GB1509958A - Control circuit for controlling the resting frequency of a frequency-modulated colour television sub-carrier - Google Patents

Abstract

1509958 TV synchronizing systems THOMSON-CSF 28 Aug 1975 [30 Aug 1974] 35622/75 Heading H3A A control circuit suitable for a SECAM (Registered trade mark) system coder and for locking the resting frequency (zero chrominance frequency) of an FM colour TV subcarrier from a modulated oscillator 7 to a reference frequency comprises a reference oscillator 9 or 10 at the reference frequency F R or F B , a switch 5 for supplying the output of the reference oscillator 9 or 10 to the input of a frequency discriminator 11 during the scan portion of line periods and the output of oscillator 7 to the discriminator during horizontal blanking intervals preceding the scan portions, the output of the discriminator being applied via a clamping circuit 18 or 21 and a sample and store circuit 19 or 22 as a correction signal to the modulated oscillator. In a SECAM system involving alternate transmission of the blue and red chrominance components, the switch 5 passes alternately frequencies f B and f R from the oscillator 7 during the blanking intervals and frequencies F R , F B from oscillators 9, 10 during the line periods. The correction signal derived via elements 18-20, 40 during one line period is applied via an adder 2 to control the oscillator 7 to frequency F B . During next line period, the oscillator is shifted towards frequency F R by a square wave S and the error signal derived via elements 21-23, 41. A generator 28 applies blocking signals for initial phase adjustment of the oscillator 7. In the modification of Fig. 2 (not shown), the error signal from the second loop is applied to a multiplicative device between the adder 2 and a clipper 3. The invention is also applicable to an oscillator 7 with a single resting frequency or to two separate oscillators for the two resting frequencies.