Chapter Four: Synthesis
Filters are normally used to remove specific frequency components from a complex sound, hence the technique is often called subtractive synthesis, mentioned previously . This is not an entirely accurate description, since filters may also add energy to portions of the spectrum. Most analog synthesizers of the '70's and '80's came with the following four basic filter types (they are carried forward today on digital instruments with many variations): Lowpass, Highpass, Bandpass and Notch (sometimes called Band Reject). Lowpass filters cut off high frequencies, pass low frequencies. Highpass filters cut off low frequencies and pass high frequencies. Bandpass filters pass a certain width or band of frequencies and cut off frequencies on either side of the band. And Notch filters cut out a notch or band in the center and passes frequencies to either side of the notch or band.
Looking at the graphs above, you will notice that most filters do not suddenly cut off sound at a specific frequency. Rather, they "roll off" the frequencies gradually, for example 12 dB's per octave. We specify a cutoff frequency (c.o.f.) at the point a specific frequency component would have lost approximately half the power (-3 dB) of unaffected frequencies, often refered to as the half-power point. A common synthesis technique is to sweep the cutoff frequency up or down to provide a 'spectral shape' to a sound over time. Cutoff frequencies are usually controlled by an envelope generator or an oscillator (timbre modulation). The portion of the sound not reduced by 3 dB power or more is called the passband, while the rolled-off portion from -3 dB down to -infinity is called the stopband.
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