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. In a second embodiment, the system provides clear and error free continuous transmission of digital signals over an analog channel simultaneously with an analog signal by periodically analyzing the analog signal and, during any given period, transmitting the digital signals in open bands, i.e.