US 4,072,983 AGranted

SECAM identification circuit

AssigneeRCA Corporation

InventorFelix Aschwanden

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Abstract

SECAM burst signals, which recur at the line scanning rate and which change on a line-to-line basis between 4.25 MHz (corresponding to a B-Y subcarrier frequency) and 4.40625 MHz (corresponding to an R-Y subcarrier frequency) are utilized to provide line-by-line color identification and color killer signals to associated demodulation apparatus. Synchronizing burst signals and color information signals are separated from the remainder of a composite video signal by means of a "cloche" filter which, during each active line scanning interval, is characterized by a resonant frequency of 4.286 MHz and a relatively low Q (wide bandwidth). During the burst portion of each line retrace interval, the response of the filter is shifted such that its resonant frequency corresponds to one of the burst frequencies (e.g., the R-Y burst frequency of 4.40625 MHz) and the Q is increased to provide a narrower bandwidth response. Consecutive R-Y and B-Y bursts applied to the filter are thereby respectively reinforced and suppressed. The higher amplitude R-Y bursts (which recur at one-half the line scanning rate) are supplied along with a SECAM switching square wave to a phase comparator. The output of the comparator is filtered to provide a color killer and phase correction signal to activate or inactivate the color circuits and to properly phase the bistable circuit (SECAM flip-flop) which generates the switching square wave.

In a system for processing color television signals of the SECAM type in which a binary circuit for producing a substantially square wave output recurring at one-half of the image line scanning rate is included for synchronizing the demodulation of at least first and second color signal carriers of different frequencies recurring alternately in a line-by-line sequence and modulated during line scanning intervals with respective color image information signals, said SECAM type signal further including first and second synchronizing burst components having frequencies corresponding, respectively, to those of said carriers and occurring during line retrace intervals adjacent in time to an associated color carrier, a color signal identification circuit comprising:

signal filtering means exhibiting a bell-shaped amplification versus frequency response during said line scanning intervals for processing said color signal carriers prior to demodulation thereof;
switching means coupled to said filtering means for modifying said bell-shaped response during said line retrace intervals to increase relatively the response at one of said burst component frequencies with respect to the response at the other of said burst component frequencies;
detection means coupled to said filtering means, responsive to burst components of increased amplitude coupled from said filtering means and relatively unresponsive to said other burst components, for producing output pulses at one-half said line scanning rate in timed relation with said increased amplitude burst components;
phase comparator means responsive to said pulses and to said square wave for providing an error correction signal representative of the time relation of said pulses and said square wave;
and
means coupled to said binary circuit and responsive to said error correction signal for synchronizing the operation of said binary circuit with respect to the time sequence of said first and second carriers so as to provide proper recovery of said color information signals.

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Abstract

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