Abstract: A frequency discriminator of the horizontal synchronizing signal for a multi-mode monitor has a plurality of filter capacitors connected to horizontal synchronizing signal lines for rejecting noises of a direct current component of the horizontal synchronizing signal. The frequency discriminator also includes a plurality of phase comparator means respectively connected to the filter capacitors for comparing local oscillating frequencies set differently according to each mode with the inputted horizontal synchronizing signal to output one signal representative of the mode of the monitor.