Dynamic de-emphasis compensation system

In a color television receiver subject to reception of composite color television signals having periodic vertical blanking intervals interposed between successive sequences of active line intervals, and inclusive of a chrominance component encoded in SECAM format;

• said SECAM-encoded chrominance component comprising:
• (a) during image portions of alternate active line intervals, a first subcarrier having a first resting frequency and subject to frequency modulation over a given frequency deviation range in accordance with a first pre-emphasized color-difference signal;
• and (b) during image portions of the intervening active line intervals, a second subcarrier having a second resting frequency and subject to frequency modulation over said given frequency deviation range in accordance with a second pre-emphasized color-difference signal;
• with a lead-in burse of oscillations at said first resting frequency preceding the image portion of each of said alternate active line intervals, and with a lead-in burst of oscillations at said second resting frequency preceding the image portion of each of said intervening active line intervals, apparatus comprising:
• an FM detector, responsive to the received signals, and subject to operation in either a first mode with center frequency tuning corresponding to the resting frequency of said first subcarrier, or a second mode with center frequency tuning corresponding to the resting frequency of said second subcarrier;
• a triggered flip-flop circuit for providing a half line rate wave output;
• a source of line rate pulses timed to encompass the periods of appearance of said lead-in bursts to the exclusion of the periods of appearance of said modulated subcarriers;
• means, responsive to said line rate pulses, and to said half line rate wave output, for controlling the operating mode of said FM detector such that said FM detector operates in respectively different ones of said modes during the image portions of successive active line intervals but operates in only said first mode during the periods of appearance of said lead-in bursts;
• identification means, responsive to an output of said FM detector, and responsive to said half line rate wave output, for controlling the phasing of said half line rate wave output;
• de-emphasis means, having an input terminal and an output terminal, said input terminal being coupled to receive a demodulated signal output of said FM detector, for subjecting said demodulated signal output to a frequency response characteristic exhibiting a response decline with increase in signal frequency over at least a portion of the frequency range occupied by said color-difference signals;
• means, responsive to line rate pulses from said source, and to said half line rate wave output, for delivering to said output terminal a differentiated version of recurring pulses which have leading and trailing edges timed to coincide with the changes of operating mode of said FM detector.