GB1173518A - Generating a Signal having a Phase Determined by a Burst of The Same Frequency. - Google Patents

Abstract

1,173,518. Colour television. BRITISH BROADCASTING CORP. 16 July, 1968 [4 July, 1967], No. 30804/67. Heading H4F. A method of producing a continuous signal at a given frequency which has an instantaneous phase equal to that of the last received of a series of bursts of said frequency, may be applied to the processing of N.T.S.C. signals produced at the output of a standards converter as described in Specification 1,052,438. In such signals the subcarrier bursts are not necessarily spaced from each other by equal line intervals and so their phases can be substantially different. The use of a standard phase lock local generator for producing the colour signal demodulation subcarrier is thus difficult, and the method of the invention produces a subcarrier which follows the burst phase variations. The bursts are thus gated through 11 and applied to synchronous demodulators 12A and 12B where they are demodulated by a subcarrier frequency reference signal directly and via a 90 degree phase shifter 14. The outputs of the demodulators represent respectively the amplitudes of the components of the burst in phase with and in phase quadrature with the reference signal. Said outputs, which only occur during the bursts, are stored for the ensuring line period in respective sample and hold circuits 15A and 16B. These may each comprise a storage capacitor across the output terminals, and an electronic switch, closed during the burst interval, between the input terminal and the capacitor. The charge on each capacitor is used to amplitude modulate the direct and 90 degree phase shifted reference signal in modulators 16A and 16B respectively. The outputs of the modulators are summed at 20 to produce the desired continuous phase following subcarrier.