Patent GB-1128151-A (GB)
Title: Colour television transmission system
IPC: H04N11/14
Filed: 1965-09-28
Abstract: 1,128,151. Colour television synchronizing arrangements. HAZELTINE-AGA LABORATORIUM G.m.b.H. 28 Sept., 1965 [3 Oct., 1964], No. 41138/65. Heading H4F. In a colour television system in which the signal to be transmitted includes a chrominance signal formed by amplitude modulating different phases of a carrier signal with two or three colour signals which are blanked for the period of the line sync. pulse and the front and back porches, at least all but one of the colour signals has the direct voltage component suppressed and the chrominance signal is blanked for a part of the blanking period of the colour signals and is arranged to have a component signal of the frequency of the carrier signal. The system provides a signal from which synchronizing information to control synchronous detection at a receiver may be obtained continuously (except for the period of chrominance blanking) by utilizing the component at carrier frequency. In known systems a carrier frequency component is produced by the direct voltage components of the colour signals. In the invention, by suppressing the direct voltage component in at least all but one of the colour signals, a carrier frequency component may be produced with a fixed phase suitable for synchronization purposes, or may be caused not to originate from the colour signals whereby a wholly independent component of fixed phase may be introduced. In one way of carrying out the invention, Fig. 1 (not shown), red, green and blue signals are supplied to a three-phase modulator through capacitor coupling which eliminate the direct voltage components. Bias voltages are also applied to the modulator stages and two of these bias voltages are adjusted so as to cause the production of a carrier frequency component of a desired phase and amplitude. The colour signals are blanked throughout the line sync. pulse and front and back porches, whilst the modulator output is blanked for the period of the line sync. pulse, the front porch and, if desired, part of the back porch. A luminance signal is formed by combining the colour signals complete with their direct voltage components. In another way of carrying out the invention (not illustrated in detail) the chrominance signal is formed from I and Q signals formulated in accordance with N.T.S.C. standards. The I signal complete with the direct voltage component is applied to one modulator, whilst the Q signal with the direct voltage component eliminated is applied to the other. In this case the carrier frequency component is due to the I signal direct voltage component and has the I signal phase which is stated to be the best phase for compatability. The carrier frequency component should have an amplitude which is less than 10% of the maximum amplitude range of the luminance signal. Reference is made (without giving details) to an arrangement in which the carrier frequency component is made proportional to the average amplitude of the chrominance signal and has a restricted rate of change. A receiver is described, Fig. 3 (not shown), which utilizes demodulation at the X and Z axes and a matrix circuit formed of three triode valves for producing colour-difference signals. Direct voltage restoration is effected by pulses applied to the three cathodes during the last part of the back porch. Colour synchronization is effected by applying the output of the first chrominance amplifier stage to a selective amplifier which has a limited bandwidth centred on the chrominance carrier frequency and which directly controls a quartz crystal oscillator. The oscillator supplies signals at appropriate phases to the synchronous demodulators. The selective amplifier output is also rectified and applied to control the second chrominance amplifier stage for the purpose of "colour killing". Reference is made to arrangements at a receiver for eliminating the colour synchronizing component from the chrominance signal by cancellation with an anti-phase component, particularly where the signal is applied directly to a cathoderay tube. Reference is also made to an arrangement for restoring the direct voltage component at carrier frequency by gating the chrominance signal during the blanking interval, preferably at the end of the back porch, and producing thereby a voltage of fixed phase and amplitude which is supplied to the chrominance signal.