Patent GB-909593-A (GB)
Abstract: 909,593. Colour television. VALENSI, G. July 15, 1960 [July 16, 1959; Sept. 25, 1959], No. 24730/60. Class 40 (3). [Also in Groups XL (a) and XL (c)] The invention relates to a television receiver for the reception of a signal comprising a luminance component and a chrominance component and in which an image is produced on a projection screen EP with the aid of cathoderay light modulators TC1, TC2 of the type including an oil film deformable by electronic scanning, one modulator TC2 functioning to produce a detailed black-and-white picture in response to the luminance component, and the other modulator TC1 functioning in response to the chrominance component to produce a relatively coarse colour picture which is superimposed on the black-and-white picture. As illustrated the modulators comprise separate cathode-ray tubes. According to a modification described with reference to Figs. 1c and 1d, not shown, the two modulators are embodied in a single envelope (see Group XL (a)). The oil films are carried by transparent plates P1, P2 rotated slowly by mechanism Mec and modulate light transmitted through the films from white light sources S1, S2. Alternative arrangements are described with reference to Figs. 1e and 1f, not shown, based on reflected light, the oil films being carried by plane or concave reflective plates. The receiver shown in Figs. 1a and 3b is intended for operation with a signal according to the American N.T.S.C. standards. Reference is also made with reference to Figs. 4 and 4a, not shown, to a receiver for operation with a signal in which the chrominance component (a subcarrier) is modulated in amplitude only in accordance with coded colour information denoting colour in sixteen discrete steps comprised of eight hues each with two degrees of saturation. The receiver differs from that shown in Figs. 1a and 1b only in respect of the method of deriving hue and saturation information from the chrominance subcarrier which is effected with the aid of decoding cathode-ray tubes having specially arranged targets. A feature of the invention is a luminance weighting device comprising a pentode L in which the low frequency part of the luminance signal 1<SP>1</SP> (extracted from the received signal by filter F3) is controlled inversely in amplitude in accordance with a saturation signal S and is then recombined with the high frequency part of the luminance signal 1<SP>11</SP> (extracted by filter F2) in a mixer mxl. The saturation signal S is obtained from the chrominance subcarrier chr (which is separated from the received signal by filter F<SP>1</SP>2) by amplitude detection in a circuit DA. A potentiometer controls the extent to which signal S modifies signal 1<SP>1</SP>. In general the luminance component should be reduced to zero for 100% saturation so that modulator TC1, which is controlled by the luminance signal, contributes no white light to the projected image, except for fine details reproduced by signal 1<SP>11</SP>. The operation of modulators TCl and TC2 is based on the deformation of the oil films to form diffraction gratings. The films are scanned with conventional television rasters under the control of line and frame scanning coils h<SP>1</SP>1, v1 and h<SP>1</SP>2, v2 supplied in the normal manner by line and frame scan generators Oh, Ov, and in addition are subjected to velocity modulation of the line deflections by further signals applied to plates h1, h2. The velocity modulation causes non-uniform charge distributions to be laid down whereby the films deform to form diffraction gratings. An electron spot is stated to be 0.02 mm. wide, and 0.1 mm. long, the spot being one " line " of a grating. In the case of modulator TC2, which produces the black-and-white image, the modulating signal A 2 sinW 2 t, which originates from an oscillator O 2 , has a constant frequency and has its amplitude varied in accordance with the weighted luminance signal by modulator md 2 . A filter F4 rejects the carrier and one side-band of the modulated signal. The light from source S2 illuminates a first bar system B1 which is imaged on to a second bar system B2 so that normally no light passes to screen EP. In response to signal A 2 sin w 2 t the light is diffracted and passes bar system B2, the amount being proportional to the amplitude of the film deformations and hence the luminance signal. In the case of modulator TC1 the modulating signal A 1 sin w 1 t changes in frequency in a sequence between three values which are chosen so as to cause the production of diffraction gratings of such line spacings as to allow the passage of light respectively in the red, green and blue thirds of the spectrum. To achieve this it is necessary to correlate the grating spacings with the dimensions of slits and bars in a mask M and the Specification describes in detail with reference to Figs. 3, 3a, 3b, 3c, 3e and 3f, not shown, how this is done for a number of different arrangements of masks. Some of the arrangements include lenticular lens screens for imaging the diffraction spectra on mask M. In one method of operation the frequency of signal A 1  sin w 1 t is changed at line-scanning rate so that red, green and blue are produced in repeating sequence on successive lines throughout each television field. A chain of pulse circuits GR, CD, MV, MV<SP>1</SP>, do, dc<SP>1</SP>, MX and AM, which is triggered by line synchronizing pulses tl, generates a 3-step wave ech, each step of which occupies the duration # of one line. The wave controls a modulator md1 which switches the frequency of an oscillator O1 (by changing either an inductance or capacitance) so that the correct frequencies for signal A 1  sin w 1 t are produced. An alternative arrangement, Fig. 2f, not shown, uses three separate oscillations whose outputs are gated in turn. The signal is applied to modulator TC1 through a gate stage 1b which is controlled by the chrominance component chr so that coloured light is produced only at the appropriate points in each line. The chrominance component is amplitude limited in a stage la and then compared in a phase detector DP (described in detail in Fig. 2c, not shown), with a reference carrier wave obtained from an oscillator O which is synchronized in the conventional way with the colour " burst " by a group of circuits Asr, dp and ct. The signal C1 from detector DP is proportional to the phase of the chrominance component and thus represents hue. The signal is applied to the horizontal deflecting plates H of a decoding cathode-ray tube Td the vertical plates V of which receive the 3-step wave from modulator Md1. The tube target incorporates an apertured plate arranged as shown in Fig. 2c, and the combined effect of signal C1 and the 3-step wave is the production of a control signal C2 to operate gate 1b during each scanning line only when the colour component allotted to that line is necessary for the production of the hue denoted by the phase of the chrominance signal. Another method of operation is described with reference to Fig. 2c and 2g, not shown, in which during one interlaced field red and green appear on alternate lines and during the other field blue appears on all lines. As an alternative to the sine wave signal A 1  sin w 1 t, a triangular or square-wave signal may be used. In connection with the use of a square-wave signal reference is made to an arrangement in which the duration of the pulses forming the wave may be modulated by the low frequency luminance signal 1<SP>1</SP>. This results in a diffraction grating in which the line widths are varied with a consequent variation of the amount of diffraction light flux so causing brightness modulation of the produced colour. The low frequency luminance signal is applied through a gate controlled by the saturation signal S and arranged to be blocked except when a very saturated colour is to be produced. Focus control circuits th1, th2 (see Group XL (c)) which are described in more detail in Fig. 3 (c), not shown, are associated with the focus coils sf1, sf2 of modulations TC1, TC2 to compensate for the use of off-axis electron guns.