Patent GB-275675-A (GB)
Title: Improvements in or relating to the transmission of pictures by radio or other telegraphy
IPC: H04N7/24
Filed: 1927-08-09
Abstract: 275,675. British Thomson-Houston Co., Ltd., (Assignees of Alexanderson, E. F. W.). Aug. 9,1926, [Convention date]. Copying-telegraphs. - In a wireless copying-telegraph, signalling is effected by the presence or absence of impulses as distinct from variations in the intensity of impulses, a variable shutter being so arranged at the transmitting station that different tone-values are signalled by different numbers of impulses. Transmitter, Fig. 1. At the transmitting station the picture is inscribed on a kinematograph film 2 which is slowly traversed, and light passes from the lamp 1 to the photoelectric cell 8 through the film 2, lens 11 and screen 12, the film and screen being. at conjugate foci of the lens. The element whose image falls on the hole 13 in the screen is traversed by the rotation of the quartz or glass polygonal prism 9, and the rates of movement of prism and film are so related that successive elementary strips of the picture overlap one another. The cell 9 controls a short-wave wireless transmitter 17, the grids of which are negatively biassed by a battery 19 so that oscillation cannot take place until the input voltage reaches a threshold value corresponding to a certain intensity of tone in the picture element. Receiver, Fig. 2. The received impulses cause the mirror 40 of a galvanometer 37 to reflect the light from a source 39 through a lens 41 and rotating polygonal prism 42 on to a moving kinematograph film 45. The prism deflects the light beam so that it traverses successive strips of the film. Half tones. For each signal received the receiver, Fig. 2, records a dot of fixed intensity, so that a two-tone picture would result from the absence of the sectored disc 55, Fig. 1, at the transmitter. This disc rotates at such a rate that each of the four sectors, which are cut away to different radii, corresponds to one elementary strip of the picture. The sectors reduce the illumination of the cell 8 in varying degrees, so that while a white picture element can effect a signal in each of the sectors, a light grey element effects a signal in only three of the four sectors, and so on. As the elementary strips overlap, the tone of an element is represented by the number of signals which it effects during the rotation of the disc 55. At the receiver different tone-values are reproduced by the superposition of different numbers of marks. Atmospherics, eliminating or limiting; fading, preventing. As described above, the signals consist in the presence or absence of wireless impulses, gradations of tone being accounted for by the disc 55 and not by variations in the intensity of the impulses. The latter are all stronger than atmospherics, and the wireless receiver 28 is given such a grid bias that it only responds to signals whose intensity exceeds a threshold value which is not reached by atmospherics. The filament current is so adjusted that the output current of the valve 28 reaches saturation as soon as it begins to flow, so that the impulses applied to the galvanometer 37 have a fixed value. In addition a screen 52 may occupy the region corresponding to deflections of the mirror 40 due to atmospherics, and the aperture of the lens 41 is wide enough to allow for variations in the deflection due to the fading of signals and to the eupsrimposition of atmospherics upon signals.