instrument, and close the Track Info window when you're done.

Note that the bass track on these imported MIDI files is often an octave too low. If you'd like to adjust the bass, double-click on the timeline portion of the bass track and, in the Advanced section of the resulting pane, enter 12 in the Transpose field; then press return. This moves the bass track up an octave (12 half steps).

4. Playing the Blues Your backup band is ready. It's time to step into the spotlight and rip out a solo that'll make B.B. King toss away Lucille in shame. (But remember that someone took the time to create the MIDI files, so don't use them to record music without permission.)

Select New Track from the Track menu (or click on the plus sign [+] to the left of GarageBand's playback controls), and choose either Software Instrument or Real Instrument (depending on whether you're going to play a MIDI instrument plugged into your Mac or a real instrument such as a guitar or your voice). For MIDI input, select an appropriate sound (Big Electric Lead in the Guitar section, for example), press the spacebar to trigger the sequence, and start jamming. To play a real instrument, select the kind of instrument you're playing and an appropriate effects setting -- Heavy Blues, for instance. Press the spacebar and play (or sing) the blues.

Rattle and No Hum

You may have noticed an annoying hum coming from your home stereo speakers when you connect your Mac or music-streaming device (such as Slim Devices' Squeezebox) to the receiver, using analog cabling. If so, don't toss your speakers or stereo out the window. The likely culprit is a ground loop, caused by differences in resistance in your electrical wiring.

To break the loop and silence the hum, give Xitel's $30 Ground Loop Isolator ( www.xitel.com ) a try. Plug the cable end into an RCA input on your receiver, and then attach your Mac or audio component to the Isolator. All you'll hear are the sweet sounds of your favorite bands. -- jonathan seff

Hit the Stage

Are your GarageBand compositions starting to sound a little stale? If you've been using many of the program's included Software Instruments and are looking for a fresh sound that will make you and your MIDI keyboard stand out, check out the $54 SynthPack for GarageBand, from Britain's Advanced Media Group ( www.samples4.com ). This collection of 154 vintage synth instruments packs in some memorable classic synths from the eighties and nineties -- including MiniMoog, Xpander, Octave Cat, and Matrix 6. With all these bass, string, brass, drum, organ, choir, and other sounds, you can become a one-hit-wonder in no time. -- jonathan seff

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