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SYNC SUPPRESSION SCRAMBLING OF .. TELEVISION SIGNALS FOR SUBSCRIPTION TV
The present invention relates to subscription TV in which TV signals are scrambled by suppressing the sync intervals thereof so that the TV signals are distorted when displayed except at authorized receiving stations which are equipped to descramble the TV signals.
The invention is especially suitable for use in the transmission of premium, subscription TV programs by cable systems. Features of the invention may also be applied to the transmission and reception of premium TV by broadcast or in other networks such as in hotels, dormitories and other places of public accommodation.
Sync suppression scrambling and descrambling systems of various types have been suggested, and some systems are presently in use on those channels of cable TV ‘systems which offer premium subscription programming. These systems generally operate by the transmission of the TV signals with the horizontal sync intervals thereof suppressed. At the authorized receiving stations, the sync signals are restored by the use of a restoring signal, which may be transmitted as a pilot tone on an unused cable channel‘ or which is generated in response to the horizontal sync components of the transmitted signal or by other means. Various systems for sync suppression scrambling are described in the following patents. In most cases the suppression signals are pulses. Sine wave suppression signals have been suggested in the five patents which are listed last below. Druz, U.S. Pat. No. 2,705,740, issued Apr. 5, 1955; Weiss, U.S. Pat. No. 2,907,816, issued Oct. 6, 1959; Loughlin et al, U.S. Pat. No. 3,081,376, issued Mar. 12, 1963; Doundoulakis, U.S. Pat. No. 3,116,363, issued Dec. 31, 1963; Court et al, U.S. Pat. No. 3,184,537, issued May 18, 1965; Doundoulakis, U.S. Pat. No. 3,201,511, issued Aug. 17, l965;'Salit et al, U.S. Pat. No. 3,242,258, issued Mar. 22, 1966; Walker, U.S. Pat. No. 3,439,113, issued Apr. 15, 1969; Rieiter et al, U.S. Pat. No. 3,478,166, issued Nov. 11, 1969; Reiter et al, U.S. Pat. No. 3,530,232, issued Sept. 22, 1970; Vogelman et al, U.S. Pat. No. 3,736,369, issued May 29, 1973; Blonder, U.S. Pat. No. 3,813,482, issued May 28, 1974; Sperber, U.S. Pat. No. 4,095,258, June 13, 1978; Mistry et al, U.S. Pat. No. 4,163,252, issued July 31, 1971, Court, U.S. Pat. No. 3,729,576, issued Apr. 24, 1973; Harney et al, U.S. Pat. No. 4,024,575, issued May 17, 1977; Saeki et al, U.S. Pat. No. 4,064,536, issued Dec. 29, 1977; Uemora et al, U.S. Pat. No. 4,145,716, issued Mar. 20, 1979; and Guif et al, U.S. Pat. No. 4,145,717, issued Mar. 20, 1979. All of the above listed patents are United States patents.
The problem with sync suppression scrambling schemes which have been suggested, except for those which transmit complex coding signals and which have not been generally accepted, is the ease with which the signals can be descrambled and restored to normal amplitude so that they can be displayed at unauthorized receiving stations. For example in the case where the signals are encoded by a pilot tone or timed to the horizontal sync signals, all that is required is a simple circuit for detecting these pilot or sync rate signals and generating a restoring pulse for a gated attenuator through which the television signal passes in order to restore the television signal to normal amplitude. One such scram
- 2 bler has been offered by a company known as C&D Engineers, their model CDK 234 and 236 descrambler.
It is a feature of this invention to provide improved methods of and systems for thetransmission of premium subscription TV programs with greater scrambling efficiencies and which is more difficult to defeat and descramble without authorization, but without increasing the complexity and cost of sync suppression scrambling so as to make the use thereof impractical.
It is a further feature of the invention to provide improved methods of and systems for the transmission of encoded suppressed sync TV signals, as well as for the reception and descrambling of such signals, which can be implemented by digital circuits which further facilitates the fabrication and use of sync suppression scrambling having high security efficiency at low cost.
It is a further feature of the present invention to provide improved methods and systems for sync suppression scrambling and descrambling of TV signals which may be encoded with selectively coded timing which may be dynamically varied, so that the generation of sync restoring signals with the proper timing to descramble and normalize the TV signal is available only to authorized parties thereby making it very difficult to defeat the security of the system and discouraging the use of unauthorized descramblers.
Briefly described, sync suppression scrambling and descrambling of TV signals for subscription TV programs, in accordance with the invention, may be carried on by generating sync suppression pulses in overlapping time relationship with the horizontal sync pulses of the TV signal. Timing pulses are generated in selected time relationship with the sync suppression pulses such that each of the timing pulses and the sync suppression pulses have a selected time delay With respect to each other. This time delay may be selected, preferably digitally, from a plurality of different delays or dynamically varied. The horizontal sync pulses are suppressed to below normal amplitude, which sync detectors of conventional TV receivers at the receiving stations are capable of using. The timing pulses are transmitted with the TV signals, preferably by modulating the FM audio carrier thereof to encode the TV signals. The scrambled and encoded TV signals are transmitted from the transmitting stations which may, for example, be the head end system of a cable TV system to the receiving stations, for example on a channel dedicated to premium programming. The timing pulses are derived at these receiving stations with the selected delay to provide delayed timing pulses. Then restoring pulses are generated in proper overlapping time relationship with the horizontal sync pulses upon occurrence of the delayed timing pulses. The TV signals are restorable to normal amplitude through the use of the restoring pulses to descramble the TV signals. Digital delays are preferably interposed on the timing pulses to provide the delayed timing pulses. The selected delays may be pre-set, for example as pre-set digital delays at the transmitting point and also at those receiving stations which are authorized to receive premium programming. If desired and when the delays are to be dynamically varied, delay control signals are transmitted with the TV signals to vary the delay dynamically at the authorized receiving stations so as to correspond to the variation in delay at the transmitting point. Only at authorized receiving stations will the restoring pulses be in synchronism with the horizontal