The purpose of the filter in a subtractive synthesizer is to remove portions of the signal—the frequency spectrum—sent from the oscillators. After filtering, a brilliant-sounding sawtooth wave can become a smooth, warm sound without sharp treble.
The filter sections of most subtractive synthesizers contain two primary controls known as cutoff frequency—often abbreviated to cutoff—and resonance. Other common filter parameters are drive and slope. The filter section of most synthesizers can be modulated by envelopes, LFOs, the keyboard, or other controls such as the modulation wheel.
Filter types
There are several filter types. Each has a different effect on various portions of the frequency spectrum:
Highpass filter: High frequencies are passed; low frequencies are attenuated.
Lowpass filter: Low frequencies are passed; high frequencies are attenuated.
Bandpass filter: Only frequencies within a frequency band are passed.
Band reject filter: Only frequencies within a frequency band are attenuated.
Allpass filter: All frequencies in the spectrum are passed, but the phase of the output is modified.
