GB307309A - Improvements in or relating to picture and the like radio transmission systems - Google Patents

Abstract

307,309. Telefunken Ges. f³r Drahtlose Telegraphie. March 3, 1928, [Convention date]. Copying-telegraphs; fading, preventing.-In a picture-telegraph apparatus means are associated with the scanning device to alter the wavelength of transmission, or an intermediate modulation frequency, in the intervals between the transmission of consecutive lines of the picture. Thus a rotating contact device coupled with the pick-up device, may alter the wavelength of the transmitter by altering the magnetization of an iron-cored choke, by switching a condenser in or out of a circuit, or by short-circuiting a resistance in known manner. Alternatively an intermediate modulation frequency, obtained by a rotating perforated disc in the exploring light beam, may be altered in any convenient manner. At the receiver, where a local heterodyne oscillator is employed, the beat frequency is maintained constant by varying the oscillator frequency in synchronism with the transmitted wave, or where two wavelengths only are employed the local oscillator may be made to give the mean frequency of the two, thus giving a constant beat frequency. If the two transmitted frequencies are widely separated, the receiving tuner may be adjusted; if nearly the same the receiving device may remain unaltered.