This equalization is necessary due to the material impossibility of engraving the groove of the vinyl record directly from the sound of the piece of music resulting from the studio mix that includes frequencies between 20 Hz and 20,000 Hz. The waves of the low frequencies, compared to those of the high frequencies, grooves of such width would be engraved to occupy the entire surface of the disk in a short time. The problem has therefore been overcome by using filters that compress the low frequencies in the engraving phase and, at the same time, exalt the high frequencies, produce narrower grooves and, consequently, are long enough to allow wider incisions.
The Recording Industry Association of America standardized its equalization curve in 1953, replacing the equalization curves used up to that point. The RIAA equalization curve is a rather complex mathematical formula that calculates the time constant of three frequency bands, low, medium and high and determines a constant expressed in dB or decibels.
Since 1976, the International Electrotechnical Commission or CEI has improved the equalization curve by introducing the RIAA / IEC, a new version of the calculation of the original RIAA curve to which an extra time constant has been added that modifies the most serious sounds, effectively introducing the advent of surround.
When playing the disc, an exactly opposite equalization is applied that allows the reproduction of the sound similar to the original one. The RIAA de-emphasis curve transcribed on a diagram resembles a very open "S"; the center of it is the frequency of 1000 Hz, zero equalization point. The frequencies above this point are attenuated, while those below are enhanced. Equalization is carried out by a filter, which can be achieved by a simple electronic circuit formed by capacitors and resistors where the accuracy of the values of the filter components determines the degree of fidelity of the returned signal.
Being this equalization curve made a standard by the RIAA only in 1953, it is quite complicated to find a right de-equalization curve for the first 33 revolutions put on the market and then reproduce them correctly unless you have period electronics or electronic filters variables.
