A video signal is modified so that a television receiver will still provide a normal color picture from the modified video signal while a videotape recording of the modified video signal produces generally unacceptable pictures. This invention relies on the fact that typical videocassette recorder automatic gain control systems cannot distinguish between the normal sync pulses (including equalizing or broad pulses) of a conventional video signal and added pseudo-sync pulses. Pseudo-sync pulses are defined here as any other pulses which extend down to a normal sync tip level and which have a duration of at least 0.5 microseconds. A plurality of such pseudo-sync pulses is added to the conventional video signal during the vertical blanking interval, and each of such pseudo-sync pulses is followed by a positive pulse of suitable amplitude and duration. As a result, the automatic gain control system in a videotape recorder will make a false measurement of video level which causes an...
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1. A method for processing a video signal so as to inhibit the making of acceptable videotape recordings therefrom while producing a normal color picture on a television receiver, wherein the video signal has a blanking interval which includes a sync pulse, said method comprising the step of adding a plurality of ordered pseudo-sync pulse and positive pulse pairs to said video signal following said sync pulse and within said blanking interval, whereby said added pulse pairs cause an automatic gain control system in a videotape recorder to sense an erroneous indication of video signal level and produce a gain correction that results in an unacceptable videotape recording.
2. A method in accordance with claim 1 wherein the amplitude of said pseudo-sync pulses is equivalent to that of said sync pulse of said video signal, and said pseudo-sync pulses each have a duration of at least 0.5 microseconds.
3. A method as in claim 1 wherein said positive pulses are added to said video signal at times within said blanking interval not used by said receiver for black-level signal restoration, and thus do not cause black level depression in such a receiver receiving the processed video signal.
4. A method in accordance with claim 1 wherein said blanking interval is the vertical blanking interval of said video signal.
5. A method in accordance with claim 4 wherein:
- said pseudo-sync pulses each have a duration of at least one microsecond and are at least 25 in number for each of said blanking intervals, and adjacent pseudo-sync pulses have an interval therebetween of at least 6 microseconds; and
- said positive pulses each have a duration of at least 3 microseconds and an amplitude at least equal that of the peak white level of said video signal.
6. A method in accordance with claim 5 wherein said amplitude of said positive pulses is raised to the inherent clipping level of the transmission medium for the processed video signal.
7. A method in accordance with claim 4 wherein said pulse pairs are contiguous within said vertical blanking interval.
8. A method in accordance with claim 4 wherein said pulse pairs are randomly scattered throughout said vertical blanking interval.
9. A method in accordance with claim 4 wherein said pulse pairs are scattered throughout said vertical blanking interval in a non-contiguous non-random pattern.
10. A method in accordance with claim 1 wherein said blanking interval is an inserted interval of at least 63.5 microseconds to thereby prevent display by said receiver of said positive pulses due to inadequate retrace blanking circuitry thereof.
11. A method in accordance with claim 1, further comprising the step of adding a non-paired positive-going pulse immediately after a selectable percentage of said sync pulses, selection of said percentage being one of either random, pseudo-random or predetermined.
12. A method in accordance with claim 11, further comprising the step of raising the amplitude of the black-level reference signal of said video signal to thereby compensate for black level depression effects on a receiver caused by the processed video signal.
13. A method in accordance with claim 1, further comprising the step of reducing the amplitude of said sync pulses of said video signal.
14. An apparatus for processing a conventional video signal, having a blanking interval which includes a sync pulse, so that it produces a normal color picture on a television receiver but inhibits the making of acceptable videotape recordings thereof, comprising:
- input means for receiving said conventional video signal;
- video signal modifying means, responsive to said input means, for adding a plurality of ordered pseudo-sync pulse and positive pulse pairs to said conventional video signal following said sync pulse and within said blanking interval, and for outputting the same as a modified video signal; and
- output means, responsive to said modifying means, for outputting to said television receiver and to a videotape recorder said modified video signal, wherein said television receiver produces acceptable quality pictures from said modified video signal, but an automatic gain control system of said videotape recorder senses an erroneous video signal level due to said modified video signal and produces a gain correction that results in an unacceptable videotape recording of said modified video signal.
15. An apparatus as in claim 14, wherein said modifying means includes:
- oscillator means, responsive to said conventional video signal, for outputting pulses based on said sync pulses;
- pseudo-sync pulse means, responsive to said oscillator means, for adding said pseudo-sync pulses to said conventional video signal; and
- positive pulse means, responsive to said pseudo-sync pulse means, for adding said positive pulses to said conventional video signal.
16. An apparatus as in claim 15, wherein said oscillator means includes:
- sync separator means for outputting only said sync pulses of said conventional video signal;
- first monostable means, responsive to said sync separator means, for outputting a pulse stream corresponding to said sync signals;
- counter means for counting said pulse stream and outputting signals indicative thereof; and
- pulse generator means, responsive to said counter means output, for generating said oscillator means output pulses.
17. An apparatus as in claim 16, wherein said pulse generator means includes a programmable ROM for causing said response to said counter means output to be in one of a random, pseudo-random and predetermined pattern, depending on the programming of said ROM.
18. An apparatus as in claim 15, wherein:
- said pseudo-sync pulse means includes a monostable multivibrator and transistor responsive thereto for producing and adding said pseudo-sync pulses to said conventional video signal, and means for varying duration of said pseudo-sync pulses; and
- said positive pulse means includes a monostable multivibrator for producing and adding said positive pulses to said conventional video signal, and separate means for varying duration and amplitude, respectively, of said positive pulses.
19. An apparatus as in claim 14, further comprising vertical blanking interval extension means for extending said blanking interval in time and for limiting said adding of said ordered pulse pairs to the extended portion of said vertical blanking interval.
20. An apparatus as in claim 14, wherein said video signal modifying means adds said pulse pairs only at times within said blanking interval which are not used by said television receiver for black-level signal restoration, thereby preventing said modified video signal from causing black level depression in pictures produced by said television receiver.