Patent GB-887562-A (GB)
Title: Improvements relating to phase modulating circuits
Inventor: GARGINI ERIC JOHN
IPC: H04N11/12
Filed: 1957-01-30
Abstract: 887,562. Multiplex radio signalling. ELECTRIC & MUSICAL INDUSTRIES Ltd. April 30, 1958 [Jan. 30, 1957], No. 3214/57. Class 40(5). [Also in Group XL(b)] In a colour television system of the kind described in Specification 864,452 in which hue information is transmitted by phase modulation of the main carrier (which is amplitude modulated by brightness information) and saturation information by amplitude modulation of a subsidiary carrier wave, the effect of sudden phase reversals, which may be due to sudden changes in the scene from one colour to another or may be caused by "noise" when the colour content of the scene is low and which produce the effect of "pinholing" in the reproduced picture when the main carrier wave, at any point in the system, is passed through a narrow-band tuned circuit which is unable to follow rapid phase reversals, is removed by effecting the phase modulation of the main carrier such that it represents only magnitude changes in phase and transmitting an indication of the sign of such phase changes by means of a subsidiary carrier wave. In the arrangement shown the main carrier is generated by an oscillator 2 operating at 60 mc./s. and from this oscillation a 4.5 mc./s. signal is produced by combining the former in mixer 16 with the output of a 55.5 mc./s. oscillator 17. The 4.5 mc./s. signal is then supplied via a cathode-follower 20 and frequency doubling circuit 14 to the suppressor grid of a valve 12 to the control grid of which is supplied a 9 mc/s. signal consisting of a frequency doubled 4.5 mc/s. signal which is amplitude modulated by the saturation and phase modulated by the hue of the image being televised. The valve 12 is arranged as an amplitude limiter and supplies an output signal representative only of phase differences between the two input signals. This output signal is then supplied via a cathodefollower 10, low-pass filter 9 and a second cathode-follower 8 to the cathode of a variable reactance valve 5 connected across the frequency determining circuit of the oscillator 2. In this -manner the phase of the latter oscillator is controlled by the phase of the signal supplied to the control grid of valve 12, the effect of frequency doubling of the initial phase and amplitude modulated 4.5 mc/s. signal being to convert phase changes of 90 degrees into phase changes of 180 degrees and phase changes of 180 degrees into changes of 360 degrees and that of the frequency doubling circuit 14 being to divide the phase changes by 2 so that the actual phase change produced in the 60 mc/s. signal generated by oscillator 2 is restored to its initial magnitude there being now no distinction between oscillations initially 180 degrees out of phase so that phase reversal of the 60 mc/s. signal does not occur. The output of oscillator 2 is then amplitude modulated by brightness information and also by a portion of the initial phase and amplitude modulated 4.5 mc/s. signal which thus becomes a subsidiary carrier wave. At a receiver the phase modulation of the main carrier is detected with reference to the phase of the subsidiary carrier wave so that phase reversals in the initial signal are restored. Specification 863,604 also is referred to.