A co-channel communications system is disclosed which permits a digital data signal to be simultaneously transmitted with a communications medium signal such as telephone voice or television video. The data signal or signals are converted into very low multifrequency signals consisting of fundamental...http://www.google.co.uk/patents/US4425642?utm_source=gb-gplus-sharePatent US4425642 - Simultaneous transmission of two information signals within a band-limited communications channel
