Apparatus for descrambling transmitted scrambled television signals

Claim 1

Apparatus for descrambling transmitted scrambled television signals which include a scrambled video signal having selected fields inverted in polarity about a predetermined turn-up level in accordance with a first scrambling pattern, a scrambled screen control signal transmitted in lieu of said video signal for a predetermined period comprised of a plurality of fields during which adjacent portions of successive fields are alternately inverted about said predetermined turn-up level in accordance with a second scrambling pattern, and descrambling signals including key code signals respectively signifying said first and second scrambling patterns and an identification signal having a level proportionate to said predetermined turn-up level, the apparatus comprising: monitor means; means for detecting said key code signals signifying said first and second scrambling patterns and providing corresponding first and second descrambling pattern signals; means for detecting the level of said identification signal as an indication of said turn-up level of said scrambled video signal and scrambled screen control signal, as received; means receiving the detected identification signal level for generating therefrom a controlled turn-up level and including control means actuable for adjusting said controlled turn-up level; and descrambling means including inverting means receiving said controlled turn-up level and being operative in response to said second descrambling pattern signal to selectively reinvert about said controlled turn-up level the inverted portions of the scrambled screen control signal for descrambling the latter and displaying the descrambled screen control signal on the monitor means while said control means is actuated to adjust said controlled turn-up level in the sense for eliminating any flicker from the display, whereupon, said inverting means is made operative in response to said first descrambling pattern signal to selectively reinvert said inverted fields of said scrambled video signal with reference to the adjusted controlled turn-up level for accurate display on said monitor means of the resulting descrambled video signal.