Patent GB-1241128-A (GB)
Title: Television receiver
IPC: H04N5/46
Filed: 1969-03-25
Abstract: 1,241,128. Television. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 25 March, 1969 [26 March, 1968], No. 15556/69. Heading H4F. In a linear signal handling stage for a television receiver, the ratio of the output to the input signal amplitudes is controlled by a beam current limiting voltage applied to the stage. As shown, Fig. 1, in a S.E.C.A.M. colour receiver the luminance signal Y is applied to a variable gain amplifier 13 comprising a current generator 41, 43, the output of which is shared between the transistors 37, 39 of an emitter coupled differential amplifier in dependence on the relative levels of a reference voltage 49 and a control voltage supplied by distributer 19. The resistors 47 and 49 are proportioned so that a picture is displayed even when transistor 37 is cut off. The output is A.C. coupled to the display device 31, the reference level being established in known manner. The arrangement may be modified so that the D.C. component of the output signal at the collector of transistor 37 is independent of the beam current and contrast adjustments so that the output may be D.C. coupled to the display device. This may be achieved by arranging a second emitter coupled differential amplifier in parallel with the first differential amplifier, the emitters being connected to current source which supplies a current corresponding to a reference D.C. component in the luminance signal and controlled in the reverse sense by the beam current limiting signal. The distributer 19 is fed with a beam current limiting voltage derived from a high voltage generator coupled to the horizontal scan oscillator both of which are included in unit 25. A contrast and saturation adjusting device 20 and is also connected to distributer 19. The chrominance signal Chr is applied to a sequential-to-simultaneous converter 57 to produce sub-carrier signals modulated with red and blue colour difference signals which are applied to limiters 69 and 71 which are also controlled by distributer 19. The signals are applied to demodulators 81, 83, the outputs of which are applied to a matrix 89 to derive the green colour difference signal. In a second embodiment, Fig. 2 (not shown), for a PAL or NTSC colour receiver, the luminance signal is applied to a variable gain amplifier (13) similar to that of Fig. 1 but modified by providing the emitter-coupled differential amplifier transistors (37, 39) with separate collector resistors (97, 99) and cross-coupling the collectors by a potential divider (101, 103) to prevent complete picture extinction, the tapping point of the divider being selected so that the voltage is independent of the control signal. The sync. signals for the scan generators are derived from this tapping point. The chrominance signal is applied to an amplifier (104) and then to a variable gain amplifier (107) arranged in the same way as that shown in Fig. 1. The output is applied to a demodulator (131) and the resulting red and blue colour difference signals are applied to a matrix (89) to derive the green colour difference signal. The colour sync. signal at the collector of the controlled transistor (115) of the emitter coupled differential amplifier is independent of the control signal level and may be used for AGC of the chrominance amplifier (104). The variable gain amplifier controlled by the beam current limiting voltage may also be utilized in black-and-white receivers.