Graphic equalizers are used to control the frequency response of an audio system. An equalizer contains a number of fixed-frequency bandpass/notch filters with a gain control for each filter. Resonance and nulls in the frequency response of an audio system are easily compensated with proper adjustment of the equalizer.
The following file here, contains detail information about designing a Digital-Controlled Graphic Equalizer using LMC835, a plastic 28-pin dual-in-line package integrated circuit. A single LMC835 contains enough step-variable resistors for a stereo, 7 band equalizer with 1 dB steps covering a g12 dB range. (source: national.com)