Patent GB-737171-A (GB)
Title: Improvements in or relating to television systems
Inventor: JESTY LESLIE CONNOCK
IPC: H04N9/093
Filed: 1952-01-16
Abstract: 737,171. Television. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Dec. 11, 1952 [Jan. 16, 1952], No. 1293/52. Class 40 (3). A colour television system comprises means for developing, at fixed points during each scanning line at both the transmitter and the receiver, colour registration signals which are dfferent and separable from the picture signals and which are employed to control the distribution of the developed picture signals among the different colour channels at the transmitter and the application of the received colour signals to the picture reproducing tube at the receiver. Alternatively the colour registration signals may be compared with master synchronizing signals to derive correction signals which control the line scanning velocity at both transmitter and receiver. Fig. 1 shows the arrangement at the transmitter where the object 1 is imaged on the picture signal generating tube 7 via a colour line filter 4, the elements of which run at right angles to the direction of line scanning and comprise parallel strips in a repeating sequence of R (Red), G (Green), B (Blue), but with alternate blue strips replaced by opaque bars. A bias light source 6 reflected, via a semi-silvered mirror 3, on to the filter 4 ensures that an image of the opaque bars is formed at the tube 7 even if those parts of the image which coincide with the bars are not illuminated. With this arrangement the signal output of the tube is as shown in Fig. 2, where R, G, B represent signal levels representative of the component colours present in the object 1, BL the level corresponding to unilluminated portions of the object (determined by the bias light 6) and S the colour registration signals which are produced by scanning the images of the opaque bars in filter 4. If the bias light is made strong enough to produce peak white signals and is either switched on just before and during the scanning of the opaque bars or, is arranged to illuminate only very narrow strips adjacent each opaque strip, the colour registration signals may be produced as peak white as shown in Fig. 3. This allows the full range of light intensity which the camera can handle to be employed for production of the picture signals. The colour registration signals produced in this manner (e.g. as shown in Fig. 2), are then supplied via an inverter 1 (Fig. 4), to a first (Schmidt) trigger circuit comprising valves V1, V2, the anode circuit of the latter of which includes a damped circuit L1, C3, R7 resonant at the repetition frequency of pulses S (so that, if some of the latter pulses are obscured by random noise in the tube 7, " replacement " pulses are supplied) and arranged to supply a sinusoidal wave to a second trigger circuit V3, V4, the former valve, V3, being arranged to conduct on the positive peaks of this wave so that a rectangular pulse, for each peak, is produced across R14 in the anode circuit of V4. These pulses which, by means of a delay line D1 across R14, are arranged to have a duration which is a little less than the time required to scan one strip of filter 4 (Fig. 1), are then fed via cathode follower V5 to a further delay line D2 from tappings a, b, c, d, e on which pulses coincident with the scanning of strips R, G, B, R, G respectively of filter 4 are obtained. The latter pulses are then employed as sampling pulses for the five sampler circuits SC1 ... SC5 (Fig. 5) (to each of which the picture signal train (Fig. 2) is supplied), the outputs of which are fed to separate colour transmission channels RC, GC, BC, via low-pass filters FR, FG, FB, respectively, filter FB having a cut-off frequency of one half that of the other filters since only half as many blue as red or green filter strips are scanned in a given time. At a receiver co-operating with the transmitter described but operating in what is termed an " unlocked " manner the reproduced image on the picture tube P1 (Fig. 6), is directed via a colour line filter P4 (similar to 4, Fig. 1) on to a half-silvered mirror P5, the light transmitted by which is again projected by means of suitable optical devices from focal point P7 to a viewing screen and the light reflected by which is employed to generate, via photo-electric cell P6, registration pulses corresponding to the scanning of the opaque lines in filter P4. These pulses are then supplied to a circuit similar to Fig. 4 which produces five time-spaced pulses a, b, c, d, e and these latter pulses are employed to control samplers RSC1 ... RSC5 (Fig. 9) to which the receiver Red, Green and Blue channesl RC, GC and BC are connected as shown, so that colour signals in the correct sequence are derived in conductor COC for application to the picture tube P1. To ensure that the registration pulses are produced even when the portion of the picture being scanned is dark the tube P1 may be biased so that light (of low intensity) is always produced or, alternatively, a brightening pulse derived by any known means from the registration pulses may be employed to brighten the light produced by tube P1 just prior to the scanning of the opaque strips. In a further alternative the registration pulses are produced by means of a screen P8 (Fig. 7) provided with narrow transparent strips of the same pitch as the opaque strips in filter P4, the scanning of the latter and former strips being either simultaneous or, in order to prevent the brightening pulses applied to P1 from being visible in the projected picture, the scanning of the opaque strips in P4 is arranged to occur just prior to the scanning of the transparent strips in P8. A circuit for ensuring that at the start of each line scan the light produced by the picture tube is bright enough to produce a first registration pulse from which the succeeding brightening pulse may be derived is described in connection with Fig. 10 (not shown). In an alternative method of operation, termed the " locked " method, the repetition frequency of the registration pulses produced at both the transmitter and receiver is compared with a constant frequency source common to the whole system, any departure from this constant frequency being employed to correct the line scanning velocity at either the transmitter or receiver as required (Figs. 13, 14 and 15, not shown). The colour filter P4 (Figs. 6 and 7) may be replaced by a screen of suitable fluorescing and for the registration pulses more fluorescing phosphors placed inside the picture tube. The invention is also adaptable to existing monochrome television systems. Specification 713,641, [Group XL (a)], is referred to.