Voltage controlled filter

A voltage-controlled filter ( Engl. Voltage-controlled filter , short VCF ) is an electronic filter , wherein the filter characteristic by means of a control voltage can be varied. VCFs are elementary circuit components of analog synthesizers and effects units in music electronics . In modular synthesizers , they correspond to separate assemblies that can be switched between Voltage Controlled Oscillator (VCO) and Voltage Controlled Amplifier (VCA). The type of filter used in synthesizers is mostly low-pass filters , more rarely high-pass , band-pass or all-pass filters . In most cases, the upper limit frequency is influenced .

In the digital age, VCFs and LFOs are mostly modeled digitally; instead of the control voltage, a data stream is then effective.

application

The filters are usually controlled time-dependently via envelopes , but can also be controlled by e.g. B. affect a Low Frequency Oscillator (LFO).

In addition to the change in timbre due to the filter cut-off frequency, an adjustable filter resonance is typical of VCFs. From a technical point of view, this corresponds to a feedback in the area of ​​the filter cut-off frequency of the output signal. Depending on the phase shift through the filter, the signal is amplified or weakened depending on the frequency.

Among the most famous VCFs probably count the cascade filter of Robert Moog . It is constructed as a transistor cascade that functions as a 24 dB low-pass filter and creates a special timbre.

Another well-known example is the filter of the Roland TB-303 , an analog bass synthesizer, made famous by techno music with bass runs that, depending on the setting of the filter cutoff frequency and resonance, tend to "bubble" or rather "screech".