VCF voltage controlled filter
One of the main modules within a synthesizers is the Voltage Controlled Filter (VCF) the filter modifies any harmonics of a signal when passed through it and before the signal arrives at the output.
To apply filtering to an audio wave it needs to be connected by a patch cord or directly hardwired from the VCO output into the audio input of the (VCA) voltage controlled filter this allows us to hear sound.
Filter Cutoff Frequency
The attenuator of the filter cutoff frequency needs to be moved to the up position which will open the filter and allow the audio signal to be increased through the cutoff which in turn will allow the output mixer and audio signal to be increased to its required level.
Many synthesizers have a resonance circuit ( sometimes called emphasis) this circuitry highlights certain harmonics within the VCA section it filters the output signal back into the input of the voltage controlled filter.
The amount of resonance control is controlled by the rotary attenuator in a looping cycle.Harmonic frequencies within many instruments contain complex timbres offering many possibilities to explore the resonance circuit on an analog synthesizer.