Patent GB-1165564-A (GB)
Title: Television Circuit.
IPC: H04N11/14
Filed: 1966-10-11
Abstract: 1,165,564. Colour television. MOTOROLA Inc. 11 Oct., 1966 [24 Oct., 1965], No. 45343/66. Heading H4F. In a colour television receiver for an N.T.S.C. type of transmission the colour representative signals E R , E B  and E G  are obtained either by means of respective phase detectors balanced for detecting a given phase of the chrominance modulated sub-carrier but unbalanced for conducting luminance frequency components i.e. unbalanced-balanced demodulators, or by means of balanced phase detectors to which unbalanced luminance frequency components and balanced chrominance modulated sub-carrier signals are applied. In detail, opposite phases of the demodulated composite video signal, in which the chrominance modulated sub-carrier overlaps the frequency range of the video frequency luminance components, are applied to the positive and negative input terminals of an unbalancedbalanced demodulator, Fig. 2. This demodulator includes diodes 64 and 66 which are alternately rendered conductive by opposite phases of a reference signal from transformer 68, the reference signal comprising oscillations at a particular phase e.g. associated with the red signal in the chrominance modulation, and being derived from the received burst signals. In addition to this demodulated chrominance at a particular phase that is developed in output filter 73, a particularly selected amplitude of the luminance components will appear in the detector due to the unbalanced condition caused by resistor 75. Thus the conduction of diodes 64 and 66 are not equal and opposite for the video frequency chrominance components and a portion of the luminance signal E Y  will be conducted which combines with the demodulated chrominance E R -E Y  to develop E R  at the output of filter 73. For variations in the received signal adjustment of the unbalance is necessary to adjust the amplitude of the chrominance signal components with respect to the luminance signal components. Alternatively, adjustment of the unbalance is effected by means of variable resistor 83, Fig. 3, so that the demodulator conducts the proper amplitude of luminance components to be combined with the demodulated chrominance components for the direct production of the red representative signal. When using unbalanced-balanced demodulators a spurious brightness or luminance transient may be produced due to the unbalanced condition and modulation of the sub-carrier reference signal by the luminance. According to a first embodiment, to eliminate this spurious luminance transient signal the demodulated composite video signal is coupled through a filter (102, Fig. 4, not shown), which passes the luminance and chrominance modulation components to the unbalanced-balanced demodulators, and through a filter (110) which only passes the luminance components to balanced demodulators (114) associated with the unbalancedbalanced demodulators, each balanced demodulator being arranged to produce an output spurious signal of correct phase for cancellation of the spurious signal from the respective unbalancedbalanced demodulator. In a further embodiment, Fig. 5, the production of spurious signal components due to luminance modulation of the sub-carrier is avoided by applying the chrominance components in balanced manner to balanced demodulators 20B, 20C, 20D<SP>1</SP> and by applying the luminance components in an unbalanced manner to the balanced demodulators. The demodulated composite video signal from amplifier 17 and phase equalizer 100 is coupled via amplifier 125 and emitter follower 132 to the interconnection of resistors 134 and 135 connected between emitter followers 137 and 138. The composite signal is also coupled to a phase splitter 130 including transistor 140, the emitter electrode of which is coupled to filter 144 which passes only the luminance components, and the collector electrode of which is coupled to filter 142 which passes both the luminance and chrominance components. The outputs of filters 142 and 144 are coupled through resistors 146 and 147 to a variable resistor 48 the adjustable arm of which is coupled via a phase splitter 150 to the emitter followers 137 and 138. Accordingly opposite phases of differing portions of the composite video signal are applied to variable resistor 148, which portions include opposite phases of the luminance components which thus cancel leaving only the chrominance components the amplitude of which may be adjusted by resistor 148. The input to the balanced demodulators thus includes the same phase of the luminance components applied through resistors 134 and 135 and the opposite phase of the chrominance components. The luminance components E Y  will thus be conducted into the demodulators and combined with the demodulated chrominance components e.g. E G -E Y , so that the colour representative signal E G  is directly produced at the output. An effective chroma control comprises variable resistor 128 by means of which the amplitude of the luminace components translated through circuit 132 may be adjusted. The Specification describes an alternative circuit arrangement, Fig. 6 (not shown), which may replace the channel comprising phase splitters 130 and 150 or may be substituted for the entire circuitry between phase equalizer 100 and emitter followers 137 and 138. In a still further embodiment, Fig. 7 (not shown), a response correction circuit is included between amplifier (17) and the balanced demodulator circuits which includes a variable control for adjusting the amplitude of the chrominance modulation components. The response correction circuit includes a differential amplifier (229), (230) and the variable control comprises a potentiometer (224) connected to the differential amplifier having applied to its ends the luminance and modulated sub-carrier components and the luminance components respectively. Monochrome signal reception is possible in these circuits although it is preferably to open circuit the modulated chrominance components channels to reduce spurious noise components.