One feature often associated with low-pass filters is the regeneration control, sometimes called resonance, or emphasis. This control works very much like the feedback that occurs in a P. A. system when the microphone is placed too close to the speakers and the sound output from the speakers is again picked up by the microphone. In a filter, the amount of regeneration produced can be controlled by the user.
As regeneration is increased, extra amplitude, or emphasis, is given to the cutoff frequency. Stated another way, as regeneration is increased, the more the frequencies at and around the cutoff frequency are emphasized. The extra amplitude (emphasis) given the cutoff frequency results in a change in the entire response curve of the filter.
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Example 21-1: The regeneration control of a low-pass filter adds emphasis to the frequencies at and around the cutoff frequency.
The frequencies below the cutoff frequency will be attenuated. Typically this occurs so that the circuitry of the filter will not be overloaded by the increased amplitudes at the cutoff frequency.