GB1086352A - Improved colour television systems and apparatus - Google Patents

Abstract

1,086,352. Colour television. COMMUNICATIONS PATENTS Ltd. Sept. 16, 1964 [April 27, 1964], No. 17346/64. Heading H4F. In a colour television system in which transmission of luminance information takes place as an amplitude modulation of a carrier wave and the transmission of colour information takes place as an amplitude modulation of a suppressed carrier wave, the carrier waves having the same frequency but a phase quadrature relationship, the carrier waves are derived from a common oscillator and the phase quadrature relationship between the carrier waves is established prior to their application to respective ones of two modulators one of which is arranged to modulate the carrier wave applied thereto in accordance with luminance information signals and the other of which is arranged to modulate the carrier wave applied thereto in accordance with colour information signals. The synchronizing signals are amplitude modulated on to the same carrier as the luminance signals and the colour signals comprise two colour-difference components transmitted linesequentially or at a faster sequential rate. The luminance signals are distorted by means of two correcting signals prior to transmission to minimize the visible effects of distortion introduced into the signals by receiving apparatus. One of the correcting signals is arranged to compensate for apparent amplitude errors in the luminance signal due to the presence of the colour signal and comprises a direct current signal representative of the amplitude of the colour signal which is passed, via a network which introduces an inverse non-linearity into the signal, to a subtractor device to modify the luminance signal. The other correcting signal is arranged to compensate for apparent amplitude errors in the luminance signal due to the asymmetrical pass-band response of receivers, and this correcting signal is produced by means of receiving apparatus having a pass-band response similar to that of a receiver and arranged to receive a part of the output signal from the transmitting apparatus and provide a demodulated video signal at its output which is arranged to modify the luminance signal in a subtractor device. Keying or identifying signals may be transmitted for operating display delay line recognition circuits in the receivers e.g. bursts of low-frequency alternating current during suppressed lines of alternate field synchronizing pulses. In a receiving apparatus, Fig. 3 (not shown), the amplified signals are passed to an envelope detector and a synchronous detector. The envelope detects both the luminance and colour signals to derive a composite signal which is utilized as the luminance signal, and the synchronous detector is arranged to detect the colour signals. The oscillator which controls the synchronous detector via a 90-degree phase shift network is controlled by bursts of carrier wave having a frequency corresponding to that of the carrier wave of the luminance signal which are passed by a gate during synchronizing pulse intervals. In a further receiving apparatus, Fig. 4 (not shown), the received picture signals are subjected to a frequency-changing process in a mixer fed by an oscillator. The frequency of this oscillator is changed if the phase of the oscillations produced by an uncontrolled reinsertion carrier oscillator device, which controls the synchronous demodulator that detects the colour signal, is not in phase quadrature with the unsuppressed vision carrier. If the phase of the uncontrolled oscillator is not correct then the detected colour signal will include residual synchronizing pulses which are associated with the luminance signal and these are gated out if present by a gate opened during the synchronizing pulse intervals and fed to an integrator and reactance modulator to vary the frequency of the oscillator which controls the mixer. The polarity of the sequential colour signals may be altered at a sub-sequential frequency rate, e.g. half picture rate, to reduce the effect of luminance cross-talk.