Patent GB-1138828-A (GB)
Title: Colour television receiver
IPC: H04N9/66
Filed: 1967-01-04
Abstract: 1,138,828. Colour television receivers. MOTOROLA Inc. 4 Jan., 1967 [9 Feb., 1966], No. 475/67. Heading H4F. In a colour television receiver a first colour representative signal (the red colour difference signal) is applied to the image reproducer in a substantially linear manner, whilst a second colour representative signal (the blue colour difference signal) is applied in a non-linear manner such that variations of the signal from a reference level (the zero amplitude level) which represent a decrease in reproduced colours give rise to a greater amplitude of change than corresponding variations in the opposite direction which represent an increase in reproduced colour. The receiver is also arranged to translate a third signal (the green colour difference signal) in a non-linear manner whereby it produces unequal amplitude changes in the same way as the second signal. The invention is said to result in a more pleasing reproduction of red, yellow, green and flesh tones by reducing the amount of blue provided for these tones, and also avoids any appearance of green in red, blue and magenta by reducing the green provided for these tones. The invention is described as applied to a receiver for an N.T.S.C. type of signal in which the chrominance subcarrier is demodulated with the aid of a single valve 54 which also functions as the subcarrier reference oscillator. The oscillator section is formed between the cathode 55, control grid 57 and screen grid 58, whilst the subcarrier to be demodulated is applied from transformers 42 to suppressor grids 61, 63. The connection to grid 61 includes a 43.5‹ phase advance circuit 84, 86, 88 and the connection to grid 63 includes a 44‹ phase retard circuit 80, 82 so as to establish the correct phase relationship to cause demodulation of B-Y and R-Y signals. The oscillator section is locked to the subcarrier synchronizing "bursts" which are applied from gate 44 via crystal filter 56. Cathode circuit 68, 70, 69 serves to establish bias voltages for the suppressor guide. In accordance with the invention the bias voltage for suppression grid 63, which is taken via resistor 78, establishes the operation on a wholly linear portion of the suppressor voltage, anode current characteristic, whilst the bias voltage for suppressor grid 61, which is taken via resistor 71, establishes the operation on a non-linear portion of the characteristic such that the anode output signal for B-Y exhibits the desired property described above. The current flowing in the screen 58 is related inversely to the currents flowing in the anodes 65 and 67. Thus a signal representing G-Y may be developed in the screen circuit. Since the current in anode 67 varies non-linearly, the G-Y signal exhibits a non-linear variation similar to the B-Y signal. The signals are coupled to the control grids 92, 94, 99 via CR parallel circuits 91, 93; 96, 98 and 95, 97 which provide both A.C. and D.C. coupling. Potentiometer 106 is a master brightness control. Reference is made (without giving details) to an arrangement in which separate amplifiers follow demodulators for the colour signal, the invention being applied by biasing the amplifier for the B-Y signal so that it operates non-linearly.