Graphic equalizer

A graphic equalizer is the device for adjusting relative loudness of the audio signals at various frequencies.  It allows tailoring of amplitude versus frequency output of hi-fi sound equipment. Equalizers are taken in use in recording studios and by the serious hi-fi stereo enthusiasts.In the graphic equalizer, there are many independent tone controls, each one affecting a different part of audible spectrum. The controls are slide potentiometers with the calibrated scales. Each potentiometer controls gain within the certain portion of the spectrum. The slides move up and down, or left to right, in some cases,. When potentiometers are set such that the slides are all at same level, the audio output or response is flat, which means, no particular range can be amplified or attenuated with respect to whole AF spectrum. By moving any one of the controls, user can adjust gain within the certain frequency range without affecting gain outside which range. The positions of controls on front panel provide an intuitive graph of the output or response curve.

The given figure is a block diagram of a graphic equalizer having 7 controls. (Many equalizers have over 7 sections.) In this hypothetical example, the input is fed to the audio splitter which breaks the signal into 7 paths of equal and independent impedance and prevents interaction among circuits. The 7 signals are fed to audio attenuators, also called as bandstop filters, each filter is having own gain control of it. The center frequencies of attenuators in this instance are at 30, 100, 300, and 900 Hz,

                 Figure--A graphic equalizer consists of a splitter, several audio bandstop filters, several gain controls,and a combiner.
 
and 2.5, 7, and 18 kHz. The slide potentiometers are shown as the variable resistors following the attenuators. Finally, signals pass through an audio combiner (called as mixer); the composite is sent to output.

The design of the graphic equalizer is not an easy task at all. The main challenge is to ensure that controls do not interact with each other. Judicious choice of the filter frequencies and responses is also significant. If filters employ active devices such as operational amplifiers, they should not introduce any distortion into signal wave forms. The active devices should not generate significant audio noise. Graphic equalizers are placed at low level points as they are not built to handle high power. In the multichannel circuit such as a stereo sound system, a separate graphic equalizer is used for each channel. In the stereo equipment, there are usually 2 equalizers, one for left channel and the other for right channel.