5. Synthesizer Filters
A more creative type of filtering is sometimes desirable, and so some manufacturers have taken particularly the low pass filters commonly found in analogue synthesizers and put them into a separate box so that you can pass a regular audio signal through them rather than a synthesizers oscillator.
These have a much sharper cutoff slopes, typically 24dB per octave, giving a much more prominent filter sound, especially when used with resonance, where the cutoff point itself is emphasised, sometimes to the point of self-oscillation. This gives the characteristic wah synth sound made by classic synthesizers like the MiniMoog and is particularly popular in dance music where often a drum loop or even the whole mix will be fed through the filter. As these are commonly analogue devices they can come with control voltage inputs for linking filters together, or interfacing them with analogue synthesizers - for example, the filter cutoff can then be controlled automatically by being swept by a synthesizers low frequency oscillator.