Tutorials and Topics - Peabody Computer Music

Tutorial 31Processing:Comb filteryt = axt + bx(t-d) + cy(t-d)The fundamental frequency of the comb filter depends on the delay time, and theintensity of the filtering depends on the other three parameters. Note that the scalingfactor for the feedback (the right inlet) should usually not exceed 1, since that wouldcause the output of the filter to increase steadily as a greater and greater signal is fed back.Trying out the comb filterThe tutorial patch enables you to try out the comb filter by applying it to different sounds.The patch provides you with three possible sound sources for filtering—the audio inputof your computer, a band-limited pulse wave, or white noise—and three filteringoptions—unfiltered, comb filter with parameters adjusted manually, or comb filter withparameters continuously modulated by other signals.Choose a sound source and route it to the desired filtering using the pop-up menus• Click on the buttons of the preset to try out some different combinations, withexample parameter settings. Listen to the effect of the filter, then experiment bychanging parameters yourself. You can use MIDI note messages from your synth toprovide pitch and velocity (frequency and amplitude) information for the pulse wave,and you can use the mod wheel to change the delay time of the filter.A comb filter has a characteristic harmonic resonance because of the equally spacedfrequencies of its peaks and valleys of amplification. This trait is particularly effectivewhen the comb is swept up and down in frequency, thus emphasizing different parts ofthe source sound. We can cause this frequency sweep simply by varying the delay time.Band-limited pulseThe effects of a filter are most noticeable when there are many different frequencies in thesource sound, which can be altered by the filter. If we want to apply a comb filter to apitched sound with a harmonic spectrum, it makes most sense to use a sound that hasmany partials such as a sawtooth wave or a square wave.237