GB738386A - Improvements in or relating to colour television transmission - Google Patents

Abstract

738,386. Television. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. June 23, 1953 [July 2, 1952], No. 16681/52. Class 40 (3). In a colour television transmission system in which the object to be televised is imaged on the camera tube via a multi-colour strip filter, the different colour strips have different " pitches" " so that in the camera tube output the signals corresponding to different colour components are modulated at different and therefore separable frequencies determined by the different pitches of the colour strips. The filter comprises two superimposed sections, one consisting of cyan strips and the other of yellow strips, each section having transparent strips between the colour strips and in the described embodiment the yellow strips are twice as wide as the cyan strips and the filter is positioned with the strips at right angles to the direction of line scanning in the camera. Fig. 1 (Prov.) shows the manner in which the two sections are superimposed, the cyan strips being designated R (red signal producing) and the yellow strips marked B (blue signal producing) so that the colour sequence of the composite filter when viewed by transmitted light is " green "; " yellow " ; " cyan " ; " white." With " ideal " filter strips (i.e. the cyan strips fully suppressing red and the yellow strips fully suppressing blue) the distribution of red and blue is as shown in Figs. 2 and 3 (Prov.), but in practice, due to filter imperfections, the actual modulation signals are contaminated by " colour crosstalk " and signals of the form shown in Figs. 4, 5 and 6 (Prov.) are obtained for the green, red and blue primaries respectively. This " cross-talk " may be reduced either by employing a suitable additional filter, e.g. a composite filter having a small gradation in magenta (minus green) to eliminate the green strip frequency modulations and small gradations in cyan and yellow in the case of the red and blue or, alternatively, by employing a suitable electrical filter network which performs a matrix transformation to produce the true primary colour modulations. With an " ideal " filter the camera output signal is of the form of a normal video signal with the green component emphasised (i.e. G+¢R+¢B-which may be corrected to a signal of correct brightness, i.e. ¢(G+R+B) by including a uniform magenta filter with a transmission factor of ¢ in the optical path) together with Rand B components which are in effect modulated on two different carrier frequencies determined by the pitch of the cyan and yellow strips in the composite filter. These latter signals are separated by conventional electrical filter networks and the arrangement therefore provides a colour television signal comprising red, blue and brightness signals (Fig. 7, Comp., not shown).