Considerations in the choice of the optimum data transmission systems for use over telephone circuits

The transmission properties of telephone circuits are first considered and different modulation methods are then compared on the basis of their relative performance under these conditions. Phase modulation is found to be superior to other methods and the best arrangement is where the signal carrier is synchronized to the modulating waveform at the transmitter and both sidebands are transmitted. For general applications over all types of telephone circuits differentially-coherent detection has important advantages over the more conventional coherent detection. Various methods of transmitting the timing information are considered, leading to the choice of a synchronous system in which the timing waveform in the receiver is extracted from the detected signal waveform or its equivalent. 94 references.