Frequency Response and Equalization: In audio systems i.e. AF amplifiers, microphones, loud speakers, various types of recording and reproduction of sound, the response is not uniform to various frequencies of Audio Signal. Due to this, frequency distortion results. The quality of the sound is not good. Fidility will be poor.
In AF voltage amplifiers, the voltage gain falls at low frequencies (BASS) due to reactive coupling elements. At high frequencies (Trebble), the gain falls due to inter-electrode capacitances and stray capacitances.
In Accoustic devices i.e., microphones, loud speakers, the frequency response is not uniform. When sound is recorded/ reproduced either by Disc/ Tape/ Film/ CD it is observed that frequency response is not uniform.
To cite an example, while recording/reproducing sound on magnetic tape, the dimension of head-gap has an effect on its performance.
Similarly while recording/replay of sound on Disc (Grammaphone), the frequency response is not uniform.
Equalizer: This is a network having opposite frequency response of the audio circuit/ device, which when included in the circuit gives an output of uniform frequency response. Thus, fidility of the audio system improves. For example, pre-emphasis and De-emphasis networks are equalizers.
Pre-emphasis and De-emphasis Curve:
The high frequencies have low energy content. So, Emphasize the signals above 1 KHz at 6db per octave. During playback, the higher frequencies are more attenuated. Curves are shown in figure.
In sterio-systems, para-meteric or graphic equalizers are adopted. In parameteric equalization, a boost up to ± 15 db is possible. If there is an unwanted peak in response at a particular frequency, it can be cut-down.
In graphic equalization, slider-type Potentio meters are arranged on front panel of the equipment. The AF is divided into a number of bands (3 or 6 or 8 or 10 bands). Each band of frequencies is provided with a control. The user will be able to adjust the shape the frequency response curve as per his desire.
This is a process of compensating for the losses at both low and high frequencies. Electronic circuits can be used to over-come the frequency dependant losses while recording and replay. When it is done while recording, it is called ‘pre-equalization’ and if it is done in player it is called ‘post-equalization’.
A process called pre-emphasis is adopted to enhance low intensity sounds prior to recording. While replay, a process called De-emphasis is adopted to obtain original signal. Other wise fidility will be lost. Hence, combined process of pre and de-emphasis can be called equalization.
Equalization has to be standarised for simple playback. There are CCIR, IEC, DIN and NAB standards. The basic difference between the CCIR and NAB standard is the bass boost at low frequencies for NAB. Bass boost improves hum rejection. But CCIR standard prefers simple equalization. Currently Dolby’s method of equalization is popular.
In pre-emphasis circuit, the capacitor which is in series offers low reactance at higher frequency. Thus, attenuation for ‘Trebble’ is low.
In De-emphasis circuit, the capacitor is in paralleland output is developed across it. The capacitive reactance is low at trebble and output is less for high frequencies.