Patent GB-621560-A (GB)
Title: Colour television systems
IPC: H04N9/09
Filed: 1945-10-10
Abstract: 621,560. Television. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Oct. 10, 1945, No. 26489. Convention date, Aug. 21, 1942. [Class 40 (iii)] In a colour television system, groups of signals representative of the primary colours of the object are transmitted together with signals representative only of the intrinsic brightness of these colours. An image of the object to be transmitted is focused by lens system 21 via a four-sector colour filter 25, on to the mosaic electrode 31 of an iconoscopetype tube 27, the video output from signal plate 29 being fed to an amplifier 39 (Fig. 3). Three sectors of the colour filter contain red, green and blue filters and the fourth sector an orthochromatic filter. The output from 39 is fed to a brush 41 bearing on a commutator device 40 mechanically coupled to the motor 23 driving the colour filter 25. The four segments 43, 45, 47, 49 of commutator 40 are connected to collector rings 51, 53, 55, 57 from which video signals are collected by brushes 59, 61, 63, 65 and fed to amplifiers 67, 87, 89, 91, the mechanical phasing of the colour filter and commutator being such that these amplifiers receive video signals corresponding to orthochromatic, red, green and blue images respectively. The output of amplifier 67 is fed to delay devices 69, 71, 73, which introduce delays equal to the time required to scan one, two and three picture fields respectively, and thence to reversing switch devices 75, 77, 79. The outputs from the reversing switches developed across potentiometers 81, 83, 85 supply a controlling bias for gain control units 95, 97, 99, to which the outputs of amplifiers 87, 89, 91 are applied. Combining amplifier unit 93 supplies the outputs from 67, 95, 97, 99 successively to the transmitter channel and also to a monitoring device 125. The polarity of the biassing voltage developed by the reversing switch devices may be reversed by means of switches 101 which introduce or cut out an additional valve stage. The selected polarity is determined by the requirement for either increasing or decreasing the intensity of the red, green and blue video signals in units 95, 97, 99 according to the light and shade intensities of the orthochromatic image, the signals due to one scanning of the latter image being available, because of the delay devices, to supply controlling bias for each of the amplifiers 95, 97, 99 during the period of scanning of the red, green and blue images respectively. In an embodiment (Fig. 4) light from the object is directed through red, green and blue filters, 141, 155, 161 respectively, on to the mosaic screens of three iconoscope-type tubes 139, 153, 157 by means of half-silvered mirrors 135, 137. The density of each filter is such as to compensate for the loss of light in the half silvered mirrors and thus equalize the general intensity of the optical image on each mosaic screen. The outputs from the image tubes after amplification in amplifiers 164, 165, 166 are fed to brushes 170, 171, 172 and thence by way of segments 176, 177 in commutator 167, 168, 169 to inner commutators, from which brushes 180, 181, 182 convey the signals to the amplifiers 184, 185, 186 respectively, and brushes 193, 197, 195, which are connected together, to the three amplifiers simultaneously. The segments 176, 177 extend for 270 degrees and 90 degrees respectively, so that the signals fed to amplifiers 184, 185, 186 consist of those corresponding to red, green and blue picture fields respectively, occupying three-fourths of the time for one complete rotation of the commutator and, during the remainder of the rotation, those corresponding to an orthochromatic image applied to the three amplifiers simultaneously. The outputs from 184, 185, 186 are transmitted over three separate channels. The orthochromatic image signals used in both systems may result from the response of the tube mosaic in the infra-red, the visible or the ultra-violet portions of the spectrum. One form of receiver suitable for use with the embodiment of Fig. 4 may comprise a single cathode-ray tube having a laminated luminescent target, responsive to the various picture colours and scanned by a plurality of electron beams, or alternatively, a plurality of separate cathode-ray tubes, each having a target adapted to reproduce one of the colours utilized in the system may be used, the various images being superimposed on a single viewing screen. In a further form the receiver may comprise a plurality of cathode-ray tubes each reproducing a black-and-white image, a rotary colour filter supplying the necessary colour to the image produced by each tube. In a modification of the transmission system the three coloured images may be transmitted in cyclic order followed by simultaneous transmission of three orthochromatic picture fields. In a further modification the three coloured images may be simultaneously transmitted, followed by the cyclic transmission of equalized coloured images. Instead of mechanical commutation, electronic switching methods may be utilized.