Abstract: The present invention relates to the transmission of information over a communication link (17), such as a telephone line, and to the encoding and decoding of data and to the storage of such encoded information. The system allows for transmission of real time video images over a twisted pair telephone line. The apparatus includes a data encoding circuit (11, 23) having a tone generator (24) for producing a fixed frequency tone within an audio frequency range and a harmonic generator (25) for phase modulating an input frequency. The composite signal is applied to a communication link (17) where a remote decoder (18, 21, 32, 35, 36) has a phase detector circuit (18, 32) driving a voltage controlled frequency conversion circuit (21, 36) to decode the encoded data and reproduce the high frequency signals.