Abstract: A modem adapted to enable data transmission over information communication links, such as a voice-grade telephone line. The modem includes a modulator where a binary data source and a clock pulse source are connected to an exclusive OR gate which provides an output pulse train including pulses having widths which are a function of the binary data. The pulse train is applied to a flip-flop and thereafter the output of the flip-flop is introduced into a low-pass filter to provide a sinusoidally varying frequency signal. This signal from the low-pass filter is demodulated by a demodulator circuit which in a first embodiment of the present invention employs a counter implemented in the form of digital circuitry. In a second embodiment of the present invention, this demodulator includes a shift register operating in conjunction with a phase locked loop and a controlled oscillator.