GB1066798A - Improvements relating to demodulating circuits for colour television receivers - Google Patents

Abstract

1,066,798. Colour television; phase-changing networks. TELEFUNKEN PATENTVERWERTUNGS G.m.b.H. Oct. 29, 1963 [Dec. 31, 1962; March 20, 1963; Aug. 22, 1963], No. 42549/63. Headings H3U and H4F. Colour television.-The invention relates to a colour television receiver demodulating circuit for use with signals having a sub-carrier formed by the combination of two quadrature components amplitude modulated by two colour dependent signals, the phase of the quadrature component for one colour signal being changed through 180 degrees for alternate lines with the object of reducing a shift in a reproduced hue consequent upon phase errors imparted to the sub-carrier during transmission. In accordance with the invention the circuit is arranged to combine signals due to two consecutive lines so that the shifts in hue, which are in opposite directions due to the 180 degrees phase shifts, cancel in the combined signal. In a first embodiment, Fig. 2, the received sub-carrier, which appears at terminal 25 is applied to a first pair of quadrature synchronous demodulators 26 directly and to a second pair 27 via a one-lineduration delay line 5. The demodulators are associated with local demodulating signal sources 28, 29 which include means for periodically reversing the phase of one of the quadrature demodulating signals in step with the reversal in the transmitted signal. The simultaneously appearing outputs I 1 , I 2 due to one quadrature component as applied to adder 34 which derives the mean value (i.e. the adder output is a signal the elements of which represent the average of corresponding elements of the two consecutive line periods) and likewise the outputs Q 1 , Q 2 due to the other quadrature components are applied to adder 35 which develops the mean value. A conventional matrix 36 develops colour difference signals B-Y, R-Y and G-Y. In a second embodiment, Fig. 3, the received sub-carrier, which appears at terminal 4, is applied by a first path 9 directly to an adder 8, by a second path which includes a one-lineduration delay line 5 to both adder 8 and a further adder 13 and by a third path 14 which introduces a 180-degree phase reversal to adder 13. The method of operation is illustrated in Figs. 4-8 (not shown), where Figs. 4 and 5 show the signals on alternate lines formed by reversing the I component, Fig. 6 shows the signals applied to adder 8, and Figs. 7 and 8 show the signals applied to adder 13 on alternate lines. In adder 8, the I components cancel to leave only the Q components, whilst in adder 13 the Q components cancel to leave the I components which however reverse in phase in alternate lines. The resulting signals are of suppressed carrier form, and the carrier is restored by a local source 15 which is applied directly to adder 8 and to adder 13 via a phase shifter 16 which is switched to Π90 degrees on alternate lines under the control of a pulse generator 17. The final signals are demodulated by conventional amplitude demodulators 11 and 20. Suitable resistive networks for adders 8 and 13 and the method of connecting to delay line 5 are described with reference to Figs. 10 and 12 (not shown). Impedance networks.-In Fig. 3, 7 is a phase shifting circuit for providing fine adjustment of the phase of the sub-carrier passed to adders 8 and 13. Fig. 13 illustrates one suitable arrangement in which a bandpass network 207 provides a fixed phase shift of 90 degrees but is shunted by a potentiometer 210 so that a variable phase shift between 0 and 90 degrees may be obtained by adjustment of tapping point 211 as illustrated at (F1-F7) in Fig. 14 (not shown). Fig. 15 (not shown) illustrates the use of two such networks in tandem to obtain an adjustable phase shift over a range of 180 degrees, Fig. 17 (not shown), shows a 90-degree phase shift network with separate input and output potentiometers (216), (217) coupled to a common output (215) via transistor isolating stages and Fig. 18 (not shown) shows the use of two 90-degree networks in tandem with a transformer (221) for connection between potentiometer (210) and the output (215).