The voltage controller filter or VCF basically filters out certain frequencies from the sound generating modules of a synthesiser, in its most basic form it can be described as a sort of tone control but by turning up the filters feedback or resonance as it is more usually known, the filtering effect becomes more pronounced and can produce twangs and wah effects.
The VCF is usually used in conjunction with its own envelope generator where the attacks, decay, sustain and release controls are usually used to control the cut-off frequency of the VCF. You can of course also use other modules to control the filters frequency and by connecting an LFO’s output to the VCFs control input, you can create all sorts of auto wah effects.
VCF’s also come in different varieties; the most popular type is the low pass filter which as the name implies allows frequencies to pass through below the cut-off level which usually has a manual control as well as voltage which is controlled by the control voltage input to the VCF. As the control voltage is increased the cut-off point goes higher allowing more frequencies through.
Another type of filter is the high pass type which not surprisingly is the exact opposite to the low pass filter but otherwise functionally the same. There are also band pass filters which filter out low and high frequencies allowing a band of frequencies between the two cut-off points to get through the width of the allowable frequencies are again altered by a control voltage.
The band pass filter also has its arch nemesis, the band reject filter which as its name implies does the opposite to the band pass and rejects a band of frequencies, again the width of which depends on the control voltage.
The filter can only filter out sounds that are there in the first place so if you put a sine wave through it you won’t produce anywhere near as much of a pronounced effect as if you start with a sonically rich wave form such as that of a square wave or ramp wave.
Filters do tend to have their own character and various synthesiser manufacturers have had their own ideas of what a filter should sound like. Dr Robert moog used what is known as a diode ladder in the construction of his synthesiser filters. They were also technically known to filter at the rate of 24 decibels per octave which in practice means they were quite ferocious in terms of filters. Some engineers also claimed that the moog filter introduced a small amount of distortion by slightly overdriving the input stage! However they did it though they certainly produce a distinctive sound that many including me have known and loved.
Some manufacturers opted to use 12 decibels per octave which gave a different type of sound, only really to be described as more electronic. The synthesiser VCF certainly has and continues to produce much conjecture among synthesists as to which is best, as almost all of the traditional synthesiser manufacturers filters have their own sound.
I prefer to sit on the fence and just say they all sound different!
The Waldorf 2-Pole Analog Filter is a stand alone filter from the respected German manufacturer, Waldorf. It can high,low and band-pass filter and contains an envelope follower and an LFO to modulate it as well as external control voltage inputs. You also get a drive input which really can enhance the sound you put through it in quite pronounced way that only a true analog filter, as this is, can do.
Not cheap but a quality device.