GB1245615A - Improvements in or relating to synchronization in colour television receivers - Google Patents

Abstract

1,245,615. Colour television. BLAUPUNKTWERKE G.m.b.H. 4 Oct., 1968 [5 Oct., 1967], No. 47171/68. Heading H4F. In a P.A.L colour television receiver, Fig. 1, the output of the local colour subcarrier oscillator 20 is fed to the colour synchronous demodulator 17 and has its phase for the demodulation of the (R-Y) signal changed over by 180 degrees on alternate lines by the output of a bi-stable oscillator 21 normally switched by the line fly pulses 57, Fig. 2. The swinging burst 30, Fig. 4a, separated out and amplified at 26, is fed to phase discriminator 25 for comparison with the output of oscillator 20 such that the discriminator produces a synchronous sawtooth 44, Fig. 4b, at half-line frequency. As usual the average value of this sawtooth controls the phase of the oscillator output. The sawtooth is also fed to a gate switch 22, via a control signal amplifiers in an inverted amplifier from 49, Fig. 4c. The output 52, Fig. 4d of the bistable oscillator 21 is also fed to the gate switch where it is added to the sawtooth at the base of transistor 50. If the phase of the change-over square wave from bi-stable oscillator 21 is correct, the highly positive half of the square wave is coincident with and overcomes the negative going trailing edge of the sawtooth 49, so that transistor 50 is not enabled. If, however, the phase is incorrect, the sawtooth trailing edge is coincident with the only slightly positive square wave half so that the trailing edge predominates and the transistor is enabled to give a pulse 64, Fig. 4e, which, applied to input 66 of the bi-stable oscillator 21, switches it to the correct phasing. Such switching is shown at 67, Fig. 4d. In the case of black and white transmission the input to the phase discriminator 25 is due to noise 87 which produces a sawtooth output 88 at the line frequency. The output of the bistable oscillator is then continually out of phase such that the trailing edges of the sawtooths 91 continually predominate to produce a continuous train of pulses 93 from transistor and the continuous switching of the bi-stable oscillator such that its output is substantially maintained at the highly positive level. The train of pulses 93 operate the colour killer circuit 23.