High-Speed Digital and Analog Parallel Transmission Technique Over Single Telephone Channel
A new technique for parallel transmission of digital and analog signals over a single telephone channel is presented. The modulation scheme treated in this paper is quadrature amplitude modulation, wherein the digital and analog information are mapped separately as in phase and quadrature components. In the realization of the high-speed parallel transmission, adaptive equalization and carrier phase control are significant problems. In the hybrid QAM modem a two-dimensional error signal cannot be measured accurately at the demodulator output. This causes difficulty in implementing the conventional decision directed circuitry, which is usually employed in the adaptive equalizer and the carrier phase control circuit. The new decision directed schemes which are applicable to parallel transmission are derived by using an error signal measurement defined independent of the analog part. Computer simulation for the parallel transmission scheme of 4.8 kbit/s digital and 2.4 kHz sampled analog sequences is carried out for a typical dispersive telephone channel. Simulation results show that this scheme is useful for high fidelity analog signal transmission.