An analog scan signal having a repetitive background waveform is converted into a multi-level signal through a peak hold circuit (2), a converter (14,83,93), a digital memory (4,4') and a discriminator (6). The peak hold circuit detects and holds the peak voltage of the current scan lines. The background waveform is normalized by the peak voltage and converted into a digital signal in the converter and stored in the memory. The analog scan signal is discriminated in the discriminator on the basis of a reference voltage produced with the normalized background waveform read out of the memory and converted in the converter and with the current peak voltage supplied from the peak hold circuit.