System for simultaneous analog and digital communications over an analog

Abstract

At a transmitting end, frequencies used to a construct a digital signal are substantially removed from an analog signal by a notch filter circuit to produce an interim signal which is then combined with the digital signal as by combining to produce a composite analog signal that is transmitted to a receiving end. At the receiving end the frequencies used to construct the digital signal are substantially removed from the composite analog signal by a notch filter circuit. In this way the digital signal can be transmitted simultaneously with the analog signal without errors that could be introduced by the analog signal, and with only a slight change to the frequency spectrum of the analog signal. This can be used in telephone communications for sending caller ID digital data over the same line carrying a voice signal. Caller ID data is sent FSK encoded and only those frequency used for the mark and the space are attenuated in the received voice signal before it reaches the telephone's speech circuit. For PSK encoded data method, a single notch filter removes the carrier frequency from the voice signal at the receiving end. In this way, the user is not subjected to the audible frequencies used to transmit the caller ID data, but the overall fidelity of the voice signal is only insignificantly reduced.