You can trigger another type of loop during a live performance besides standard audio loops: MIDI sequences that play as a loop are far more flexible and malleable than any audio loop, no matter how you chop up and process the audio. Nasty-sounding sonic artifacts often crop up when trying to significantly alter the performance or time-correct an audio loop, but a MIDI performance can play at any tempo with no loss in audio quality.
Sequencing used to be constrained to the studio because producing a decent mix from a MIDI sequence required a ton of gear. The sequencer, whether a stand-alone hardware unit or a computer software program, had to be connected to a stack of MIDI modules for the sounds; then, the audio outputs were patched into a mixing console and processed through outboard effects units, just to get a stereo mix. Today, this entire system can be housed in a single laptop computer or a diminutive groove box and easily carried to a gig.
MIDI loops are great because you can do cool things to change them up — things that are impossible or difficult to do to an audio loop. For example, if your groove box has an awesome MIDI drum loop, but you can't stand its bass-drum sound, simply reassign the performance's bass-drum note (generally MIDI Note Number 36, C1) to a different bass-drum sound. Perhaps a chord within a sequence is perfect, but the assigned synth preset lacks personality. No problem: Just write an automation curve into the sequence that sweeps the filter cutoff parameter in time to the beat. Maybe you have an ideal bass-line sequence, but it doesn't swing right with the drum beat. Well, it's a snap to change the feel of the bass performance by quantizing it to match the drum groove.
You can cook up your own sequences or work with third-party MIDI sequences (sometimes called MIDI samples), such as the Keyfax Twiddly.Bits series. Writing your own sequences usually produces the best results because you are able to customize performances to match your musical style and the capabilities of your gear. However, when you step onstage, it's ideal to have hundreds of MIDI samples at your fingertips, and composing all of those sequences personally can most definitely be a time-consuming endeavor. Incorporating third-party sequences can really help flesh out your performance library quickly, even if you just use them as compositional springboards from which to write your own sequences.
Several years ago, manufacturers began implementing a unique new feature that allowed you to trigger a MIDI sequence in much the same way that you could trigger an audio sample. Many software programs (such as Emagic Logic, Propellerhead Reason and Ableton Live 4) and a few workstations (such as the Korg Karma and Triton) let you assign individual sequences to specific MIDI Note Numbers for triggering and playback. For example, play a note on your keyboard, and the sequence assigned to that note will begin playing, just as if it were an audio loop. Generally, you have a variety of triggering options: You can press the note once to begin playback and again to stop playback; you can hold down a note, and the sequence will play for as long as you hold that note; or you can set the next sequence to begin playing only after the current sequence plays through.
For mixing different sequences on the fly (commonly called live sequencing), you must be able to trigger and play more than one sequence simultaneously. (All of the products mentioned can do this.) The sequences can be fully arranged and mixed multitrack performances or individual parts. Working with lots of short MIDI loops — one to two bars in length and with many variations — provides the widest and most flexible palette with which to perform live sequencing.
To keep multiple sequences synched to each other and any musical cohorts you're playing with, you will need to decide who and what will be your system's master clock. You have two choices: Either your sequencer is the master and everybody follows it or your sequencer is slaved to incoming MIDI Clock. The clock source you choose depends on your live setup. For groups in which one MIDI sequencer plays all of the loops, that sequencer should be set to internal clock, and all of the live musicians should follow its tempo (even the drummer). A group that has several live sequencing members, all with their own sequencer, should choose a dedicated hardware module (such as a groove box, which, from personal experience, is always more dependable onstage than a computer) to act as the master MIDI Clock source for all of the other sequencers. Employ a MIDI router (such as the Edirol UM-880) to distribute the groove box's MIDI Clock to all of the sequencers; avoid using MIDI Thru jacks, as they often do not pass through MIDI Clock correctly.
If you plan to mix MIDI sequences on the fly with a DJ, then you will need to match your sequencer tempo to the DJ's songs. A tap tempo feature allows you to adjust the sequencer's tempo in real time by simply tapping a key to the beat that you're hearing. If your timing is good, this method can produce tight beat matches fast. For software programs or workstations that don't have this feature, employ a groove box with a tap tempo key (such as with the Korg Electribe series) as your master clock, and from it, output MIDI Clock to your sequencer. If your timing isn't so hot, you can try an audio-to — MIDI Clock converter, such as the one onboard the Korg SX-1. Converters do not always produce tight beat matches because calculating an exact tempo is difficult when a beat is too busy, so counting on a converter for all of your beat matching probably isn't wise.
And remember, the only thing less dramatic than pressing a single key to trigger a sequence is taking a nap onstage. Just because you have all of that power at your fingertips doesn't mean those are the only things that you should be moving.