GB1148448A - Demodulator for colour television signals - Google Patents

Abstract

1,148,448. Colour television. FERNSEH G.m.b.H. 18 July, 1966 [17 July, 1965], No. 32093/66. Heading H4F. The invention relates to a P.A.L. transit time demodulator in which the quadrature phase components a and b of the colour carrier F are separated by a circuit arrangement comprising a one-line-duration delay line 10 together with subtracting and adding circuits 11 and 21. If the components a and b (which are suppressed carrier signals) are detected in the customary manner (i.e. by carrier reinsertion and rectification or synchronous detection), in the presence of incoming carrier phase errors, the detection components vary in amplitude with the phase error but remain in the correct ratio, thus preserving the correct pictum live but causing a saturation error. In accordance with the invention, the saturation error is prevented by the use for detection of oppositely poled peak rectifiers 13, 14 and 23, 24 whose outputs are commoned in adding stages 15 and 25, the carrier components a and b being applied to the rectifiers during only a portion of each carrier cycle by operation of switches 12 and 22. The selected portion lies within the range Π20 degrees to Π70 degrees, and is preferably Π45 degrees, referred to the maximum value of the colour carrier reference signal. Variation of the phase of the incoming carrier within this range then produces no saturation error. The switches are controlled by the carrier reference signal generator 1, one output from which is established in quadrature phase by circuit 3 and another output from which is subjected to periodic phase reversals at line frequency (in step with the corresponding phase reversal of the I modulation axis used in the P.A.L. method of operation) by means of circuit 2 and switch 4. The carrier signals are converted into signals of appropriate duration to operate switches 12 and 22 by shaping circuits 5 and 6. In a first detailed circuit, Fig. 3 (not shown), each switch comprises a transistor which functions to short-circuit the signal channel except during the selected portion of the carrier cycle. In a second detailed circuit, Fig. 4 (not shown), each switch comprises a transistor whose emitter-collector circuit is connected in the signal channel, the transistor being rendered conducting for the duration of the selected portion of the carrier cycle. In a third detailed circuit, Fig. 5 (not shown), each switch comprises a rectifier bridge arrangement. Reference is made (without giving details) to the use with the invention of a system in which the phase of the signal from generator 1 is caused to follow the phase errors of the incoming carrier signal. With the use of such a system greater phase errors may be tolerated.