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9.4.2.1. 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. Delay
Fig. 3 shows how the comb filters change with the delay time.
9.4.2.2. 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.
Delay Figure 2: COMB FILTERS.
Input signal mixed with a 2 msec.
delayed signal. (Delayed signal has
10dB less amplitude. Max. filter
gain is +2.5dB, and max. depth is -
3.) Reducing the amplitude of the
delayed signal reduces the comb
filters' effect.
Delay Figure 3: Comb
filters get closer as delay
time increases.
Delay Figure 4: Comb
filters noticeably affect
your sound.
Section Nine: Suggestions for Optimal Use of the GRAPHI-Q