FORMANT FILTER
for
Eurorack
Modular
Synthesizers
Instruction Guide
v2.2
128.5 mm
60.6 mm
12 HP
Inspired by the human voice, the Grendel Formant Filter is an effect
processor that creates vocal-like vowel tones from external audio
using subtractive synthesis. It contains four voltage-controlled
analog bandpass filters and a low pass filter pre-set to 160 Hz. The
filters are configured in parallel, all receiving the same input signal
and with their outputs mixed together internally. A dedicated CV
processor is included to modulate the characteristics of the BPFs in
special patterns. Each BPF covers a specific range of frequencies,
creating a stack of animated filters that simulate the resonance of the
mouth as vowel tones are spoken.
MODE 2 : 4 bandpass filters + 160 Hz low pass filter
MODE 1: 4 bandpass filters only
BYPASS: No filter effect (gain knob is active)
THE GRENDEL FORMANT FILTER
Choosing a frequency-rich signal source such as a sawtooth
waveform or pink noise will give the strongest effect. The human
vocal cords produce a unique waveform (the glottal pulse) which can
be approximated by an asymmetric triangle wave with its leading
edge faster than the trailing edge. Using an oscillator which has
variable symmetry may achieve more natural-sounding results.
Note that a pure sine wave input gives weak results because it lacks
overtone content which the Formant Filter needs to produce its
effect. For the most vocal-like tones, your source oscillator should
have a fundamental frequency in the range of 50 – 1000 Hz. Complex
signals such as percussion loops, string pads, or even another vocal
will be worth experimenting with as well. Pre-distorting your source
audio with fuzz or overdrive can add harmonics and give a stronger
effect.
CHOOSING SOURCE MATERIAL
CONNECTING AUDIO
To hear the effect, patch the output of your signal source to the
Formant Filter's audio input, and connect the Formant Filter's audio
output to your audio monitoring chain via a mixer. Use the Gain knob
(at the bottom) to adjust the input level to the filter. This patch is good
for droning sounds, and also recommended when you are playing a
keyboard instrument as the audio source. Or, you can patch a VCA
after the filter to create the classic subtractive synthesis signal chain
– VCO > VCF > VCA with CV/Gate control.
This module does not add any gain boost to the signal. To process a
low level signal such as a microphone, you will have best results by
connecting a preamplifier between the source and this input.
A peak indicator light is located above the Gain knob. This LED
illuminates to indicate that the Formant Filter input is being
overdriven. To maintain a clean sound, back the Gain knob down
slightly if the peak LED lights.
PEAK INDICATOR
