Method and apparatus for scrambling and unscrambling communication signals

• US 3,919,462 A
• Filed: 08/15/1973
• Issued: 11/11/1975
• Est. Priority Date: 08/15/1973
• Status: Expired due to Term

• Current Assignee: Unisys Corporation
• Original Assignee: Albert F. Hartung, Charles T. Barooshian, Edward J. Zacharski, Frank W. Lehan
• Inventors: Hartung, Albert F., Lehan, Frank W., Barooshian, Charles T., Zacharski, Edward J.
• Primary Examiner(s): Wilbur, Maynard R.
• Assistant Examiner(s): Buczinski, S. C.
• Application Number:
  US05/388,439
• Time in Patent Office: 818 Days
• Field of Search: 178/5.1,DIG.13,22 325/34
• US Class Current: 380/235
• CPC Class Codes: H04N 2007/1739 the upstream communication being transmitted via a separate link, e.g. telephone line
• CPC Class Codes: H04N 7/1675 Providing digital key or authorisation information for generation or regeneration of the scrambling sequence pseudo-random number generators in general G06F7/58
• CPC Class Codes: H04N 7/1716 by inverting the polarity of active picture signal portions
• CPC Class Codes: H04N 7/17363 at or near the user terminal

Abstract

Method and apparatus for scrambling and unscrambling television video and audio signals in a subscription television system in which program selections of subscribers are known at a central transmitting site, and control signals encoded into vertical blanking intervals of the video signals are addressed to receivers authorized to receive unscrambled transmissions, to selectively control unscrambling at those receivers. In an unscrambler at each subscriber'"'"''"'"'s receiver, the control signals are decoded, and, if addressed to the particular subscriber'"'"''"'"'s receiver, operate to enable or disable the unscrambler, or to frequently vary its mode of operation, thereby greatly increasing the security of the system and deterring viewing of scrambled transmissions. Video scrambling and unscrambling are effected by inversion of selected horizontal lines of a transmitted television picture, and possible modes of scrambling and unscrambling include inversion of alternate groups of equal numbers of lines, inversion or non-inversion selected on a lineby-line basis, with an appropriate control signal being transmitted with each line, and inversion or non-inversion in a preselected sequence, as determined by synchronized logic at the receivers and the transmitting site.

Claims

1. For use with a receiver in a subscriptioN television system, a method of unscrambling television signals transmitted in a scrambled form having selected portions containing only video information inverted according to a preselected mode from among a plurality of modes of scrambling, each mode being characterized by a sequence of alternating inverted and non-inverted groups of lines, said method comprising the steps of:

receiving the scrambled signals together with control signals and identifying codes both encoded on a common video signal carrier, the identifying codes serving to identify receivers authorized to unscramble the scrambled signals;

decoding the encoded control signals and identifying codes;

comparing each of the identifying codes with one associated with the receiver;

unscrambling by selective inversion of the inverted video portions of the scrambled television signals, according to the same preselected mode as used for scrambling; and

controlling said unscrambling step, if said comparing step results in a match, in accordance with the decoded control signals, said controlling step including controlling the preselected mode in accordance with mode selection signals included with the control signals.

2. A method as set forth in claim 1, wherein said controlling step includes enabling and disabling selective inversion in response to particular control signals transmitted for that purpose.

3. A method as set forth in claim 1, further including the step of unscrambling audio signals received in said receiving step, by applying a frequency shift to a carrier signal modulated by the audio signals.

4. A method as set forth in claim 1, wherein:

the control signals are encoded as digital pulses in vertical blanking intervals of composite video and synchronization signals; and

said decoding step includes gating the digital pulses into a register.

5. A method as set forth in claim 4, wherein:

the control signals include an unscrambler synchronization pulse; and

said method further includes the step of synchronizing said step of unscrambling with a corresponding scrambling step, in response to the unscrambler synchronization pulse.

6. A method as set forth in claim 4, wherein:

said controlling step further includes enabling and disabling selective inversion; and

the particular signals transmitted to effect said step of controlling the preselected mode are effective to select desired unscrambling modes at all receivers having selective inversion enabled.

7. A method as set forth in claim 1, wherein said step of unscrambling includes:

counting portions of the video signals corresponding to lines of the television picture in a counter having a plurality of digits; and

inverting the video signals only when a particular digit of the counter is in a particular state; and

said step of controlling the preselected mode of unscrambling is effected by varying selection of the particular digit from the counter, whereby selection of a digit of low significance in the counter results in frequent inversion of the video signals and selection of a digit of high significance in the counter results in less frequent inversion.

8. A method as set forth in claim 7, wherein said step of inverting the video signals includes selecting the particular digit and its particular state in such a manner as to produce rolling patterns of inverted lines in the television picture.

9. A method as set forth in claim 7, wherein the control signals are encoded in vertical blanking intervals of the video signals, and said method further includes:

periodically receiving unscrambler synchronization signals also encoded in vertical blanking intervals; and

resetting the line counter in response to receipt of an unscrambler synchronization signal, thereby synchronizing unscrambling operations with complementary scrambling operations.

10. A method as set forth in claim 9, wherein said step of unscrambling includes genErating an inversion control signal from the state of the particular digit in the counter and from the state of a horizontal synchronization pulse signal, whereby inversion is performed only between horizontal synchronization pulses and only when the particular digit is in the particular state.

11. A method as set forth in claim 1, wherein:

said step of controlling the preselected mode of unscrambling includes encoding an inversion indicator signal for transmission with each line of the television picture; and

said step of unscrambling includes decoding said inversion indicator signal and inverting the video signals only when said inversion indicator signal is in a particular state.

12. A method as set forth in claim 11, wherein:

said inversion indicator signal is encoded at a selected location with respect to each horizontal synchronization pulse; and

said step of decoding said inversion indicator signal includes measuring elapsed time from the start of a horizontal synchronization pulse to the location of said inversion indicator signal, and gating said inversion indicator signal into a register.

13. A method as set forth in claim 11, wherein said step of unscrambling includes generating an inversion control signal from said inversion indicator signal and from a horizontal synchronization pulse signal, whereby inversion is performed only between horizontal synchronization pulses and only when said inversion indicator signal is in the particular state indicating inversion.

14. A method as set forth in claim 1, wherein:

said step of controlling the preselected mode of unscrambling includes storing said unscrambler mode selection signals in register means; and

said step of selectively inverting includes;

scrambling the signals stored in the register means according to a predetermined pattern;

deriving an inversion indicator signal from the scrambled stored signals; and

inverting the video signals only when the inversion control signal is in a particular state.

15. A method as set forth in claim 14, wherein the register means include shift register means having a plurality of digits, and said step of scrambling the signals stored in the register means include:

periodically shifting the signals in the shift register means a predetermined number of times between successive horizontal synchronization pulses separating the lines;

logically generating a digit from the plurality of digits in the shift register means; and

gating the generated digit into the shift register means during each of said shifting steps.

16. A method as set forth in claim 15, wherein said step of logically generating a digit includes performing successive exclusive OR functions on the plurality of digits to obtain a single-digit result.

17. A method as set forth in claim 14, wherein said step of deriving an inversion indicator signal is effected by sensing a particular digit in the shift register means.

18. A method as set forth in claim 14, wherein:

the encoded control signals include an unscrambler synchronization signal; and

said method further includes the step of synchronizing said step of selectively inverting with a corresponding inverting step performed in scrambling the video signals, by resetting the shift register means to a starting value on receipt of the unscrambler synchronization signal.

19. A method as set forth in claim 14, wherein said step of unscrambling includes generating an inversion control signal from the inversion indicator signal and from a horizontal synchronization pulse signal, whereby inversion is performed only between successive horizontal synchronization pulses and only when the inversion indicator signal is in the particular state indicating inversion.

20. For use with receivers in a subscription television system, a method of unscrambling television video signals scrambled by inversion of only video portions of the signals correspOnding to preselected lines in a television picture, said method comprising the steps of:

receiving and decoding control signals encoded from time to time on a common video carrier with the video signals, receiver identifying codes being also encoded on the same common video carrier, the control signals including an unscrambler enabling and disabling signal, and unscrambler mode selection signals;

enabling and disabling unscrambling operations at selected receivers identified by the identifying codes in response to the unscrambler enabling and disabling signal;

selectively inverting the inverted portions of the video signals, when unscrambling operations are enabled, according to a mode determined by the unscrambler mode selection signals and characterized by a sequence of alternating inverted and non-inverted groups of lines; and

varying the mode of unscrambling on receiving and decoding further unscrambler mode selection signals.

21. For use with receivers in a subscription television system, apparatus for unscrambling television video signals scrambled by inversion of only video portions corresponding to preselected lines in the television picture, said apparatus comprising:

means for receiving and decoding identifying codes and control signals both encoded from time to time on a common video signal carrier for transmission with the video signals, said control signals including unscrambler mode selection signals;

means for comparing received identifier codes with an identifier code generated at a receiver location;

means for enabling and disabling unscrambling operations in selected receivers according to the determination of said comparing means; and

means for selectively inverting said video signals, when unscrambler operations are enabled, according to a mode determined by said unscrambler mode selection signals.

22. Apparatus as set forth in claim 21, wherein said means for selectively inverting includes:

means for counting portions of the video signals corresponding to lines of the television picture, said counting means having a plurality of digits; and

means responsive to said counting means for inverting the video signals only when a particular digit of said counting means is in a particular state; and

said means for selectively inverting being operable to vary selection of the particular digit from said counting means, whereby selection of a digit of low significance in said counting means results in frequent inversion of the video signals and selection of a digit of high significance in said counting means results in less frequent inversion.

23. Apparatus as set forth in claim 22, wherein said control signals are encoded in vertical blanking intervals of the video signals, and said apparatus further includes;

means for receiving and decoding periodically transmitted unscrambler synchronization signals also encoded in the vertical blanking intervals; and

means for resetting said counting means in response to said unscrambler synchronization signal, thereby synchronizing unscrambler operations with complementary scrambling operations.

24. Apparatus as set forth in claim 22, wherein said means for selectively inverting includes means for generating an inversion control signal from the state of said particular digit in said counting means and from the state of a horizontal synchronization pulse signal, whereby inversion is performed only between horizontal synchronization pulses and only when said particular digit is in said particular state.

25. Apparatus as set forth in claim 21, wherein:

an inversion indicator signal is encoded for transmission with each line of the television picture; and

said means for selectively inverting includes means for decoding said inversion indicator signal and inverting said video signals only when said inversion indicator signal is in a particular state.

26. Apparatus as set forth in claim 25, wherein:

said inversion indicator signal is encoded at a selected location with respect to each horizontal synchronization pulse; and

said means for decoding said inversion indicator signal includes means for measuring elapsed time from the start of each horizontal synchronization pulse to the location of said inversion indicator signal, register means, and gating means for gating said inversion indicator signal into said register means.

27. Apparatus as set forth in claim 25, wherein said means for selectively inverting includes means for generating an inversion control signal from said inversion indicator signal and from a horizontal synchronization pulse signal, whereby inversion is performed only between horizontal synchronization pulses and only when said inversion indicator signal is in the particular state indicating inversion.

28. Apparatus as set forth in claim 21, wherein said means for selectively inverting further includes:

register means for storing said unscrambler mode selection signals;

means for scrambling the signals stored in said register means according to a predetermined pattern;

means for deriving an inversion indicator signal from the scrambled stored signals; and

means for inverting said video signals only when said inversion indicator signal is in a particular state.

29. Apparatus as set forth in claim 28, wherein said register means includes shift register means having a plurality of digits, and said means for scrambling the signals stored in said register means include:

means for shifting the signals in said shift register means a predetermined number of times between successive horizontal synchronization pulses separating the lines;

logic means for generating a digit from the plurality of digits in said shift register means; and

means for shifting said generated digit into said shift register means each time said stored signals in said shift register means are shifted by said means for periodically shifting.

30. Apparatus as set forth in claim 29, wherein said logic means for generating a digit includes exclusive OR gate means connected to said plurality of digits to obtain a single-digit result.

31. Apparatus as set forth in claim 28, wherein said means for deriving said inversion indicator signal includes means for gating a particular digit from said shift register means.

32. Apparatus as set forth in claim 28, wherein:

certain coded control signals include an unscrambler synchronization signal; and

said apparatus further includes means for synchronizing said means for selectively inverting with corresponding means used in scrambling the video signals, by resetting said shift register means to a starting value on receipt of said unscrambler synchronization signal.

33. For use in a subscription television system with a plurality of subscriber'"'"''"'"'s receivers identified by addresses, a method of scrambling and unscrambling television signals, comprising the steps of:

inverting at a transmitter selected video portions of a composite video signal, the inverted portions representing lines of a television picture selected according to a preselected scrambling mode from among a plurality of modes, each mode being characterized by a sequence of alternating inverted and non-inverted groups of lines;

encoding with the video signal on a common video carrier control signals to control unscrambling at receivers of authorized subscribers, and address signals identifying authorized receivers;

transmitting and receiving the video, control and address signals on the common video carrier;

decoding the encoded control and address signals; and

controlling selected inversion of the video signals at a receiver in accordance with the scrambling mode, and in response to the control signals if the address signals match the address of the receiver, said controlling step including varying the scrambling mode in response to particular settings of the control signals.

34. A method as set forth in claim 33, wherein said steps of inverting and controlling selected inversion include inverting alternate groups of equal numbers of lines.

35. A method as set forth in claim 33, wherein the control signals include an inversion indicator signal encoded with each line transmitted.

36. A method as set forth in claim 33, wherein:

said steps of inverting and controlling selected inversions include generating an inversion control signal for each line of the picture, at the transmitter and at the receiver; and

said method further includes the step of synchronizing said steps of generating an inversion control signal at the transmitter and the receiver.

37. For use in a subscription television system with a plurality of subscribers'"'"''"'"' receivers, a method of scrambling television signals at a transmitter, comprising the steps of:

selectively inverting video portions of a composite video signal corresponding to lines of a television picture selected according to a preselected scrambling mode from among a plurality of modes, each mode being characterized by a sequence of alternating inverted and non-inverted groups of lines;

encoding for transmission with the video signals on a common video carrier, control signals to control unscrambling operations, and address signals designating authorized receivers; and

varying the preselected scrambling mode and further encoding control signals to correspondingly vary unscrambling operations.

38. A method as set forth in claim 37, wherein said step of selectively inverting includes inverting alternate groups of equal numbers of lines.

39. A method as set forth in claim 37, wherein the control signals include an inversion indicator signal encoded with each line transmitted.

40. A method as set forth in claim 37, wherein:

said step of selectively inverting includes generating an inversion control signal for each line of the picture; and

said method further includes the step of generating a synchronizing signal to synchronize unscrambling operations at the receivers with said step of generating an inversion control signal.

41. For use in a subscription television system, a method of unscrambling at a subscriber location television signals transmitted in scrambled form to a plurality of receivers, said method comprising the steps of:

receiving the scrambled television signals at unscrambling means connected with a receiver at the subscriber location, the television signals including a composite video and synchronization signal having only video portions scrambled, and having unscrambler control signals and subscriber identifying codes encoded into the composite signal for transmission on a common video carrier;

decoding the encoded control signals and identifying codes;

comparing each received and decoded identifying code with an identifying code generated at the unscrambling means;

controlling the unscrambling means in response to the decoded signals accompanying the transmitted identifying codes, provided said comparing step results in a match, said controlling step including, as determined by the particular control signals, enabling the unscrambling means to unscramble the video portions of the television signals in accordance with an unscrambling mode also indicated by the control signals, and changing the unscrambling mode from time to time as indicated by further control signals, thereby to increase the security of the system and to deter unauthorized viewing; and

unscrambling the video portions of the scrambled television signals in response to said controlling step.

42. A method as set forth in claim 41, and further including the step of synchronizing said unscrambling step with a corresponding scrambling step, in response to a synchronizing signal periodically included for transmission with the control signals.

43. A method as set forth in claim 41, wherein said controlling step includes selectively Controlling channel selection at the subscriber location in response to channel selection signals included with the control signals.

44. A method as set forth in claim 41, wherein said unscrambling step includes the step of inverting selected video portions of the composite signal corresponding to selected lines of video information, the selection being in accordance with a mode of unscrambling determined by said controlling step and characterized by a sequence of alternating inverted and non-inverted groups of lines.

45. A method as set forth in claim 44, wherein:

said unscrambling step includes unscrambling audio signals; and

said step of unscrambling audio signals includes applying a frequency shift to a carrier signal modulated by the audio signals.

46. A method as set forth in claim 44, wherein said step of inverting includes:

counting portions of the composite video and synchronization signal corresponding to lines of video information in a counter having a plurality of digits; and

inverting the video signals only when a particular digit of the counter is in a particular state; and

wherein said step of changing the unscrambling mode is effected by varying selection of the particular digit from the counter, whereby the selection of a digit of low significance in the counter results in frequent inversion of the video signals and selection of a digit of high significance in the counter results in less frequent inversion.

47. A method as set forth in claim 46, wherein said step of inverting includes selecting the particular digit of the line counter, and its particular state, in such a manner as to produce rolling patterns of inverted lines in the television picture.

48. A method as set forth in claim 46, wherein said step of synchronizing includes:

periodically receiving unscrambled synchronization signals also encoded into the composite signal; and

resetting the line counter to a predetermined value in response to receipt of an unscrambler synchronization signal, thereby synchronizing unscrambling operations with complementary scrambling operations.

49. A method as set forth in claim 48, wherein said step of resetting the line counter resets the counter to zero.

50. For use in a subscription television system, apparatus for unscrambling at a subscriber location television signals transmitted in scrambled form, said apparatus comprising:

receiving means, for receiving the scrambled television signals at the subscriber location, the television signals including a composite video and synchronization signal having only video portions scrambled and having unscrambler control signals and subscriber identifying codes encoded into the composite signal on a common video carrier;

decoding means, connected with said receiving means, for decoding the encoded control signals and identifying codes;

comparing means, connected with said decoding means, for comparing each received and decoded identifying code with an identifying code generated at the subscriber location;

unscrambling means for unscrambling the video portions of the composite signal in accordance with a mode of operation of said unscrambling means indicated by the control signals, if said comparing means finds a match; and

control means, for controlling said unscrambling means in response to the decoded control signals, if said comparing means finds a match, said control means including means for selecting different modes of operation of said unscrambling means as indicated by further control signals received from time to time, thereby increasing the security of the system and deterring unauthorized viewing.

51. Apparatus as set forth in claim 50, and further including means for synchronizing operation of said unscrambling means in response to a synchronizing signal periodically included for transmission with the control signals.

52. Apparatus as set forth in claim 51, and further including means for sElecting a television channel for viewing at the subscriber location, in accordance with channel selection signals included with the control signals.