Patent GB-773380-A (GB)
Title: Improvements in or relating to producing multiplex television signals
IPC: H04N9/00
Filed: 1954-11-11
Abstract: 773,380. Television. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Nov. 11, 1954 [Nov. 14, 1953], No. 32711/54. Class 40 (3). In a colour or stereoscopic television system at least one aspect of the object is imaged on the picture signal generating tube through a grid structure so that the tube output signals contain a carrier frequency wave (determined by the " pitch " of the grid and the line scanning frequency) modulated by signals representative of this aspect. Fig. 1 shows one embodiment producing signals representative of the R (red) and G (green) aspects of an object in which the grid structure 7 comprises a colour filter constructed, as shown in Fig. 2, of an interdigitated series of colour filter strips a and b, the strips a passing both red and green light and the strips b passing red light only. When this grid is interposed in the object light path of the tube 1 a mathematical analysis of the signal developed across the load impedance 5 shows that it comprises, as shown in Fig. 3, (1) signals representative of the R and G aspects occupying a frequency range extending from zero to an upper limit f0 determined by the " pitch " of the grid strips and the (optical) definition of the image focused thereon, and (2) signals representative of the G aspect modulated on a carrier frequency f determined by the " pitch " of the grid strips and the line-scanning frequency. The optical definition of the image (which is adjusted by means of lens 6) focused on the grid and the " pitch " of the latter are selected (for a given line scanning velocity) that signals (1) and (2) do not overlap, so allowing these signals to be separated by means of suitable filters. As shown in Fig. 1, the signals (2) are selected by means of a band-pass filter F1, the green signals being recovered by detection in D1 and the signals (1) after selection by a low-pass filter F2 are combined in A1 with the green signals to provide, by subtraction, the red signals. The grid structure may consist of alternate opaque and transparent strips and be positioned in one of two different optical paths between the object and the tube to provide stereoscopic signals (Fig. 4, not shown) or, for two-colour television, each path may include an appropriate colour filter. To produce three-colour television signals the object light path is split up, by means of the optical system H1, into three separate paths LI, L2, L3 in two of which (L2, L3) colour filters 14, 15 transmitting red and blue light respectively and grid structures 16, 17 of different " pitches " and comprised of alternate opaque and transparent bars are positioned, the paths being then recombined by means of the optical systems H2 and focused on the camera tube 1 by means of lens 18. As in the arrangement of Fig. 1, lens 13 is arranged to produce a slight de-focus of the object so as to reduce the frequency band occupied by the " modulated " signal output of the tube and allow easy separation by means of filters. Fig. 6 shows the signal output developed across the load impedance 5, the band extending up to f3 and comprising the red and blue " unmodulated" components ER, EB, together with a luminance component EL (due to the optical path L1) being separated by means of a L.P. filter F3 and supplied to an adding device A2 in which it is combined with the detected lower sideband of the red-modulated component (derived via B.P. filter F4 and detector D4) and the detected upper sideband of the blue-modulated component (derived via B.P. filter F5 and detector D5). The outputs of detectors D4 and D5 then provide the red and blue component signals directly and by suitably combining the signals supplied to adding device A2 a luminance signal alone or a combination having desired fractions of EL, ER and EB may be produced at terminal L. In the embodiment of Fig. 5 the function of the colour filters 14, 15 and the grids 16, 17 may be combined by employing colour-selective grids similar to that shown in Fig. 2. By employing a composite colour selective grid (Fig. 7a, not shown) the light paths L2 and L3 may be combined. Specification 747,644 is referred to.