State Variable Filter

1. Cutoff Knob Sets the filter’s cutoff frequency.
2. FM Input Exponential audio-rate frequency modulation input, with FM depth knob.
3. LP/HP Knob Controls the mix of low-pass and high-pass filter on the Out jack. 12 dB/octave low-pass when fully counterclockwise, notch (band-stop) when centered, 12 dB/octave high-pass when fully clockwise.
4. Q Knob Sets the quality factor, or resonance. Self-resonant when set fully clockwise.
5. Growl Knob Adds some bite.
6. Audio Inputs Signal to be filtered (stereo), with input level trim knob.
7. BP Outputs Signal filtered with 6 dB/octave band-pass filter (stereo).
8. HP Outputs Signal filtered with 12 dB/octave high-pass filter (stereo).
9. LP24 Outputs Signal filtered with steep 24 dB/octave low-pass filter (stereo).
10. Main Outputs Signal filtered according to the LP/HP knob setting (stereo).

Overview ⚓︎

The state variable filter (SVF) is a versatile filter design that provides low-pass, high-pass, band-pass and band-stop (notch) outputs from a single input signal.

Our take on this classic filter design adds an audio-rate FM input, a 24 dB/oct low-pass output, a low-pass/high-pass balance and a unique growl feature that can slightly sweeten or completely mangle the output signal.

Because of its versatility and its stability under heavy modulation, this should be the default filter for most patches.

Usage ⚓︎

Input and Outputs ⚓︎

Audio inputs and outputs come in pair to allow stereo processing. For mono signals, only use the left input and output jacks.

The State Variable Filter expects an audio source to be connected to its In jack pair, and will output four different filtered signals on its output jack pairs.

When adding a new State Variable Filter module to a patch, its main output (labelled Out) will be a low-pass filter with a slope of 12 dB/octave.

Using the LP/HP knob, it is possible to change between a low-pass and a high-pass filter, with a kind of band-stop (notch) filter in-between.

The BP output is a 6 dB/octave band-pass filter.

The HP output is a 12 dB/octave high-pass filter.

The LP24 uses a second low-pass filter built into the module to produce a very musical 24 dB/octave lowpass filter, ideal for fat basses and smooth leads.

Cutoff ⚓︎

This describes the effect of the Cutoff knob when the Growl knob is fully counterclockwise and there is no audio-rate FM input.

For low-pass and high-pass outputs, the cutoff frequency is the point at which higher or lower frequencies will start to be attenuated.

For the band-pass output, the cutoff frequency is at the center of the passband.

For the notch-like output when the LP/HP knob is centered, the cutoff frequency is at the center of the stopband.

For some sounds such as analog leads, it can be useful for the filter cutoff frequency to track the current note’s pitch; otherwise, the low notes will sound too bright and the high notes will sound dull. This is done by connecting the Keyboard Pitch output into one of the Cutoff modulation inputs, and adjusting the modulation depth to taste. With a modulation depth of 100%, the cutoff frequency will perfectly track the keyboard.

Q ⚓︎

This describes the effect of the Q knob when the Growl knob is fully counterclockwise and there is no audio-rate FM input.

The Q knob controls the so-called quality factor of the filter, sometimes called the resonance. A higher Q will add emphasis at the cutoff frequency.

At its default value (when double-clicking on the knob), there is no emphasis and the low-pass or high-pass attenuation at the cutoff frequency will be -3 dB.

Higher values may cause internal clipping, shown by the Over LED. This is not necessarily bad: it can add some grit to filter sweeps.

Lower values will cause the frequencies close to the cutoff frequency to be attenuated.

When the Q knob is fully clockwise, the filter enters self-oscillation. In that mode, a sine wave can always be heard at the cutoff frequency even when no input signal is connected.

When the LP/HP knob is centered, the behaviour of Q becomes a bit counterintuitive for the main filter output. In that mode, the cutoff frequency is at the middle of a frequency band that is removed from the signal. Therefore, higher Q will remove a narrower band, which will be harder to hear. Conversly, a lower Q will remove a larger band and will make the effect of the filter more obvious. Since this can sound really nice, especially when the cutoff is modulated, we made our Q knob go much lower than what you can find in typical state variable filters.

Growl ⚓︎

Growl is a unique feature that adds some distortion to the signal by opening a feedback path from a point in the internal audio signal path to the cutoff frequency control voltage. The higher the Q setting, the more obvious the Growl effect.

At very high Q and Growl settings, digital aliasing can make new inharmonic frequencies appear. As these unexpected frequencies are fed back into the cutoff frequency control, everything quickly turns to chaos. Good and noisy chaos.

The Growl feature interferes with the value displayed by the cutoff frequency. Increasing the Growl will usually lower the cutoff frequency, but it will also boost some harmonics above that frequency, so the low-pass output may sound brighter.

FM ⚓︎

The FM input allows audio-rate exponential frequency modulation of the filter cutoff. This complements the Cutoff knob’s input jacks. If you need precise CV-rate modulation, say for pitch tracking, use the cutoff inputs. If you want audio-rate modulation, use the FM input.

Audio-rate FM is best heard on low-pass, high-pass and band-pass outputs when the Q knob is past its midpoint and the cutoff frequency is not too high.

This input provides an attenuator knob. Too much FM may introduce digital aliasing artefacts, especially at high Q and high input signal frequency.

Gain Staging ⚓︎

If the Over LED turns on, it means that the filter is saturating internally. Although this adds a musical and desirable distortion, you may want to avoid it. In that case, lower the input gain until the Over LED turns off, and compensate by increasing the output gain. Conversely, if you wish to have more distortion, you can increase the input gain and lower the output gain.