Patent GB-851430-A (GB)
Title: Colour television on projection screen
IPC: H04N9/16
Filed: 1958-08-12
Abstract: 851,430. Colour television. VALENSI, G. Aug. 12, 1958 [Aug. 12, 1957; March 14, 1958; July 17, 1958], No. 25943/58. Class 40(3). [Also in Groups XX and XL(a)] In a colour television receiver employing large screen projection, means are provided for producing a detailed black and white picture in response to a luminance signal and superimposing thereon a less detailed coloured picture produced in response to a chrominance signal, a luminance weighting device being provided to control a low frequency band of the luminance signal corresponding in width to the chrominance signal inversely in accordance with picture saturation so as to cause the production of the correct ratio of coloured to white light. Two embodiments are described, the first employing a black and white projection cathode-ray tube and a colour picture producer in the form of a fixed light source operating in conjunction with a birefringent colour modulator and an optical scanning system, and the second employing two projection cathode-ray tubes. The receivers operate with a coded chrominance signal in the form of a subcarrier, the amplitude of which represents hue and saturation in accordance with a limited number of discrete areas of the colour triangle, although a modification is described to permit operation with the N.T.S.C.-type of signal. First embodiment, Fig. 2. The incoming signal, Fig. 1(a), is directed into separate channels in accordance with the bands B1 2 , B 3  and B 2  + B1 2 , the low frequency band B 3  of luminance signal being equal in width to the band B<SP>1</SP> 2  containing the chrominance sub-carrier. The luminance signal l<SP>1</SP>in band B 3  passes through a variable gain amplifier L and is thereafter combined in mixer M with the remainder of the luminance signal l<SP>11</SP> in band B 2  + B<SP>1</SP> 2  and applied to modulate a black and white cathode-ray tube 0<SP>1</SP> which projects on to viewing screen EP by way of Schmidt optical system Á<SP>1</SP>, #<SP>1</SP>. The chrominance signal chr after detection and gain stabilization is applied to the horizontal deflection plates P, P<SP>1</SP> of decoding cathode-ray tubes Tdc and Tds (see Group XL(a)), which have targets so shaped as to produce signals C and S representative of hue and saturation. In the particular example considered the hue signal may have any one of eight possible values corresponding to eight predominating wave lengths and the saturation signal may have either of two values corresponding to two degrees of saturation. The colour picture projector comprises a light source #, such as an electric arc, electric colour modulators K, K<SP>1</SP> (see Group XX) based on birefringent crystals, and a scanning mirror drum TM. The hue signal C is applied to control modulator K directly; and the shape of the target in decoding tube Tde is arranged so that each step in the signal is at a level to produce the correct degree of retardation to allow light of the desired predominating wavelength to pass. The resulting light is not saturated. The hue signal is also applied to control modulator K<SP>1</SP> via an amplifier AC<SP>1</SP> which is rendered conducting only when the saturation signal S is at the value indicating the higher degree of saturation, this action being effected by applying the saturation signal to control the amplifier suppressor grid gc<SP>1</SP> via a valve T having a suitable threshold bias. A second similarly controlled amplifier AC<SP>11</SP> introduces a desired amount of D.C. signal. Modulator K<SP>1</SP> operates to modify the spectral distribution of the transmitted light so as to increase saturation without changing the predominating wavelength. In accordance with the invention, the saturation signal is also applied to control the gain of amplifier L so as to vary the amplitude of luminance l<SP>1</SP> applied to modulate tube 0<SP>1</SP>. Thus, with a saturated coloured area the light as regards low definition detail is contributed solely by the colour projector. The luminance signal in band B 2  + B<SP>1</SP>2 however continues to provide high definition detail. With an area of low saturation, modulator K<SP>1</SP> is inoperative and the colour projector produces light which is not saturated whilst tube 0<SP>1</SP> contributes white light in response to both parts l<SP>1</SP> and /11 of the luminance signal. In black and white areas signals C and S are zero and the colour projector ceases to operate. Refer ence is made to the modulation of light source # to correct for variations of luminance in areas of saturated colour. Second embodiment, Fig. 3, the incoming signal V is again divided into bands B<SP>1</SP>2, B 3  and B 2  + B<SP>1</SP>2, the luminance components /1 passed through a variable gain amplifier L to a mixer M where it is combined with component l<SP>11</SP> for application to a black and white projection cathode-ray tube. 0<SP>1</SP>, and the chrominance signal chr applied to the horizontal deflecting plates in decoding cathode-ray tubes Tds, Tdc to develop hue and saturation signals C and S. In accordance with the invention, the latter of these signals controls the gain of amplifier L. A filter F4 and gate #selects a subcarrier frequency reference level signal and applies it to an amplifier AC to stabilize the amplitude of the chrominance signal. In this embodiment the colour picture producer comprises a cathode-ray tube 0 having a multilayer screen (see Group XL(a)), the colour of the light from which may be either red, green or blue in dependence upon the combination of potentials applied to the final anode A and beam intensity modulator W. A sinusoidal signal of frequency f3 chosen in accordance with the desired definition in the colour picture is derived from the subcarrier frequency f2 by division in a stage DM and applied through amplifiers A 1,  A 2  and gates C 1 , C 2  to modulate the anode and modulator of tube 0 in antiphase. In the absence of any control applied to gates C 1 , C 2 , the tube would produce red, green and blue colours cyclically in the sequence indicated in Fig. 3a. To produce a desired colour, the gates which are normally closed, are arranged to be opened for intervals throughout each cycle to causes the production of appropriate amounts of each primary colour. To this end, the sinusoidal signal of frequency f3 is applied to vertical deflecting plates in decoding tube Tdc and the target is constructed as shown in Fig. 3c so that, at each horizontal position occupied by the beam in accordance with the chrominance signal chr, as the beam is oscillated vertically, a hue signal C comprising pulses of appropriate duration and position in the cycle is produced. As illustrated, the decoding tube is arranged for use with a chrominance signal having twenty-seven possible amplitude values corresponding to nine predominating wavelengths each with three possible degrees of saturation. Modifications are referred to in which the luminance signal l<SP>1</SP> is applied to modulate the width of the pulses forming the hue signal C or the amplitude of the sinusoidal signal applied to modulator W of tube 0. Operation with N.T.S.C.-type signal. For this purpose the second embodiment is modified as shown in Fig. 3f. The received signal V is applied to a decoding device DC to derive red, green and blue representative signals and these are applied to plates A, B, C, in a cathode-ray tube COM in which the beam is oscillated by a sinusoidal signal of frequency f3. The plates, which are normally negative, only collect electrons when receiving a colour representative signal with the result that pulses are produced across a resistor at the appropriate times in the cycle to control gates C 1 , C2.