Electronic music wouldn’t sound much different from its electric sister if filters hadn’t been invented. A filter is a frequency amplifier (or “tamer”) device, i.e. an effect that operates on the frequencies of a sound – it can boost them, cut them or mute them, but it can’t create ones that don’t exist in the original. So it’s basically just an EQ, although one that gets used for sci-fi sounding effects rather than for improving the quality of recordings (like during the mixing process).
There are 3 main types of filters: the Low-Pass (the most popular in electronic music) progressively cuts frequencies starting from the most trebly ones, while the High-Pass works the other way around. Both these filters have “cutting slopes” (called Q) that can be adjusted, so that the high or low frequencies are cut in a more or less radical (or steep) way. A Band-Pass filter instead incorporates both filters by only allowing a central frequency range to be played – therefore cutting both some high and some low frequencies.
Another concept you should be familiar with is Resonance, which is actually just another filter – a Peak Filter in this case – that boosts frequencies right where the high or low pass filter are starting to cut them, creating some kind of wah-like effect that doesn’t seem to ever get old when applied to analog synths.
A lot of the fun of playing with filters is in playing them live, or – when recording – creating automation that modify the sounds in unexpected ways. The sonic possibilities opened by filters are endless – try them on anything, mastering the filters is crucial for any electronic musician. – PDG