Category Archives: Theory

Comb Filters

A few days ago, someone asked me something interresting : why a Feedback Comb filter was producing a lowcut effect…
I took a deep breath and a few minutes to think and, I remembered myself the theory about Comb filtering and especialy this part :

Fundamental frequency

The fundamental frequency of a comb filter is the inverse of the delay time. For example, if the delay time is 2 milliseconds (1/500 of a second), the accentuation occurs at intervals of 500 Hz (500, 1000, 1500, etc.), and the attenuation occurs between those frequencies. The extremity of the filtering effect depends on the factor (between 0 and 1) by which the feedback is scaled.

Source : MSP Tutorial

If I use a Comb+ filter (feedforward) and if my delai is 2 milliseconds, the first “1” will be at 500Hz and the first “0” will occurs between 500Hz and 1000Hz…

If I use a Comb- filter (feedback) with the same settings shown above, the first “0” will be at 500Hz and the first “1” will occurs between 500Hz and 1000Hz…

That’s it ! The Comb- filter is acting like a lowcut because its starts with an attenuation and not an accentuation…

I’ll try to edit this post later and give it some visual example…

Stay tuned !