Calculating Comb Filter Frequencies
The frequencies of the reinforcements and cancellations depend on the delay time (the time
difference between the arrival time of the original signal and the delayed signal). The frequency of
the first cancellation occurs at 1/(2t) Hz, where t = the delay time in seconds. The cancellations
are separated by (1/ t) Hz. Fig. 11 shows how the comb filters change with the delay time.
Comb Filter Amplitude
If the original signal and the delayed signal are the same amplitude, the reinforced frequencies
increase in amplitude by 6 dB, while the out-of-phase frequencies cancel completely to -
4
dB.
Comb filters cause a lot of problems. The frequencies that are reinforced are prone to excite
feedback, while the out-of-phase cancellations make the program sound thin and over equalized.
Try this simple experiment to hear what comb filters do to your sound.
Section 8: Digital Delay
16
Fig. 9: COMB FILTERS. Input
signal mixed with a 2 msec.
delayed signal. (Delayed
signal has 10dB less ampli-
tude. Max. filter gain is
+2.5dB, and max. depth is -3.)
Reducing the amplitude of the
delayed signal reduces the
comb filters' effect.
Fig. 10: Comb filters get closer
as delay time increases.
Fig. 11: Comb filters notice-
ably affect your sound.
