Abstract: A full duplex communication system including a pair of full duplex modems. Each modem has both a call originate mode of operation and a call answer mode of operation. In the call originate mode outgoing data is transmitted by modulation of a carrier frequency F2 and incoming data is received by demodulation of a data modulating carrier of frequency F1. When in the answer mode of operation, the data is received on frequency F2 and transmitted on frequency F1. Means are provided for injecting a carrier frequency F6 in the outgoing data transmission path. The frequency F6 is near the midpoint between F1 and F2. The amplitude is such that the distribution of power within the transmission band is balanced so that the power in the frequency band set aside for single frequency signaling systems does not exceed the power in the remainder of the transmission band.

Abstract: The disclosed full-duplex communication system includes a pair of full-duplex modems, one at each end of a two wire circuit. Each of the modems includes a transmitter for transmitting over the two wire circuit a stream of synchronous data at a first carrier frequency, and a receiver for receiving over the two wire circuit a stream of synchronous data at a second carrier frequency. The receiver includes means for coherently demodulating the stream of synchronous data. In a preferred embodiment, an asynchronous-to-synchronous converter converts asynchronous input data into a stream of synchronous data for transmission by the transmitter. Also, the preferred embodiment transmits the data and receives the data as double side band suppressed carrier quadrature amplitude modulation. The system includes a remote test capability for automatically looping back test data for comparison with the transmitted test data.