Modular Techniques December 2020: Filters as Waveshapers
Using an Extra Filter as a Waveshaper
A general topic I’ve been focusing on this year is how to take the standard waveforms produced by typical VCOs and LFOs, and create fresh new variations on them.
The most recent technique I’ve been playing with is patching an extra filter directly after the VCO, and having that new VCF track the VCO’s pitch. This allows the VCF to alter the VCO’s waveform and therefore its harmonics & timbre before it gets sent off to the normal VCF/VCA or wavefolder/LPG patch chain for further treatment. This technique excels in particular at creating vocal-like sounds. Some of you are probably already familiar with this trick, but for those who aren’t, read on.
Since you may want to jump right in and try this out, here is the linked index to the movie above; after it I will share where the inspiration for this came from, and potential ways to extend it:
Since you may want to jump right in and try this out, here is the linked index to the movie above; after it I will share where the inspiration for this came from, and potential ways to extend it:
00:00 introduction
01:07 the basic patch
02:20 low pass filter with sawtooth
05:00 mixing in the unprocessed wave
07:38 high pass filter examples
08:54 notch filter examples
09:22 bandpass filter examples
10:32 alternate filter (Doepfer Wasp)
11:24 phase inversion revisited
13:55 VCO modulating cutoff
15:15 summary
Basic Application
This idea came from using the Birdkids theBateleur VCO. It features a resonant low pass filter internally patched after its suboctave waveform. At its most basic level, this allows you to tame the natural square wave of the suboctave generator to have fewer, smoother high harmonics, including filtering it all the way down to just its fundamental harmonic.
This technique can be patched together using any VCO and VCF that can track a normal pitch control voltage. I’ve had a lot of success patching a 2 pole multimode VCF after an analog VCO, which is what is demonstrated in the video above.
Increasing the resonance helps you focus on and emphasize a specific band of harmonics, or even a single harmonic: perhaps the fundamental to strengthen the bass; perhaps the second harmonic to add warmth; perhaps the third to emphasize a perfect fifth above the fundamental; perhaps an “odd” harmonic like the 9th or 11th to create an off-kilter interval without having to patch in a second VCO.
Choose the filter mode to based on which harmonics you want to reduce in level – for example, low pass to reduce the high harmonics, or high pass to reduce the lower harmonics. Or, you can use a notch output with minimal resonance to remove a small band of harmonics, creating an ear-catching “hole” in the sound. This approach also helps a single VCO emulate the sound of two VCOs tuned to an interval, tracking each other perfectly.
This added filter is particularly good at creating sounds akin to the voice or acoustic instruments, in addition to new sounds. A resonator-type filter module – such as a triple bandpass, or a filter pair in series or parallel – can modify the VCO’s harmonics in even more interesting ways. Many early electronic instruments and synthesizer keyboards had resonators built in; it’s a practice we’ve gotten away from, but which is still very useful. Another approach is to try a filter with multiple inputs – such as a Steiner-type filter, or a Three Sisters – to mix together multiple waveforms by taking a different range of harmonics from each one.
If your filter has any interesting “quirks” – such as a strengthened fundamental when you overdrive its input, or an unstable warble when you increase its resonance near self-oscillation – these can also add a unique character to your VCO’s sound.
Keep in mind that you can still envelope a waveshaper or a second filter after this VCO/VCF pair to create your normal per-note articulations. I will also often mix in a second waveshape or oscillator to fatten up the sound.
Adding Articulation
The original patch I was using with theBateleur varied its filter’s cutoff using keyboard velocity or the output of a sample & hold triggered by a seqeuncer. You can also patch performance controls such as aftertouch, a mod wheel, or a pressure sensing pad to the cutoff to directly articulate which harmonics are being emphasized while you hold a note. A chaotic modulation sources patched to this extra filter’s cutoff can be used to emulate the “instability” of some older instruments.
In any of these cases, try attenuating this control voltage so it only moves the cutoff across the range of 2-4 harmonics; this will make it sound more like human articulation than a wild, synthetic sweep (that’s what your normal filter is for).
Remember that this second CV needs to be summed with the pitch CV, so that this extra filter tracks the VCO withthese added variations. Fortunately, many filters have both a dedicated 1v/octave input as well as a general-purpose modulation input. Otherwise, you can use a simple CV summer or mixer. The precision of this summer or mixer is not as critical as when directly controlling the VCO’s pitch.
It’s Not Just For Sawtooth Waves
Don’t restrict this idea to just simple analog VCOs – definitely try it with complex oscillators, digital wavetableVCOs, and more.
I read recently on a forum where a user complained that their wavetable VCO sounded more like an organ – and indeed, when you don’t modulate a wavetable, the result is the same as using the drawbars on an organ to create a different set of harmonics. The trick is to add life to these harmonics by modulating their mix. Most wavetable VCOs allow you to crossfade between adjacent waveforms, but they pre-choose those waveforms for you (unless you spend some time with an editor). Using this filter-as-waveshaper technique allows you to choose the initial waveform you want, and then create your own variations on it.
And by the way, this technique can also be used with many polyphonic hardware or software synths that include more than one filter in a patch. Use one of the filters after a sound source and have it track the keyboard; then envelope and modulate the additional filter(s) to get your normal sonic variations per note. Start with a serial arrangement of the filters, as this will be most obvious, but you can also try parallel filters for a more subtle effect.
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