GB1155468A - Colour Television Signal Amplifier - Google Patents

Abstract

1,155,468. Colour television receiver. INTERNATIONAL STANDARD ELECTRIC CORP. 13 Sept., 1966 [15 Sept., 1965], No. 4O879/66. Heading H4F. In a colour television receiver for PAL type signals in which the colour burst alternates between 135 and 225 degrees with respect to B-Y from line to line, i.e. a " swinging burst ", the colour saturation control circuit is such that amplitude variation of the chrominance signal is rendered ineffective during the period of the burst pulses, and the colour subcarrier oscillator is synchronized by the phase-constant component of the burst signal. The colour saturation control circuit which is inserted in the chrominance signal path between the IF- demodulator and the time demodulator stage of the PAL-decoder comprises a switching diode 12, Fig. 2, earthed at its cathode and connected at its anode to a steady source of positive biasing potential U b via a resistor 14 and to a source of negative line fly-back pulses via a resistor 13. Connected to the common junction point of these elements are potentiometers 9 and 10 and resistor 15 to which is fed via capacitor 8 the chrominance signal from, for example, a chrominance signal amplifier. The output to the PAL- decoder is taken via capacitor 11. Diode 12 is normally conductive but is blocked by the negative fly-back pulses which interrupts the connection of potentiometers 9 and 10 to ground and thus renders them ineffective for amplitude control of the chrominance signals during the burst pulses. Potentiometers 9 and 10 may alternatively be bridged by means of a switch which is closed by the line fly-back pulses. In a second embodiment, Fig. 3, the chrominance signal is fed to the base of transistor 17 via capacitor 16 and an inductively coupled band-pass filter 18-24, 30 is provided in its collector circuit. The chrominance signal is removed via capacitor 31. The base of transistor 17 receives in addition to the chrominance signal a manually variable voltage U v applied via resistor 28 and coil 27, and also positive line fly-back pulses applied via a capacitor 29 and the coil 27. The resistor 28 is of a value such that the voltage U v is practically ineffective during the presence of positive line fly-back pulses and thus the amplification of the burst signal is determined by the amplitude of the pulses. In the absence of such pulses, the amplification of the chrominance signal is determined by the magnitude of the variable voltage U v .