GB749118A - Electrical system - Google Patents

Abstract

749,118. Television. PHILCO CORPORATION. May 28, 1953 [May 29, 1952], No. 14871/53. Class 40 (3). In a (dot-sequential) colour television system in which a colour image is reproduced by means of a modulated electron beam scanning transversely a repeating series of colour reproducing " triplets desaturation of the reproduced colour image is prevented by ensuring that the electron beam is ineffective to produce luminosity when scanning strips of the "triplet" which are not appropriate to a particular colour scanning cycle. The desaturation which, in the absence of the arrangement according to the present invention, is brought about, as shown in Figs. 2A and 2B, by the combined effect of the finite widths of the strips R, G, B, &c. of the colour reproducing screen and of the nature of the modulating signal comprising the brightness 'component 25 and the colour component (in this case, the red component) 26, due to the latter component being effective to produce luminescence of those parts of the green and blue strips which are adjacent to the red strips (and of the red and blue and the red and green strips during green and blue scanning cycles respectively), is avoided by modifying the modulating signal so that, as shown in Fig. 2C, the colour component 26 has zero value at the instants at which the scanning beam commences and terminates the scanning of a desired colour strip and has a negative value whilst traversing the other two strips of each "triplet." In accordance with the invention, one embodiment of which is shown in Fig. 1, the modified signal is derived by the algebraic addition of the luminosity signal, derived from source 10 via L.P. filter 11, and a signal derived via a L.P. filter 15 and produced by rectifying in 14 the colour-sub-carrier wave derived from source 10 via B.P. filter 12, the rectifier being so poled that its output signals have a polarity which is opposite to that of the signals from filter 11. The difference signal thus produced is then combined with the output of filter 15 (after amplification in 13 if necessary) and supplied to the modulating electrode 16 of a colour reproducing image tube 17. In a second embodiment employing a "two-gun" picture reproducing tube in which the luminosity signal is supplied to one " gun " and the colour signal to the other " gun at a rate determined by the scanning of "indexing" stripes in the tube screen structure the colour signal are recovered by homodyne detection and subtracted from the luminosity signal prior to the application of the latter to its respective " gun " (Figs. 3 and 4, not shown). The modification of the luminosity signal may be effected at the transmitter instead of at the receiver if all receivers are known to require the same modification and the invention may be employed in systems in which the monochrome and chromaticity signals are not formed with equally time-spaced intervals of colour intelligence (as in the embodiments described above) by suitably altering the spacing and/or disposition of the colour reproducing elements of the picture tube screen. Specification 745,353 is referred to.