Q107 State Variable Filter (VCF)

Basics
Most waveforms contain many different frequencies. When an oscillator produces a sawtooth waveform, it can be thought of as a single sine wave, and additional sine waves which are at multiples of the fundamental frequency and are at lower amplitudes. These additional frequencies are called harmonics and different waveforms have different amounts. The Q107 Filter changes the way a waveform sounds by attenuating (lowering the amplitude) of these harmonics. This effect is especially useful when changing over time. The frequency at which attenuation starts (or close enough) is called the cutoff frequency. Cutoff frequency can be controlled manually or by voltage control. Resonance (also known as Q, Regeneration, or Emphasis) has the affect of bringing out frequencies near the cutoff frequency.

Frequency Control
The frequency control gives you about 10 octaves of cutoff frequency response. The control inputs are added to the manual control's value to create the final cutoff frequency. All of these signals work together at the same time to set the filter's cutoff frequency.

It's very common to have your filter track the keyboard so that the response is the same over all frequencies. This is accomplished by using the 1V/Octave frequency control input. Simply patch your keyboard pitch voltage into a multiple then out to your oscillators and to the filter.

The other frequency control input normally comes from an Envelope Generator or from an Oscillator. You can attenuate, amplify or invert the incoming control signal right on the filter instead of having to use another module. The 0 to +5 volt outputs of the Q109 Envelope Generator will give you a total of 10 octaves of range when the attenuator is full on.

The Sequential Controller can also be used to control the filter frequency. You could use one bank to control an oscillator and another to control the filter at the same time.

Resonance Control
Resonance is the emphasis of frequencies near the cutoff frequency and has a great effect on the sound. The range of the resonance control and inputs is very large. The manual control and the input signal are combined to create the resonance amount. If you have too much resonance the filter might scream and clip. We let you decide if this is good or not. You can increase the maximum resonance possible by turning down the input signal level control so that there is more room for resonance without clipping.

Self-Oscillation
Turning the resonance to maximum without an input signal will cause the filter to self-oscillate. This allows the filter to act like an oscillator. Self-oscillation is not very precise in a state-variable filter so don't expect perfect tracking or waveform amplitude. For this reason, state-variable filters in most other synthesizers are designed to prevent self-oscillation but our attitude is to let you decide.

Signal Inputs
There are 2 signal inputs which are mixed together - one is full strength and the other is adjustable. Normally you will use the adjustable input since turning the signal level down will allow higher resonance. You can also attenuate the full strength input by patching the signal through a Signal Processor first.

Outputs
There are 4 different outputs on the filter, each with it's own affect on the input signal's harmonics. Since harmonics are higher than the fundamental frequency, it's common to use a low pass filter to remove them. Most filter sweeps used are low pass responses. Other responses are made available for your experimentation.

When using the notch response, use a low resonance or the notch will be so narrow that you can't hear the effect.

A sine wave has almost no harmonics and will only respond to the filter by lowering its amplitude. Sawtooth and Ramp waveforms have the most harmonics and respond quite nicely to filtering. Square and Pulse waveforms also have a great deal of harmonics and respond well to filtering.

Noise and Hum
Filters are especially noisy and susceptible to interference and hum from the power supply. Place the module as far away from the power supply as possible for lower hum.

A typical synthesizer patch will have a VCA (Q108) as the final module. This has the effect of gating off noise in the modules up the chain including the filter. If you use the filter as the final output without a VCA, much more noise will be heard.

It's also important to remember that the outputs of a modular synthesizer are designed to be very hot (10 volts peak-peak or more) in order to improve the signal to noise ratio, so turn down the outboard equipment (mixers, etc) and turn up the output levels of the synthesizer for best results.

The Most Important Tip
Experiment!