Modular synthesis employs the use of separate modules in sequence as a method of sound synthesis. More specifically it is a form of subtractive synthesis; a process in which the timbre of a sound is formed by attenuating unwanted frequencies to 'carve out' the desired sound. This post is designed to give you a fundamental understanding, demonstrated with the use of a virtual modular synthesizer. I recently installed VCV Rack, an open-source modular synth, and with its aid will show you how to create your own patch by investigating its foundational modules: VCO, VCA, LFO, ADSR...
VCO - Voltage Controlled Oscillator
Most often the first module required is a VCO. This is what generates the initial waveform, dependant on whichever output is chosen from the bottom: sine wave, triangle wave, saw wave, or square wave (SIN, TRI, SAW, SQR). In a real analogue module a patch lead would be plugged into one of the four options and the signal could be taken elsewhere; with this software however, obviously it's replicated with a mouse movement. The frequency of the wave can be controlled using the FREQ dial, with fine-tuning performed using FINE. When using a square wave, the P. WIDTH dial can be used to control the 'pulse width', which affects the waveform quite audibly. Finally the FM- and PWM-CV dials (frequency and pulse width modulation respectively) are used to control the amount of modulation on the signal, somewhat similar to a wet/dry concept; both will be discussed further whilst investigating LFO's. The input section also features a V/OCT or 'voltage per octave'. Although this could also be patched into an LFO, typically it is used as an input to connect some kind of MIDI device, such as a musical keyboard. I noticed that when patched into the CV of the MIDI module, different notes would be played with different keys, rather than the computer keyboard acting purely as a gate.
[TIP: insert a fundamental module called a SCOPE to visualise any of these waveform changes].
Ultimately though, the biggest issue with a VCO is it has no level control...
VCA - Voltage Controlled Amplifier
A VCA is capable of taking the signal IN and adjusting its level by sliding the green bars up or down, before it can be taken out again from the black section at the bottom. Though this too can be modulated by an LFO being patched into the CV (control voltage) plug. Further control is gained by selecting whether the voltage is affected in either a liner (LIN) or exponential (EXP) way.
AUDIO INTERFACE
By including an audio interface and appropriately routing your outputs, this is the simplest sound that can be created; stereo if it is sent to both channels 1 & 2.
[TIP: by holding the command key, you can insert a second cable to the same port]
LFO - Low Frequency Oscillator
LFO's are introduced to create movement in the sound, altering a parameter on another module in accordance with a low frequency (or slow) oscillation. Essentially they can be creatively used to modulate the sound in numerous other modules. One such example, touched on earlier, could be to patch a wave from the LFO into the VCO's PWM or 'pulse width modulation', which would change the square wave's waveform over time, correlating to the frequency setting on the LFO. Similarly, if patched into the VCO's FM, the output waveform will change in frequency over time, with respect to the parameters on the LFO, as well the FM CV level on the VCO.
In the below example, the pulse width of a square wave on the VCO is being modulated by a sine wave from the LFO. Simultaneously, the control voltage (level) is being controlled on the VCA by a saw wave from the same LFO.
To get more complex, this LFO featured has a further two parameters available, FM1 & FM2, that can themselves be controlled by yet more LFO's. Furthermore, its square wave can also be pulse width modulated. By stacking LFO's (or other modules) with each other, the possibilities really do become endless; and so you start to see how complicated and experimental one can get with modular synthesis.
MIDI
The above example results in a continuous (and somewhat annoying) sound. Instead of having to constantly turn down the level on the amplifier, it is a great idea to insert a MIDI module. Using this, the GATE can be patched into the CV of the VCA. By selecting the appropriate inputs (in my case computer keyboard, QWERTY; though there are endless external possibilities), the sound is gated until a key is pressed, indicating the signal to be sent.
ADSR - Attack, Decay, Sustain, Release
In order to shape the way the signal is presented, a standard ADSR envelope can be employed. By patching the GATE from the MIDI module into the GATE input of the ADSR module, and then OUT from there into the CV of the VCA. This way, the voltage is controlled, whenever a key is pressed, according to the shape of the ADSR module, with the waveform dependent on the VCO, which is being modulated by the LFO.
Although again, this is just one possibility. Instead you may want a continuous evolving sound, and the MIDI GATE could be used to initiate a certain level of frequency modulation, or the pulse width of an LFO or VCO.
FINAL THOUGHTS
It my appear complicated, but with a little knowledge at a time, and a lot of experimentation, modular synthesis can yield interesting and unique results. In its easiest form, what we have covered here could be simplified to:
VCO - makes sound (according to a voltage driven oscillator)
LFO - automates parameters (responding to a low-frequency oscillation)
VCA - controls level of sound or parameter (by controlling the voltage)
ADSR - controls sound envelope
In a future synthesis blog, I will investigate some more basic concepts: shaping sounds with filters, introducing sequencers, inserting effects units, and further patch possibilities.
Stay Tuned
- TA
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