Patent GB-381254-A (GB)
Title: Improvements in or relating to television, photo-telegraphic and like systems
IPC: H04N7/12
Filed: 1931-07-14
Abstract: 381,254. Secret transmission. WILSON, J. C. and BAIRD TELEVISION, Ltd., 133, Long Acre, London. July 14, 1931, No. 20196. [Class 40 (iv),] In a television or like system incorporating a radio or wire transmission channel that is capable of passing only frequencies within a certain range, in order to utilize more fully the available band width, means are provided for condensing the spectrum of the transmission into a narrower frequency band than would otherwise be necessary for carrying all the frequencies essential to the building-up of a reproduction having a prescribed degree of fidelity. For this purpose, at the transmitter, a series of frequency changers 14, 16, 18 .. are provided, each of which, by means of selective filters 8, 10, 12, .. is adapted to pass one of the predominant components of the image wave form. The output of each frequency changer is a sinusoidal current of an amplitude proportional to that of its selected frequency component, but of a suitable altered frequency, and the several currents are re-combined to form the picture &c. wave sent out from the transmitter 20. At the receiver, to re-compose the picture, the original frequencies are restored by separating the altered frequencies by filters 24, 26, 28, .. which either are associated with frequency changers 30, 32, 34 .. of inverse ratio to the frequency changers 14, 16, 18, .. or are designed to apply the separated voltages to resonant circuits of the original frequencies fed from an overloaded triode oscillator. In the system illustrated, other elements of usual form represented are the subject of transmission 2, photo-cells 4 and amplifier 6 at the transmitter, and, at the receiver, amplifier 36, Kerr cell or other light modulator 46, and a mirror or like scanning element 40. The basic frequency component, corresponding to the number of complete pictures per second, is not transmitted, but is introduced, in re-composing the picture, by a generator 38, the amplitude being regulated by a volume control 42 and the phase being adjusted by rotating the stator of the generator about the shaft 44. If desired, also, one or more of the predominant frequencies may be transmitted unchanged and re-combined at the receiver with the frequencies subjected to change. The picture may also be reconstituted and automatically synchronized by utilizing at the receiver a generator of the picture and/or line-traverse frequencies, e.g. a phonic wheel having a number of teeth equal to the number of picture strips and driven by the shaft of the reconstituting device. The generated frequency or frequencies is or are fed to a thermionic valve to saturate the latter, and the resultant harmonics are triggered individually by the incoming separately filtered frequencies, whereby a picture will be reconstituted the position of which on the screen is independent of the relative rates of movement of the reconstituting and the transmitter scanning devices. The two latter elements may, however, be synchronized by any known method, e.g. as described in Specifications 320,639 and 336,655, [both in Class 40 (iii), Telegraphs, Electric]. Specifications 230,576, 235,619, 236,978, 253,957, 265,640, 266,591, 269,658, 289,307, 299,076 and 299,402, [all in Class 40 (iii), Telegraphs, Electric], also are referred to.