Monday 4 May 2009

Modular synth sequencing, without a sequencer

This article presents a technique for producing step sequencer style patterns in a modular synth, without using a sequencer. We'll again be using the Aspect semi-modular synth. Download Aspect for Windows, Mac OS X, or Linux in Audio Unit, VST Plugin and Standalone application formats here. (Note: Only registered Aspect owners will be able to open the patches that accompany this tutorial.)

Start with a blank program configuration by clicking File / New Program.

A sequencer connected to a synth will trigger a succession of notes, where the volume contour of each note is controlled by the synth's amplifier envelope. This triggered envelope can be emulated by instead using a repeating modulation source to shape the note volume. We'll use a percussive volume contour: a quick attack, and slow decay. A Sawtooth LFO (Low Frequency Oscillator, in this case LFO1) will provide the shape we need, but from the following display we can see a problem: Aspect's Sawtooth LFOs ramp upwards, which is equivalent to a slow attack, and instant decay.

Sawtooth LFO modulation source

This is easily corrected by linking the LFO's output into an Inverter component (we'll use Inverter1 in this example). As you'd expected, this generates the shape we need (see diagram below.)

Inverted Sawtooth LFO modulation source
Which, when connected to the Output Amp Modulation and Synced to a rate of 1/16, produces the following 16th note pattern:

Download the gated patch for Aspect by clicking here.

The remaining 2 LFO components will be used to make the notes within our sequence play at different pitches. Both LFO2 and LFO3 are set to Synced Square waves. LFO2 is set to a rate of 1/8, and LFO3 to a rate of 1/4. when we align all three LFO signals we can clearly see a short sequence that repeats every four 16th notes:

Repeating LFO modulation pattern

The LFOs now need to be connected up as modulation sources to their required destinations. LFO2 should go to Oscillator1's second Pitch Modulation slot, with a depth of 2.00 semitones. (Remember that holding down the Shift key whilst dragging a rotary control will allow for more precise changes. This is useful when exact modulation depths are required.) LFO3 should be connected to Oscillator1's third Pitch Modulation slot, this time with a depth of 7.00 semitones.

With LFO2's Square modulation output causing Oscillator1's Pitch to move between 2 semitones up and 2 semitones down every 16th note, and LFO3 causing a modulation of 7 semitones, up and down, each 8th note, we get the following jaunty pattern repeated every four 16th notes:

+9 semitones
+5 semitones
-5 semitones
-9 semitones

When this is played through Aspect's lowpass filter, set to a moderate resonance, sounds something like:

Download the sequencer patch for Aspect by clicking here.

Due to Aspect's flexible semi-modular architecture, even without a dedicated sequencer interesting melodic patterns can be created. We've explored the simplest example - using LFOs to control a note's pitch - but for the more dedicated sound designer, Aspect's audio rate modulation sources and Patch section components provide the building blocks for a wealth of complex sequencer-style patterns.

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