GB1459855A - Video transmitter stabilization system - Google Patents

Abstract

1459855 Amplitude modulation TEKTRONIX Inc 20 Feb 1974 [22 March 1973] 7661/74 Heading H3R In a television transmitter a reference signal is added to the video signal and components of this and/or the video signal are compared with the signal after detection to provide signals for pre-correcting the video signal transmitted. An adding circuit 7, Fig. 3, adds to the composite video signal a vertical interval reference signal (VIRS) representing the levels of various components of the composite signal as shown in Fig. 2, to a particular line during the vertical interval. The modified signal is applied via a processing amplifier to modulate a transmitter 2, the output from which is applied over directional coupler 3 to aerial 4. A part of the output is applied to demodulator 5 which feeds indicator, e.g. an oscilloscope and also automatic gain control circuit 8, which supplies control voltages a-f to processing amplifier 1, and also a reference pulse via a line to demodulator 5. In the automatic gain control circuit, the demodulated video with superimposed zero carrier reference signal applied to terminal 9, Fig. 4 is passed over capacitor 10 and the D.C. level restored by back porch clamp 12, operated by sync. separator 11. The luminance component of VIRS is then fed directly to a number of gates 18, while the chrominance components are separated out by means of band pass filter 15 and applied to phase and amplitude decoders 17, 16 respectively, the outputs from which are applied to a burst phase gate and a chroma gate respectively. The output from sync. separator 11 is used to synchronise a first timing circuit 14<SP>1</SP> which operates a zero carrier gate and also supplies a zero carrier reference drive pulse via terminal 13 to the demodulator 5, Fig. 3. The output from sync. separator 11 also synchronises a second timing circuit 14 which provides gating pulses for the remaining gates. The output from each gate is stored on a corresponding memory capacitor and applied, except in the case of the zero carrier gate to one input of a corresponding comparator. The output from the zero carrier gate forms the input to a potentiometer g-j the tapping points of which provide voltages corresponding to the various correct signal levels : the voltages from the second inputs to the corresponding comparators, excepting the phase burst comparator, the second input to which comprises a fixed D.C. voltage. The outputs from the comparators comprise the gain control voltages a-f applied to the automatic gain control circuit 8, Fig. 3. The specification also describes an alternative comparator arrangement (Fig. 5, not shown).