High-pass Filter: Modular Synth Patching in Aspect, part 1

What is Modulation?
Aspect's flexible patching and modulation system can be a source of confusion to users more used to synthesizers with a fixed architecture. In synthesis, to modulate a parameter means to change the parameter's value whilst the sound is playing. This adds interest to what would otherwise be dull, static sounds. Most synth users are familiar with the basic modulation sources - envelopes and LFOs (low frequency oscillators). One common arrangement is the modulation of an oscillator's pitch by an LFO, producing vibrato. Another is the modulation of a filter's cutoff frequency by an envelope, resulting in a sound that has varying degrees of brightness as the higher frequencies are dynamically removed.

The Aspect Semi-Modular Synthesizer
Aspect is a synthesizer in the tradition of semi-modular synthesizers like the Korg MS-10 and Korg MS-20. The appeal of these instruments is not only in their fantastic analogue sound, but in their versatility. Re-routing means that any nearly any component can be used as a modulation source. The Patch panel expands this concept by allowing modulation sources to be combined and transformed in various ways. Download Aspect for Windows, Mac OS X, and Linux, in Audio Unit, VST, and RTAS Plugin formats.

Modulating Parameters
Rather than a confusing web of patch cables strung between modulation sources and parameters, Aspect uses a wireless modulation system. Modulation sources for a Parameter are clearly displayed with both their name and Modulation Depth (the amount that a Modulation Source alters a Parameter). The Depth for a particular parameter is listed in relevant units. For example, Modulation Depth for an Oscillator is specified in semitones. In the following screenshot, we can see that Oscillator1 is modulated 12 semitones by LFO2:

The basic steps for routing Modulation Sources in Aspect are as follows:

To assign a Modulation Source to a Parameter:

Left-click the Parameter's Modulation combo box.
Select the required Modulation Source from the drop-down list.

To remove a Modulation Source from a Parameter:

Left-click on the Parameter's Modulation combo box.
Select 'Off' from the drop-down list.

To change the the Modulation Depth for a Parameter:

Left-click and drag the Parameter's Modulation Depth rotary control upwards to increase the Modulation Depth, or downwards to decrease it. Holding Shift whilst dragging allows fine resolution changes to be made.

The Inverter Components
Aspect has 2 Inverter components, Inverter1 and Inverter2, both of which perform the same function. They take a Modulation Source and output a signal that equal to the input source flipped upside down. The following diagram illustrates this concept:

One typical use of an inverted Envelope shape is in the creation of pad sounds. Here we have a pad sound through a Lowpass Filter, with the Filter's Cutoff modulated by an inverted Envelope shape. This creates a sound with an unconventional flourish at the end: where a pad normally grows duller during the Release stage as the Filter is closed, here it gets brighter during Release as the Filter opens.

Download the Inverter Filter Pad Sound patch here.

Inverter Filter Envelope with a Pad Sound

Here is another example of an inverted Modulation Source. This tutorial on sequencing with a modular synthesizer uses an inverted Sawtooth LFO routed to the Output Amplifier to produce a quick Attack and slow Decay repeating envelope shape.

Because all signals within Aspect can be used for Modulation purposes, even MIDI control data such as Note Pitch and Velocity can be inverted. The following Program illustrates this:

Start with an empty Program by clicking File / New Program.
Assign Note Pitch to Inverter1.
Replace Oscillator1's Note Pitch Modulation Source with Inverter1.

The Oscillator is now controlled by the inverted Note Pitch. As you move up the keyboard, the pitch of the oscillator decreases!

Inverted Note Pitch modulation in the Aspect VST Plugin

Download the Inverter Keyboard patch here.

The Multiplier Components
Aspect has 2 individual Multipliers units, Multiplier1 and Multiplier2. Both of these take two Modulation Sources and produce an output that is equal to the product (multiplication) of the inputs. This is particularly useful for producing scaled Modulation Sources. For example, assigning both LFO1 and the Mod Wheel Modulation Source to a Multiplier produces an LFO output with a Depth can be controlled with the Mod Wheel:

Set LFO1 to a rate of 5Hz.
Set the first input of Multiplier1 to LFO1, and set the second to Mod Wheel.
Set Oscillator1's second Pitch Modulation Slot to Multiplier1, with a Depth of 2 semitones (Modulation Slot 1 is left assigned to Note Pitch so that the Oscillator will still track the keyboard.)

Mod Wheel controlling the LFO Depth in Aspect

LFOs that gradually fade up to their maximum Depth is another often used synthesis technique. Multiplying an LFO by an Envelope will accomplish this. In this case, the Envelope shape should have maximum Sustain. The Envelope's Attack parameter will govern how long it takes to fade in: