Throughout the 1950s and 1960s, equ

Throughout the 1950s and 1960s, equalizers grew in popularity, finding applications in soundpost-production and speech enhancement. The Langevin Model EQ-251A, a program equalizer withslide controls, was a precursor to the graphic equalizer [1]. One 15-position slider controlled the gainof a bass shelving filter, and the other adjusted the boost or cut of a peak/notch filter. Each filter hadswitchable frequencies. Cinema Engineering introduced the first graphic equalizer [1]. It could adjustsix bands with a boost or cut range of ±8 dB in 1 dB increments. However, with graphic equalizers,engineers were still limited to the constraints imposed by the number and location of frequency bands.Around 1967, Walker introduced the API 550A equalizer in which the gain appropriately changes thebandwidth of the peak or notch. As was typical in early equalizers, this device had a fixed selection offrequencies, and variable boost or cut controls at those frequencies.In 1971, Daniel Flickinger invented an important tunable equalizer [17]. His circuit, known as “sweepable EQ”, allowed arbitrary selectionof frequency and gain in three overlapping bands.