GB1117325A - Improvements in or relating to colour television receivers - Google Patents

Abstract

1,117,325. Colour television receiver. TELEFUNKEN PATENTVERWERTUNGSGESELLSCHAFT. 5 July, 1965 [7 Aug., 1964], No. 28420/65. Heading H4F. The invention relates to a receiver arranged for the reception of a transmission which includes a chrominance signal, Fig. 1 (not shown), produced by quadrature amplitude modulation of a carrier wave by two colour dependent signals with one of the modulation axes changed by 180 degrees for alternate lines, and which also includes a reference signal in the form of a pulse of oscillation at the frequency of the carrier wave transmitted in intervals between lines and being alternately phase advanced and retarded symmetrically in relation to the other modulation axis. The receiver, Fig. 4, includes a demodulator circuit 13, 14 to which the transmission F is applied and which produces in channels 32, 33 colour dependent signals (R-Y, B-Y) in response to the chrominance signal and pulses in response to the reference signal. The receiver includes a switch 21 which changes over in alternate lines in response to signal 25 and thereby changes the phase of carrier frequency reference oscillations applied from a source 20 to the demodulator circuit 13, 14 to allow for the 180 degrees change of the chrominance signal modulation axis. Source 20 is synchronized by colour synchronizing signals which are separated by gate 18 controlled by line frequency pulses 17. In accordance with the invention the correct phase of operation of switch 21 is determined by means of the reference signal. To this end the pulses in channels 32, 33 due to the reference signal are applied to a combining circuit 26 which is arranged to produce an output of a first amplitude when the switch 21 has the correct phase and an output of a second amplitude when the phase is incorrect. A connection extends from circuit 26 to the switch, an output at the second amplitude being effective to change the switch phase. The connection includes a gate 28 which is rendered transmissive during the period of the reference signal by a pulse 27. In the example disclosed the reference signal occurs in the intervals between lines during the field fly-back interval only. The function of circuit 26 (a resistor matrix) is described more fully with reference to Figs. 2-4 (not shown). The first amplitude is preferably arranged to be zero. A residual output is produced, however, when a phase error occurs in the signal from source 20. The reason why this does not cause false operation of switch 21 is discussed with reference to Fig. 5 (not shown). According to a modification, the reference signal is amplitude modulated by a frequency, e.g. 350 kc./s., which is low compared with the carrier frequency. The receiver combining circuit, Fig. 8 (not shown), includes a resonant circuit tuned to the modulation frequency. Reference is made to a receiver which operates with a delay line demodulator to separate the chrominance signal into carrier signals bearing I and Q modulations. Reference carrier signals are subsequently added to permit demodulation. In this ease the combining circuit operates with signals in the I and Q channels to obtain an output to determine the phase of the line change-over switch.