TELEFONOPKALDEKREDSLOEB TIL HURTIG DUPLEX-DATATRANSMISSION

A 4800 bit-per-second full duplex modem is described which utilizes the full bandwidth of ordinarily available dial-up telephone circuits for transmission of data in both directions in different pass-band portions thereof. The two pass-bands overlap in about the middle of the telephone band in order to fully utilize that bandwidth for high speed data transmission. The overlap is less than that which will cause a greater error rate than desired from remote echos of transmitted data. Local echos, which are much stronger, resulting from the overlap bandwidth portion are cancelled by an echo cancelling technique. Local echo cancelling is accomplished at the baud rate, generated in each modem, for substraction from a received signal; a cancellation signal is generated from a time varying system that depends upon the modulating and demodulating carrier signals.