A phase modulation data transmission system for use over the telephone network

A synchronous system is described which transmits binarycoded digital-information at 750 or 1500 bauds over normal telephone circuits. It uses a phase-modulated carrier of 1500 c/s and both sidebands are transmitted. The receiver employs differentially-coherent detection to overcome the one serious weakness of coherent detection, which is the low tolerance of the latter to the frequency-modulation effects sometimes experienced over telephone-carrier circuits. The results obtained after using the system for more than two years over the switched telephone-network, together with the results of many laboratory tests, show a very good performance under all the various conditions of noise and distortion likely to be experienced over telephone circuits. A careful comparison between this system and equivalent f.m. systems of optimum design, has shown clearly that in applications over telephone circuits, p.m. systems of this type are superior to the equivalent f.m. systems in every important respect, both technically and economically.