The transmission of pictures by radio

The paper describes the development of picture-telegraphy equipment since Alexander Bain´s first proposals in 1842, and particularly the applications to radio working since the first transatlantic tests in 1924. The technique depended on the type of radio circuit available, commencing with low-frequency high-power transmitters, then high-frequency beam-telegraph transmitters, frequency-modulated sub-carriers on double-sideband telephone transmitters and finally frequency-modulated single-sideband transmitters. The use of conventional amplitude-modulated transmitters has never been possible, first because such methods were not admissible on low-frequency circuits and later because the final fading ratios were too high to permit satisfactory reproduction of tone values. Methods of visual recording which preceded photographic reception included the use of hot wax, ink and hot-air jets and electrolytic paper. Spurious patterns caused by mechanical imperfections and means of avoiding them are described. Synchronizing methods have developed from elementary tuning-forks to quartz crystals. The comparatively low limit to speed of operation passed from equipment difficulties to that set by the propagation of high-frequency signals through the ionosphere. The effect of multi-path transmission is discussed. Consideration of bandwidth requirements discloses the inefficiency of facsimile methods for the transmission of telegrams.