Patent GB-1428415-A (GB)
Title: Television apparatus
IPC: H04N9/64
Filed: 1973-04-09
Abstract: 1428415 Colour television MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 9 April 1973 [7 April 1972 10 July 1972 (2) 19 Oct 1972 (2)] 17009/73 Heading H4F The amplitude of the demodulated red colourdifference signal in a colour television receiver is reduced if it is such as would produce distortion in the reproduced image. The amplitude reduction may take place if the amplitude of the red colour-difference signal exceeds a threshold valve, or alternatively if a high red colour-difference signal amplitude coincides with a low luminance signal amplitude. In one embodiment, Fig. 10, the demodulated red colour-difference signal at input 64 is amplified by transistor 65. When the amplitude of the signal at the collector of transistor 65 exceeds a first threshold value, diode 67 is biased into conduction, thus reducing the gain of transistor 65 by the action of resistors 69 and 70 in parallel with load resistor 66. When the amplitude of the signal at the collector of transis- 65 exceeds a second threshold value, diode 72 is biased into conduction and the gain of transistor 65 is further reduced. Diode 72 and resistors 73 and 74 may be omitted (Fig. 8, not shown), the gain of transistor 65 then being one of only two values, determined by whether its collector voltage is below or above the first threshold value. In a further embodiment, Fig. 13, the red colour-difference signal is fed to input 61a of a circuit 61, to another input 61b of which the luminance signal is fed. Transistors 110 and 112 operate to produce at the collector of transistor 110 a (negative) signal proportional to the difference between the applied red colourdifference and luminance signals. When the amplitude of this difference signal exceeds a threshold value determined by resistors 101, 102, transistor 100 is turned on, its collector voltage rises, and transistor 104 is forwardly biased to reduce the gain of colour-difference signal amplifying transistor 82 by bringing resistor 103 into parallel with load resistor 83. In a further embodiment (Fig. 12, not shown) which is a modification of the embodiment of Fig. 13, the signal fed to the base of transistor 100 is a (negative) signal dependent on the ratio between the applied red colour-difference and luminance signals. The applied colourdifference and luminance signals are applied to the bases of respective first transistors (85, 95) the collector of each first transistor being connected to the base of a respective one of a pair of second transistors (90, 91) forming a differential amplifier, the input to transistor 100 being derived from the collector of one of these second transistor (90). The collector of each first transistor (85, 95) is also connected to the cathode of a respective diode (86, 96), a voltage being developed across each diode which is proportional to the logarithm of the input signal to the respective first transistor, the signal fed to the base of transistor 100 thus being proportional to the logarithm of the ratio between the applied colour-difference and luminance signals. In a further embodiment, Fig. 14, a low amplitude luminance signal received at 61b, coupled with a high amplitude (negative) red colour-difference signal at the collector of transistor 120 causes diode 132 to be forwardly biased, thus causing a reduction in the gain of transistor 120 as resistor 130 is effectively in parallel with load resistance 121. A high amplitude luminance signal will cause diode 132 to be reverse-biased, as will low amplitude colourdifference and luminance signals. Capacitor 123 and diode 127 form a D.C. restorer for the luminance signal.