Patent GB-1225291-A (GB)
IPC: H04N9/66
Filed: 1968-04-04
Abstract: 1,225,291. Colour television. ZENITH RADIO CORP. 4 April, 1968 [10 April, 1967], No. 16357/68. Heading H4F. Specification describes an inductorless balanced colour demodulator as used, for example, in an N.T.S.C. type receiver, Fig. 1, and comprising two colour demodulators each producing an uncorrected colour control signal and each comprising: a first and second differential amplifier (e.g. 20, 25) each including a pair of balanced transistors (e.g. 20, 20<SP>1</SP>) having input (base), output (collector) and common connected (emitter) electrodes and a load resistor (21) connecting one of the output electrodes to a bias potential (+B), there being a cross-coupling connection between each load resistor and the unloaded output electrode of the other pair of balanced transistors (e.g. between resistor 21 and collector of transistors 25<SP>1</SP>); a third pair of balanced transistors (e.g. 20<SP>11</SP>, 25<SP>11</SP>) having input (base), output (collector) and common connected (emitter) electrodes, the "output to common" electrode path forming a common high impedance load to the common connected electrodes of the respective one of said first and second differential amplifiers, and another high impedance (e.g. transistor 24<SP>11</SP>) connected to the common connected electrodes of said third transistor pair to constitute therewith a linearly operating third differential amplifier; the chrominance sub-carrier being applied in antiphase (transformer 22) to the inputs of the third pair of balanced transistors of each colour demodulator and a demodulating signal, having the same frequency as the colour burst signals being applied in antiphase (transformer 23) across the inputs of each of the first and second differential amplifiers (e.g. 20, 25) with such relative phases as to develop across the load resistors (21, 26) of a first of said demodulators, opposite polarities of a first colour control OB<SP>1</SP> signal and across the load resistors (28, 44) of a second of said demodulators, opposite polarities of a second colour control signal, OR<SP>1</SP> and a resistance matrix network (21, 26, 28, 42, 43, 44) comprising resistors included in both demodulators, to matrix said first and second colour control signal OC<SP>1</SP>. The phases of the three thus produced colour control signals OC<SP>1</SP>, OB<SP>1</SP>, OR<SP>1</SP> are shown in the subcarrier phasor diagram, Fig. 3, the phases of the demodulation signal relative to the (R-Y) axis being 14À2 degrees for the first demodulator and (84À7+14À2 degrees) for the second demodulator. The said control signals are in the nature of colour difference signals containing information relating to the Green colour control signal G 1  (phasor OG). To derive the three colour signals B 1 , R 1  and G 1  for direct application to the three grids of colour display tube 12, the three signals OC<SP>1</SP>, OB<SP>1</SP>, OR<SP>1</SP> are applied via respective transistors 80, 81, 82, to an active matrix assembly 14 comprising four common emitter connected transistors 50, 51, 52, 53, the three signals OC<SP>1</SP>, OB<SP>1</SP>, OR<SP>1</SP> being applied to bases of transistors 50, 51 and 52 and the base of transistor 53 being grounded. With this arrangement, the four output signals, OB=B 1 , OR=R 1 , OG=G 1  and OC, sum to zero, vectors OB, OR and OC being obtained due to the grounding of transistor 53, to the subtraction of vector OG from OB<SP>1</SP>, OR<SP>1</SP> and OC<SP>1</SP>, respectively. Output signal OC is not used and to convert the colour difference signals to direct colour signals, the luminance signals are applied to the base of transistor 60 forming the common emitter load of transistors 50, 51, 52, 53.