Adding a VCF to a synthesizer introduces a whole range of new timbres. We can remove high or low frequencies, boost frequencies, add resonance and sweep the filter frequency to add dynamics to our sounds
For this project I have used the filter we are probably most familiar with, the Moog Low Pass ladder filter. The structure is fairly simple. We have two Level Adj modules for the following functions.
Keyboard Tracking.
The Keyboard Tracking which would be connected to the Pitch plug on our MIDI to CV Module. This is switched on and off with the KB Track button, this also applies an offset of -5 V when selected to avoid the filter pitch jumping to a higher frequency when tracking is turned on.
Modulating the Filter frequency.
Next we have a simple level control that takes a modulation input from a source such as a ADSR envelope or an LFO. This is simply to introduce a variation in the timbre of the filter’s audio output. By setting the Mod Level slider with a Minimum of -10 V, and a Maximum of 10 volts we can take the modulation from fully inverted through no modulation to full normal modulation.
Note: Don’t worry about the mode plug on the filter- it doesn’t do anything there is only one option -low pass. No I don’t understand why it’s there either!
Input levels.
If you’re connecting more than one audio input it’s a wise move to limit the audio input level to avoid over-driving the filter, as this will result in some quite harsh, un-musical distortion. Remember that each signal adds to other signals (dependent on phase), but generally with two inputs you need to drop the audio input by somewhere around 25-40% dependent on the audio signals.
The filter won’t just saturate and produce a softer distortion it will be a hard digital clipping.
Other types of VCF.
Once you have the basic structure set up you can adapt it easily for other types of filter (The multiple output filters need a mixer on the output)
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