GB1068871A - Improvements relating to demodulating circuits for colour television receivers - Google Patents

Abstract

1,068,871. Colour television receivers; transistor delay circuits; delay networks. TELEFUNKEN PATENTVERWERTUNGS G.m.b.H. May 4, 1964 [May 2, 1963; Oct. 24, 1963], No. 18363/64. Headings H3T, H3U and H4F. In a circuit for demodulating a chrominance signal formed by the modulation of a carrier beam by colour dependent signals in such a way that the chrominance signal has alternately one or the other of two different compositions in alternate scanning line periods, the circuit including means to provide simultaneous delayed and undelayed chrominance signal of said different composition, said means includes a supersonic delay line having a delay of substantially one half of a scanning line period and the delay produced by the line is doubled by passing the respective signal through the line twice, namely in opposite directions. Reference is made to the application of the invention to both SECAM (Registered Trade Mark) and NTSC/PAL receivers, but details are given only for the application to NTSC/PAL delay line demodulators, such as those described in Specification 1,066,798. In Fig. 1 the received colour subcarrier signal at terminal 1 is applied to delay line 8 via cathode-follower 2 and a hybrid circuit comprising centre-tapped transformer 6 and network 9 which matches the input impedance of the line. The line has a delay of approximately the duration of one half of a scanning line period and the far end 18 is not terminated so that reflection takes place. The reflected signal only, corresponding to a delay of one complete scanning line period appears in the secondary of transformer 10 from which it is applied to twoeresistor networks 11. The original signal is applied to the other ends of the networks and combinations of the signals corresponding to the separated Q and I subcarrier components appear at terminals 14 and 15. An adjustable delay line 12 may be included to ensure a precise phase relationship between the original and delayed signals; it may be dispensed with if line 8 is ground accurately to give exactly a delay of one half a scanning line period. The line is formed of glass or ceramic and is driven by a quartz or barium titanate transducer 16 reacting against a piece of lead. A small departure from a total delay of exactly one scanning line period is necessary where colour carrier offset is employed, e.g. where the colour carrier frequency is an odd multiple of one quarter line frequency. In a second embodiment, Fig. 3 (not shown), a second transducer is provided at the other end of the delay line and is connected to a circuit for changing the frequency of the signal and returning it to the line. The signal arriving at the input of the line is then separated from the original signal by a suitable filter. According to an alternative arrangement (not described in detail) the signal returned along the line may be converted to another type of modulation, e.g. EM to AM, which can easily be separated one from the other. The same carrier frequency can then be used. In a third embodiment, Fig. 6, the original signal F and the reflected signal aF which appears at the input of the delay line are applied through emitter follower stage 41 to one end of a potentiometer 44 and the original signal in antiphase -F is applied to the other end. The tapping 46 is adjusted until the original signal components cancel and only the delayed component remains. The delayed component is then combined with the original signal by means of centre-tapped transformer 48 and resistor networks 40 to obtain sum and difference signals corresponding to the separate I and Q subcarrier components at terminals 14 and 15. Adjustable delay line 12 may be placed in the circuit after the potentiometer tapping 46 and made of twice the delay. Auto-transformer 42 matches the output and input impedances of lines 12 and 8. A fourth embodiment, Fig. 5, does not separate the reflected signal aF from the original signal F at the input of line 8 but forms the desired final signals by combining appropriate proportions of the input signal. Thus signal F + aF is applied via emitter follower 41 to network 31 where it is combined with -(1 + a)F, and is applied to network 31 where it is combined with -(1 - a)F. The desired amplitude relationships is established by potentiometers 35, 36 and the phase inversion is introduced by stage 37. The desired sum and difference signals (with factors o.5a which may be compensated for) appear at terminals 14 and 15.