Tag Archives: filter

Adjust the pitch of a comb filter

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This patch demonstrates how to adjust the delay time of a comb filter to make the filter correspond to a desired fundamental pitch.

The filtering formula used by the comb~ object is

y[n] = a x[n] + b x[n-(DR/1000)] + c y[n-(DR/1000)]

wherein R is the sampling rate, D is a delay time in milliseconds, x[n] is the current input sample, y[n] is the current output sample, and a, b, and c are gain scaling factors.

That formula can be shown diagrammatically like this.

In the patch we convert a MIDI-based pitch number into a frequency in Hertz, then use that to calculate the correct delay time for the filter. Using delay feedback (a past y[n] value) with feedback gain approaching 1 creates strong resonance at the comb frequency, yielding an inverted comb response pattern sort of like this,

resulting in a strong imposition of the fundamental pitch and a buzzy timbre.

 

ducker~ to suppress soft sounds

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A “ducker” is a system that turns a signal down to 0 when it’s below a given threshold. It’s useful for suppressing unwanted low-level audio, such as in a cell phone transmission when the user is not talking. Or, more to the point for musical purposes, as in a microphone signal when the musician is not playing.

In this patch the user specifies an amplitude threshold, attack time, and release time, either by sending those values in the appropriate inlets or by typing them in as arguments to the object in the main patch.

When the amplitude of the signal coming in the left inlet goes above the threshold, the >=~object sends out 1, so the rampsmooth~ object starts heading toward 1 in the number of samples specified (i.e., starts fading in the signal). So as not to lose too much of the wanted signal, that fade-in should be quick. A good attack time might be in the range of 5-40 ms, depending on the source. When the amplitude of the signal coming in the left inlet goes below the threshold, the >=~ object sends out 0, so the rampsmooth~ object starts heading toward 0 in the number of samples specified (i.e., starts fading out the signal). For many instruments, the release time might be slower than the attack time, so you might want that fade-out time to be longer than the fade-in time. A good release time might be in the range of 200 ms or more, depending on the source.  In the main patch you can use the mstosamps~ object to calculate the correct number of samples for the fade times.