871,367. Colour television. ELECTRIC & MUSICAL INDUSTRIES Ltd. July 11, 1957 [July 19, 1956; Aug. 1, 1956], Nos. 22385/56 and 23675/56. Class 40(3) Relates to a colour television receiver utilizing a cathode-ray tube with a screen formed of a repeating sequence of differently coloured phosphor strips disposed transverse to the line scanning direction. In accordance with the invention the intensity of the cathode-ray beam is modulated in accordance with brightness information whilst the scanning velocity in the line direction is controlled in accordance with colour information. The velocity is controlled by superimposing on the normal line scanning a periodic wave, and in a first arrangement the amplitude is modulated in accordance with saturation whilst the phase is modulated in accordance with hue. In a second arrangement the wave is modulated solely in phase in accordance with hue whilst the saturation information is employed to vary the focus of the electron beam. The manner of operation in the case of the first arrangement is illustrated in Fig. 2 where curve 7 indicates the normal line deflection across the screen, which is formed of repeating triplets of red, green and blue phosphor strips, and curves 8 and 9 two examples of the periodic wave which is of sawtooth shape and arranged to occur at the frequency of scanning the triplets. The result due to the combined deflection is illustrated at 10 and 11. 10 corresponds to a 100% saturated green and 11 corresponds to less saturated green brought about by reducing the amplitude of the wave. Any desired colour is produced by modulating the phase of the wave, so as to position the working stroke appropriately on the triplets, and any desired degree of saturation is produced by modulating the amplitude appropriately A receiver employing this first arrangement is shown in Fig. 1 and is arranged to operate with signals comprising (1) a main carrier modulated in amplitude in accordance with luminance information and modulated in phase in accordance with part of the hue information, and (2) a subsidiary carrier wave modulated in amplitude with saturation information and modulated in phase with the remainder of the hue information. A first output from the receiving stages proper 21 comprises the luminance signal E Y 1/y as a video signal and is appled to a matrix circuit 21a to derive an equi-energy type brightnes signal E E 1/y. This signal is applied to modulate the beam intensity in cathode-ray tube 26. A second output occurs at intermediate frequency and is applied to filters 29 and 30 to separate the main and subsidiary carriers. The main carrier is amplitude limited in a stage 31 to remove the luminance modulation and is then recombined with the subsidiary carrier in a mixer stage 32. The mixer receives a third input (produced as described below) and comprising an oscillation of frequency equal to the difference between the auxiliary carrier and the desired frequency of scanning the phosphor triplets on the screen of tube 26. The mixer output, which appears across tuned circuit 33, comprises a wave at triplet scanning frequency and on to which all the modulations of the originating waves are transferred, whereby its amplitude represents saturation and its phase hue. This wave is applied to stage 34, illustrated in detail in Fig. 5 (not shown), where its modulations are transferred to a sawtooth wave as shown at 8 and 9 in Fig. 3 and the sawtooth wave is then applied to auxiliary deflection plates 35 in tube 26, the scanning of which is otherwise effected normally by means not shown. The third input for mixer 32 (see above) is derived initially from tube 26 by means of secondary emission beam indexing strips 5. Each strip corresponds in width to a colour triplet, the strips being disposed over alternate triplets. During scanning a signal is obtained from the screen at half triplet frequency, the signal being modulated however, firstly in amplitude in accordance with the intensity modulation of the tube beam by the E E 1/y signal, and secondly in phase in accordance with the scanning modulation due to the colour information. The signal is applied to frequency doubler stage 43, which incorporates a limiter to remove the amplitude modulation, and is then passed through a bucking circuit 44 where the colour modulation sidebands are cancelled by a signal from mixer 33 which is applied in appropriate phase via delay device 44a. The output passes through a further limiter 45 and comprises an oscillation at triplet frequency bearing phase modulation due solely to scanning and tube screen irregularities. The oscillation is applied to a mixer 41 where it combines with a further oscillation at the frequency of the auxiliary carrier and results in a signal at the difference frequency which comprises the third input for mixer 32. The oscillation at the frequency of the auxiliary carrier is produced by an oscillator 40 which is synchronized by "bursts" of oscillation included in the transmitted signal. The "bursts" appear at the output of mixer 32 across a tuned circuit incorporating inductor 35a and are separated by means of a gate stage 37 and applied to control oscillator 40 by way of a phase detector 38 and reactance valve 39. Modification of the receiver to employ the second arrangement is illustrated in Fig. 6. The signal from mixer 33 is applied through a limiter 70 before application to circuit 34 so that the scanning velocity is modulated only by the phase or hue component of the signal. An amplitude detector 72 derives a signal representative of saturation and this is applied to control the focus of the beam in tube 26. The control is effected by means of a comparison circuit 73 in which the signal from detector 72 is compared with a signal indicative of the state of focus. This latter signal is derived from the indexing signal output of the tube in terms of the amplitude of the second harmonic which is selected by filter 74 and measured by detector 76. To produce white the beam is focused to cover a complete triplet and thus produces a sine wave indexing output, whereas to produce a saturated colour the beam is focused progressively down to the width of one phosphor strip and produces a rectangular pulse output of increasing second harmonic content. The amplitude modulation of the indexing signal by the brightness signal is removed by an amplitude demodulator 75 receiving a second input E E 1/y. According to modifications the normal line scan is dispensed with and circuit 34 is made a staircase waveform generator the phase of the steps in which are modulated in accordance with hue information. Reference is also made to the use of sinewave instead of sawtooth deflection. To reduce colour contamination due to the finite time occupied by return strokes of the deflection the intensity control signal may be gated appropriately or modulated by the signal from mixer 32. Specifications 863,604 and 864,452 are referred to.