Patent GB-1094701-A (GB)
Title: Improvements relating to colour television signal generators
Inventor: JAMES IVANHOE JOHN PENFOUND
IPC: H04N9/09
Filed: 1963-08-21
Abstract: 1,094,701. Colour television. ELECTRIC & MUSICAL INDUSTRIES Ltd. Aug. 13, 1964 [Aug. 21, 1963], No., 33022/63. Addition to 1,046,777. Heading H4F. In a colour television system according to Claim 8 of Specification 1,046,777, a luminance signal Y<SP>1</SP>/γ = (0.30R + 0.59G + 0À11B)<SP>1</SP>/γ derived from a single pick-up tube 1, Fig. 1, is converted to a signal Y<SP>1</SP> = 0À30R<SP>1</SP>/γ + 0À59G<SP>1</SP>/γ + 0À11<SP>1</SP>/γ over the low frequency band N by subtracting in circuit 15 a correcting signal #L = Y<SP>1</SP>/γ-Y<SP>1</SP> which is a non-linear function of individual narrow-band (band-width = N) colour component signals RN<SP>1</SP>/γ, C N <SP>1</SP>/γ, B N <SP>1</SP>/γ and is obtained with the aid of a matrix circuit 13 in which is selected the instantaneous extreme of a series of linear approximations to the function. The matrix develops the function V =  U Y N 1/ #  - v<SP>R</SP>N<SP>1</SP>/γ - W<SP>B</SP>N<SP>1</SP>/γ, where U = 1/0.59, v = 0.30/0.59 and w = 0.11/0.59, and #L is obtained from subtracting circuit 14 in accordance with the equation #L = 0À59(VG N 1/#). The matrix circuit, Fig. 4, comprises five input terminals 30 to which are supplied positive and negative versions of the gamma corrected colour component signals as indicated, a series of eight resistor matrix circuits 31 which derive linear approximations to function V which are correct at the colours white (W), red inner RI, blue B, green G, magenta (M), cyan (C), yellow (Y) and red outer (RO), and a series of transistors 32 with a common emitter load which functions to select the instantaneous most positive signal. This is the signal V which is then applied through an emitterfollower stage 33 to a combining circuit 37 (equivalent to circuits 14 and 15 combined in Fig. 1) which also receives the signals G N 1/γ and Y<SP>1</SP>/γ. Resistors 34, 35 and 36 are proportioned appropriately whereby the output signal equals Y<SP>1</SP>/γ - #L = Y<SP>1</SP>. A limiter circuit 38 is provided to prevent distortion if the final signal becomes blacker-than-black. The two linear approximations for red, termed red inner and red outer, are employed because of the shape of the surface of the non-linear function V. The transistors 32 may be replaced by an equivalent arrangement of diodes or thermionic triodes. Bandwidth restriction of the signals R N , G N  and B N  is achieved by optical defocusing, electron beam defocusing or circuit bandwidth limitation. In a modification Fig. 2 (not shown), the green pick-up tube 3 is replaced by another luminance pick-up tube producing a narrow band signal y N 1/ #  The matrix circuit is similar to that shown in Fig. 4 but is proportioned so as to develop Y<SP>1</SP> N  in terms of its input signals R N <SP>1</SP>/γ, B N <SP>1</SP>/# and Y N 1/ # . The signal Y<SP>1</SP> N  is then subtracted from Y N 1/# to develop #L which is utilized to correct the luminance signal as in Fig. 1. An application of the invention to tele-cine apparatus is described with reference to Fig. 3 (not shown). The film is scanned by a cathode-ray tube flying spot scanner and the transmitted light is directed to four photo-multiplier tubes. A part-silvered mirror directs 80% of the light to a tube which develops the luminance signal and directs the remainder of the light by way of dichroic filters to three tubes which develop the colour component signals. The signals are then processed as described in connection with Fig. 1. The invention may be applied where the luminance tube develops a signal which is not a strict luminance signal but a signal generally of the form Y L  = 1R + MG + nR. In this case the matrix circuit develops a correcting signal #L of the form Y L <SP>1</SP>/γ-Y<SP>1</SP> so that the luminance signal is converted to the form Y<SP>1</SP> for the low frequency band N corresponding to be bandwidth of the colour signals from which the signal #L is developed.