Method of controlling scrambling and unscrambling in a pay TV system

• US 4,388,643 A
• Filed: 04/06/1981
• Issued: 06/14/1983
• Est. Priority Date: 04/06/1981
• Status: Expired due to Fees

• Current Assignee: Northern Telecom Limited (Nortel Networks Corporation)
• Original Assignee: Northern Telecom Limited (Nortel Networks Corporation)
• Inventors: Aminetzah, Yehuda J.
• Primary Examiner(s): BUCZINSKI, STEPHEN C
• Application Number:
  US06/251,085
• Time in Patent Office: 799 Days
• Field of Search: 178/22.08, 178/22.09, 178/22.13, 178/22.16, 178/22.17, 358/114, 358/117, 358/122, 358/123
• US Class Current: 380/239
• 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

Abstract

Scrambling of a video signal in a pay TV system is effected in dependence upon a first variable which is changed recurrently, e.g. monthly, and a second variable which changes from field to field. For each field the second variable and other data are transmitted simultaneously with the video signal together with check data produced from the transmitted data and a third variable which is changed recurrently. The first and third variables are recurrently transmitted to each authorized subscriber station, encoded in dependence upon a subscriber number stored in the subscriber station. In the subscriber station, unscrambling is effected in dependence upon the stored first variable and the transmitted second variable after the transmitted data is checked using the check data and the stored third variable. The first variable is conveniently different for different types of television programs, the type of television program being identified by the other data transmitted simultaneously with the video signal.

Claims

1. A method of controlling scrambling and unscrambling of a video signal in a pay TV system, comprising the steps of:

storing a subscriber number at a subscriber station;

recurrently transmitting to the subscriber station a first variable encoded in dependence upon the subscriber number;

decoding the first variable at the subscriber station using the stored subscriber number, and storing the first variable before or after said decoding;

scrambling the video signal in dependence upon the first variable and a second variable;

transmitting the second variable to the subscriber station simultaneously with transmission of the scrambled video signal; and

at the subscriber station, unscrambling the video signal in dependence upon the decoded stored first variable and the transmitted second variable.

2. A method as claimed in claim 1 wherein said first variable is stored after said decoding.

3. A method as claimed in claim 1 and comprising the step of producing the first variable using a random number generator.

4. A method as claimed in claim 1 and comprising the step of producing the second variable for each field of the video signal using a pseudo-random number generator.

11. A method as claimed in claim 1 or 5 and comprising the steps of:

producing different said first variables for different types of television program;

selectively recurrently transmitting said different first variables to the subscriber station for decoding and storage therein;

scrambling the video signal in dependence upon the respective first variable, for the type of television program to which the video signal relates, and the second variable;

transmitting, with said second variable, data representing the type of television program to which the scrambled video signal relates; and

at the subscriber station, selecting the respective decoded stored first variable in dependence upon said data representing the type of television program for unscrambling the video signal.

5. A method of controlling scrambling and unscrambling of a video signal in a pay TV system, comprising the steps of:

storing a subscriber number at a subscriber station;

scrambling the video signal in dependence upon first and second variables;

recurrently transmitting to the subscriber station the first variable and a third variable each encoded in dependence upon the subscriber number;

decoding the first and third variables at the subscriber station using the stored subscriber number, and storing the first and third variables before or after said decoding;

producing check data from the second and third variables;

transmitting the second variable and the check data to the subscriber station simultaneously with transmission of the scrambled video signal; and

at the subscriber station, generating check data from the transmitted second variable and decoded stored third variable, enabling unscrambling of the video signal in dependence upon a comparison of this generated check data with the transmitted check data, and unscrambling the video signal in dependence upon the decoded stored first variable and the transmitted second variable.

6. A method as claimed in claim 5 wherein said first and third variables are stored after said decoding.

7. A method as claimed in claim 5 and comprising the step of producing said first and third variables using a random number generator.

8. A method as claimed in claim 5 and comprising the step of producing said second variable for each field of the video signal using a pseudo-random number generator.

9. A method as claimed in claim 8 and comprising the steps of additionally producing said second variable for each field of the video signal using a pseudo-random number generator at the subscriber station, synchronizing the production of said second variable at the subscriber station with said transmitted second variable, and unscrambling the video signal in dependence upon the decoded stored first variable and the second variable generated at the subscriber station in the event that the check data comparison indicates an error.

10. A method as claimed in claim 9 and including the steps of storing an accumulated error count at the subscriber station, incrementing said count in response to errors indicated by the check data comparison, and inhibiting unscrambling of the video signal in response to said count reaching a predetermined value.