Propulsive rhythms are a cornerstone of pop music, so it's no surprise that today's music tools offer a variety of ways to build rhythm loops. If loading a sampled drumbeat doesn't satisfy your craving for creativity, the looping multisegment envelopes found in a number of software instruments may be exactly what you're looking for.
FIG. 1: This screen shot shows three multisegment envelopes in Native Instruments Absynth 4. The breakpoints are the small blue squares, and data for the selected breakpoint is shown in the upper area of the screen.
The envelope generators in first-generation synthesizers (around 1970 to 2000) were used mainly to shape the tones of single notes. For this, simple envelopes worked fine, and the ADSR (attack, decay, sustain, release) envelope quickly became an industry standard. But if you wanted to hear a rhythm from a synth equipped with ADSRs, you pretty much had to play it yourself on the keyboard.
Some synths have arpeggiators, with which you can generate an automated pattern. But an arpeggiator plays a string of single notes, not an envelope-based groove. An arpeggiator's output, in other words, takes the place of a keyboard performance.
With a looping multisegment envelope, pressing and holding a single “note” on the keyboard can produce a complex, infectious pattern — a chord rhythm, or perhaps a bass line layered with a beat that includes electronic kick and hi-hat. In this column, I'll discuss the main tools needed to craft this type of envelope.
Sources and Destinations
Before looking at the envelopes themselves, let's do a quick review. An envelope generator is a source for a modulation signal. The signal is called an envelope. You don't listen to the envelope itself — you listen to the tone it's modulating (usually the sound coming from an oscillator). One or more envelopes shape the tone while it's sounding.
With a sophisticated synth, you have a number of parameters that can be modulated with envelopes, and you have several envelope generators to use as modulation sources. Typical destinations for envelope modulation include the pitch of the oscillator, the cutoff frequency of a filter, the amount of frequency modulation (FM) being applied to the oscillator's waveform, and the amplitude of the signal coming from the oscillator. Multiply that by three oscillators and three filters, and you can easily have a dozen envelope generators chugging away at once. By assigning the oscillators to different tasks, you can produce a seemingly polyphonic pattern from a single MIDI note (see Web Clip 1).
Break It Up
With a multisegment envelope generator, a complex envelope shape is defined using a series of breakpoints (see Fig. 1). Each breakpoint defines the amplitude (output level) of the envelope signal at a certain moment in time. The height of the point on the screen determines the level of the envelope, and the point's left-right position determines the time at which the envelope reaches that level. Some multisegment envelope generators have 16 breakpoints; others have 64 or more.
FIG. 2: The rhythm pattern played by Native Instruments Massive in Web Clip 2 is generated in a Performer module, which is a hybrid design incorporating elements of an envelope generator and a step sequencer. Only the first 8 of the 16 steps are active, but some steps have several envelope points.
Typically, you edit the envelope by dragging the breakpoints around with the mouse. When a few points are higher than others in the same envelope, the high points produce rhythmic accents. When some points are farther apart than others, you get sustained notes or rests.
The portion of the envelope that lies between two adjacent breakpoints is called a segment. If point 1 is high and point 2 is low, the segment between 1 and 2 will have a falling shape; the reverse is also true. If the two points are at the same height, the segment is horizontal.
These days, most envelope segments in software synths have a handle in the middle. This handle controls the curvature of the segment and can be dragged up or down with the mouse. A segment can be concave, convex, or linear. The difference in the results produced by these shapes is difficult to describe in words. Fortunately, they are easy to see and hear. If you have a synth whose envelope segments allow the curvature to be adjusted, you can easily make changes and listen to the results.
A few synths go beyond segment curvature to allow refinements such as stepped segments, which have a staircase shape; segments with built-in LFO-wave contours; and contour generators that have features of both envelope generators and step sequencers (see Fig. 2).
Some envelope generators can be set to either mono or poly mode. In mono mode, all of the voices you're playing use the same envelope(s); in poly mode, each note gets its own complement of envelopes. When poly mode is active, try playing a chord with different notes starting on different beats within the measure. This is a great way to produce complex, hypnotic rhythms.
On the Grid
When editing an envelope for rhythmic purposes, you'll probably want to switch on the snap-to-grid function. When this is active, dragging a breakpoint left or right causes it to “snap” to the nearest rhythmic value. Normally the grid will provide 16th-note rhythms, but some synths have additional grid settings.
With most synths, you can choose to edit only one point at a time or you can switch on slide mode. In slide mode, when you drag a given point left or right, all of the points to the right of it (the points that are later in time) also move. This is a convenient feature, especially with envelopes that have 20 or 30 points, as it saves you from having to edit points one at a time.
Snapping to a grid would be of little value unless the envelope generator could synchronize to the tempo of the host sequencer. Sync is often switched on by default in multisegment envelopes. A few synths, notably Native Instruments Massive, play their synced envelopes only when the sequencer's transport is running. If you audition Massive patches when the sequencer is stopped, you won't hear any rhythmic envelope patterns.
FIG. 3: u-he Zebra 2 has four multisegment envelopes. The loop start and end points are orange rather than green, and the loop is displayed as a gray bar in the time ruler (top).
Most syncable envelopes don't sense the actual beats and subdivisions of the host sequencer's bars, only the raw tempo. If the note you play or record starts slightly late with reference to the beat, all of the envelope points of the synced envelope(s) will be late. After recording a sequenced track that will play a preset that uses synced envelopes, it's normal to use quantizing to align the start of each note cleanly to the beat. It's also useful to edit the ends of the notes carefully in a piano-roll display so that the rhythm pattern cuts off at a musically meaningful moment.
You'll probably be able to choose any breakpoint as the start of the envelope loop and any later breakpoint as the loop end (see Fig. 3). When you play and hold a note, the envelope will advance until it reaches the loop end point, then jump back to the loop start point and continue.
If there are several points before the start of the envelope loop, the rhythm pattern will have a sort of “intro.” If there are several points after the end of the loop, you'll hear these only after the MIDI note ends. The end segments can create an echo or fill at the end of the pattern.
After creating an envelope, you may be able to store it as a template. You can load it into a different patch or copy it to a different envelope generator in the same patch, then edit the copy and modulate a different sound parameter with the new envelope. This is a quick way to create complex rhythms that make some type of musical sense. If you're running several envelopes within one patch, they don't all have to be the same length. One of them might be an odd number of eighth notes long, producing a cross-rhythm.
Catch a Wave
With so many options and parameters to work with, coming up with a great envelope-based rhythm takes vision and patience. I usually start by auditioning the factory patches. Some work fine right out of the box, and others may need some tweaking (see Web Clip 2).
If you don't already have a software synth equipped with multisegment envelope generators, check the Web for downloadable demo versions of the synths used in this month's Web Clips. You'll be amazed at how much fun this feature opens up.
Jim Aikin writes regularly for EM and other music-technology publications. He also teaches classical cello, writes computer-based interactive fiction, and composes and records in his PC-based home studio.