The signal read-back from a magnetic disk is processed in dependance upon the track diameter in order to cut higher frequencies for outer tracks and to boost high frequencies for inner tracks for, respectively, correcting flat signal portions and eliminating or reducing peak displacement. The circuit develops a signal-amplitude-modifying factor in form of a cosine function of the frequencies of the read-back signal as applied. Two thus modified read-back signals are developed, varying oppositely in strength in dependance upon the track diameter. These two signals are combined to obtain the selective cutting or boosting as required.