A system for detecting frequency shift keying information on a real time basis independent of the amplitude of the input FSK signal. The second derivative of the input voltage waveform signal which is in exact phase with the input signal is obtained. The gain of the differentiated voltage signal is controlled so that when the input signal has a frequency below predetermined frequency, the magnitude o2 the differentiated voltage signal will be less than the magnitude of the input voltage signal at all points except zero; and when the input signal exceeds the predetermined frequency, the magnitude of the differentiated voltage signal exceeds that of the input voltage signal. The input signal and the gain controlled differentiated signal are magnitude compared and an output signal representative of the result of the comparison and hence of the frequency of the input signal is supplied. A low pass filter introducing minimal signal delay is utilized to remove possible logic silvers at the time when both signals are zero and when there is an input frequency change.