Patent GB-277761-A (GB)
Title: Improvements in or relating to the electrical transmission of pictures
IPC: H04N3/12
Filed: 1926-06-26
Abstract: 277,761. Bronk, O. von. June 26, 1926. Copying - telegraphs; television apparatus.- In copying telegraphs and television apparatus the analysis and recomposition of a picture is effected by electro-optical means. At the transmitting station, Fig. 7, light from the picture 15 is passed through a polarizer 16 to a "photocell" 8 divided into horizontal strips which are electrically controlled so that while normally opaque to the polarized light they are stressed in succession so as to transmit one horizontal strip of the picture at a time. Similarly the cell 9 is divided into vertical strips which are electrically controlled at a multiple of the control frequency of the cell 8, so as to pass successive elements of the horizontal strip of picture which reaches them from that cell. The spot of light transmitted through the two cells is focussed by the lens 18 on a light sensitive cell 14. In the receiving apparatus, Fig. 8, the inverse operation is effected : light of constant intensity from a point source 23 is collimated at 24, polarized at 25, and divided into elements by cells 8<a>, 9<a>, similar to the cells 8, 9, and subjected to the same electric control as they. It is then analysed at 26 and received on a screen 27. The polarizers and analyzers 16, 17, 25, 26, may be made of artificial sodium nitrate. System employing Kerr cells.-The cells corresponding to 8, 9, may be Kerr cells which when electrically stressed rotate the plane of polarized light passing through them, and so enable light to pass through the crossed nicols 25, 26. A given angle of rotation must be achieved before the transmitted beam reaches the intensity rerequired to affect the light-sensitive cell 14: part of this angle is contributed by the stressed horizontal strip of the Kerr cell 8, the remainder by stressed vertical strip of the Kerr cell 9, so that the only effective element of the picture is that corresponding to the intercession of the strips. In order that the strips of cell 8 may be stressed in succession the electrodes are connected to an alternator W, Fig. 2, through impedances b - - e - - - so calculated that the amplitude is the same but the phase different for each electrode. The angle between the strips belonging to the two cells may be less than a right-angle, and at the receiving station the light intensity may be varied by a Kerr cell in front of the lamp 23, the cell acting also as an analyzer in place of the analyzer 25. Modification employing piezo-electric resonators.-In an embodiment employing piezo-electric resonators a low-frequency generator 4, Fig. 7, is tuned by a rotating condenser 6, so that the oscillations applied to the cell 8 vary continuously in frequency throughout a given range. Each strip of crystal in the cell 8 responds to a different frequency and during response it passes a horizontal line of illumination from the picture 15. Similarly the oscillator 5 generates currents whose frequency is varied by the condenser 7 rotating at a multiple of the speed of the condenser 6, the crystal strips of the cell 9 being thus resonated in succession so as to transmit successive elements of the picture. The transmitted elements affect a light-sensitive cell 14, and the carrier wave sent out by the transmitter 1, 2 is modulated by the oscillations from the valves 4, 5, the amplitude of the modulation being controlled by the cell 14. At the receiving station, Fig. 8, the crystal strips composing the cells 8<a>, 9<a>, are successively resonated by the demodulated received waves, with an amplitude depending on the modulation originally imposed by the cell 14. The strips in the cells 8<a>, 9<a>, have mechanical frequencies corresponding to those in the cells 8, 9, so that the order in which they act is determined by the tuning condensers 6, 7, of the transmitter system. A pointolite lamp or other source of constant illumination 23 is therefore able to illuminate successive elements in the screen 27 as the strips in the cells 8<a>, 9<a>, respond to the incoming signals the intensity of the illumination being governed by the amplitude of modulation of the received signals.