Patent US 3,961,277

To demodulate an incoming oscillation consisting of a succession of distinct signal frequencies, particularly in a data-transmission system using an alternation of two keying frequencies, the incoming oscillation is squared and then passed through a differentiator deriving a pair of closely spaced trigger pulses from any zero crossing of the incoming oscillation. The first trigger pulse causes the transfer of the contents of a buffer register, receiving the reading of a pulse counter, into a digital/analog converter whose output is compared with a reference voltage in a threshold circuit; the second trigger pulse resets the counter which receives locally generated clock pulses whose cadence is high compared to the signal frequencies.