4,388,643
`[11]
`[19]
`United States Patent
`
` Aminetzah [45] Jun. 14, 1983
`
`
`[54] METHOD OF CONTROLLING
`SCRAMBLING AND UNSCRAMBLING IN A
`PAY TV SYSTEM
`
`FOREIGN PATENT DOCUMENTS
`2068691
`8/1981 United Kingdom ............. 178/22.05
`
`[75]
`
`Inventor: Yehuda J. Aminetzah, Nepean,
`Canada
`
`. Primary Examiner—S. C. Buczinski
`Attorney, Agent, or Firm—R. John Haley
`
`.
`[58] Flew of
`[56]
`
`’
`'
`’
`'
`References Cited
`
`’
`
`’
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`’
`
`
`
`
`
`11 Claims, 4 Drawing Figures
`
`ENGODED DATA T0 VBC
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`aco, se
`GENERATOR
`
`ENCODED DATA
`TO 8 FROM HT
`
`TRUE
`RANDOM
`
`HT SELECTION
`
`NUMBER
` DECODER
`GENERATOR
`
`PMC Exhibit 213
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`Apple v. PM
`|PR2016-0075
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`Page 1
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`POLLING
`
`CONTROL
`UNIT
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`208
`
`[73] Assignee: Northern Telecom Limited, Montreal,
`ABSTRACT
`1571
`Canada
`Scrambling of a video signal in a pay TV system is
`L
`_: 251 085
`effected in dependence upon a first variable which is
`[21] App No
`’
`— changed recurrently, e.g. monthly, and a second vari-
`[22] Filed:
`Apr. 6, 1981
`able which changes from field to field. For each field
`the second variable and other data are transmitted si-
`~
`%i3 """"""""""""" Hmglsg multaneously with the video signal together with check
`178/2213; 358/114; 358/117; 358/122
`data produced from the transmitted data and a third
`variable which is changed recurrently. The first and
`third variables are recurrently transmitted to each au-
`thorized subscriber station, encoded in dependence
`upon a subscriber number stored in the subscriber sta-
`tion. 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 vari-
`able. The first variable is conveniently different for
`different types of television programs, the type of tele-
`vision program being identified by the other data trans-
`"med S'm“1“=“1e0“S1Y W11 the V1490 518"“-
`
`U'S' PATENT DOCUMENTS
`3,906,460 9/ 1975 Halpern .......................... .. 178/22.08
`4,068,264
`1/1978 Pires ...... ..
`353/122
`4,081,832
`3/1978 Sherman ..
`4,115,807 9/1978 Pires ...... ..
`.
`4,163,254
`7/1979 Block et al.
`4,238,854 12/1980 Ehrsam et al.
`’ 4,292,650 9/1931 Hendrickson
`4,’323,92l
`4/1982 Guillou ...... ..
`4,337,483
`6/1982 Guillou ...........................
`
`358/122
`178/22.16
`353/122
`.. 358/114
`358/114
`
`..
`
`PMC Exhibit 2134
`Apple v. PMC
`IPR2016-00755
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`U.S. Patent %
`
`Jun. 14, 1983
`
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`Apple v. PMC
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`PMC Exhibit 2134
`Apple v. PMC
`IPR2016-00755
`Page 5
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`1
`
`4,388,643
`
`METHOD OF CONTROLLING SCRAMBLING
`AND UNSCRAMBLING IN A PAY TV SYSTEM
`
`2
`rity through coding, and again it is relatively easy for
`unauthorized persons to defeat or cheat the system.
`Accordingly, a need exists to provide a more secure
`coding system for use in encoding control signals for
`This invention relates to a method of controlling 5 enabling unscrambling of a video signal in a pay TV
`scrambling and unscrambling in a pay TV system.
`system, particularly in a pay TV system which uses
`It is well known in the art of pay TV systems to
`scrambling procedures which are themselves difficult to
`scramble a video signal, in any of a variety of different
`defeat directly, i.e. without use of the appropriate con-
`ways, prior to broadcasting it by cable or transmission
`trol signals. Such a pay TV system forms the subject of
`via free space, with the intent that only authorized sub- 10 a copendingapplication Ser. No. 246,878 by J. A. Bond,
`Y. Li, and L. J. Crane filed on Mar. 23, 1981 and entitled
`scribers equipped with an appropriate unscrambler
`should be able to unscramble the video signal in return
`“Scrambling and Unscrambling Video Signals in a Pay
`for payment of a fee. The security of the pay TV sys-
`TV System”, the entire disclosure of which is hereby
`tem, i.e. the degree to which unauthorized persons are
`incorporated herein by reference.
`prevented from unscrambling the video signal, depends 15
`An object of this invention, therefore, is to provide an
`upon the nature of the scrambling process and the secu-
`improved method of controlling scrambling and un-
`rity of any coding which is employed. If a pay TV
`scrambling of a video signal in a pay TV system.
`system uses no coding, then any person obtaining an
`According to one aspect, this invention provides a
`appropriate unscrambler can unscramble the video sig-
`method of controlling scrambling and unscrambling of
`nal. Such a system is obviously insecure in that unautho- 20 a video signal in a pay TV system, comprising the steps
`rized persons are readily able to unscramble the video
`of: storing a subscriber number at a subscriber station;
`signal.
`recurrently transmitting to the subscriber station a first
`A pay TV system which incorporates coding is dis-
`variable encoded in dependence upon the subscriber
`closed in Sherman U.S. Pat. No. 4,081,832 issued Mar.
`number; decoding the first variable at the subscriber
`28, 1978. In this prior art system the video signal is
`station using the stored subscriber number, and storing
`scrambled, for example by inversion of selected groups
`the first variable before or after said decoding; scram-
`of video signal lines, in accordance with a predeter-
`bling the video signal in dependence upon the first vari-
`mined code which is punched on a card which is mailed
`able and a second variable; transmitting the second
`to a subscriber. Information as to which group of lines
`variable to the subscriber station simultaneously with
`is inverted in any particular video signal field is trans-
`transmission of the scrambled video signal; and at the
`mitted to the subscriber with the scrambled video sig-
`subscriber station, unscrambling the video signal
`in
`nal. The card mailed to the subscriber also contains a
`dependence upon the decoded stored first variable and
`the transmitted second variable.
`subscriber number which is compared with a number
`stored at the subscriber station in the form of wired
`According to another aspect, this invention provides
`jumpers, and contains an area/month code which is
`a method of controlling scrambling and unscrambling
`compared with an area/month code transmitted with
`of a video signal in a pay TV system, comprising the
`steps of: storing a subscriber number at a subscriber
`the scrambled video signal. If these comparisons are
`satisfied, decoding means at the subscriber station are
`station; scrambling the video signal in dependence upon
`enabled to unscramble the video signal using the trans-
`first and second variables; recurrently transmitting to
`the subscriber station the first variable and a third vari-
`mitted information and the code on the card, the card
`being punched to indicate acceptance of a TV program.
`able each encoded in dependence upon the subscriber
`The punched card is subsequently returned by mail for
`number; decoding the first and third variables at the
`billing of the subscriber.
`subscriber station using the stored subscriber number,
`and storing the first and third variables before or after
`Whilst this prior art system provides some security
`due to the coding employed, it is easily defeated or
`said decoding; producing check data from the second
`and third variables; transmitting the second variable and
`cheated. Thus the decoding means can readily be per-
`manently enabled, and the code on the card and the
`the check data to the subscriber station simultaneously
`transmitted information can still be used to unscramble
`with the transmission of the scrambled video signal; and
`the video signal while the punch for punching the card
`at the subscriber station, generating check data from the
`is rendered inoperable to prevent billing for a program 50 transmitted second variable and the decoded stored
`which is viewed. Furthermore, the code on the card can
`third variable, enabling unscrambling of the video sig-
`simply be used to enable unscrambling of the video
`nal in dependence upon a comparison of this generated
`signal by other apparatus without payment.
`check data with the transmitted check data, and un-
`The use of a punched card as discussed above, to
`scrambling the video signal in dependence upon the
`supply a code to a subscriber and for billing purposes, 55 decoded stored first variable and the transmitted second
`can be avoided by using a two-way transmission link,
`variable. The first and third variables are preferably
`such as the subscriber’s telephone line, for communica-
`stored after said decoding.
`tion, as described in Block et al. U.S. Pat. No. 4,163,254
`Thus in accordance with the invention first and third
`issued July 31, 1979. In the pay TV system described
`variables, DK and ICK in the embodiment described
`therein a program identification code, transmitted with 60 below, which are conveniently produced using a ran-
`the scrambled video signal, is stored in respect of each
`dom number generator so that the variables can not be
`program which is unscrambled, and the stored codes
`predicted, are transmitted recurrently (e.g. monthly) to
`are accessed via the telephone line periodically for bill-
`a subscriber station, encoded in accordance with a sub-
`ing purposes. A scramble code comparator compares a
`scriber number so that only the intended subscriber
`scramble code, transmitted with the scrambled video 65 station can decode these variables. The first variable
`signal, with a stored scramble code provided via the
`DK is used together with a second variable, PD as
`telephone line,
`to produce signals for unscrambling.
`described below, which is transmitted simultaneously .
`Thus this system provides only a small degree of secu-
`. with the video signal and which can change from field
`PMC Exhibit 213
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`30
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`4,388,643
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`3
`to field, to scramble the video signal prior to transmis-
`sion and, in the subscriber station, to unscramble the
`video signal for viewing.
`The third variable is used to produce from the second
`variable, and possibly also any other data which is trans-
`mitted therewith, check data (IC as described below)
`which is transmitted with the second variableand is
`used in the subscriber station to check that the informa-
`tion received in the respective field is correct; if so
`unscrambling is enabled. In order to permit proper
`unscrambling when the information received is occa-
`sionally faulty, preferably the second variable is pro-
`duced for each field of the video signal using a pseudo-
`random number generator, and additionally said second
`variable is produced for each field using a pseudo-ran-
`dom number generator at the subscriber station, syn-
`chronized with the transmitted second variable, and in
`the event that the check data comparison indicates an
`error the video signal is unscrambled in dependence
`upon the decoded stored first variable and the second
`variable generated at the subscriber station.
`As a protection against faulty operation, preferably
`the method includes 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 predeter-
`mined value.
`In order to enable various subscription requirements
`to be satisfied selectively, preferably the method com-
`prises the steps of: producing different said first vari-
`ables for different types of television program; selec-
`tively recurrently transmitting said different first vari-
`ables 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 sec-
`ond 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 repre-
`senting the type of television program for unscrambling
`the video signal.
`The invention will be further understood from the
`following description with reference to the accompany-
`ing drawings, in which:
`FIG. 1 shows a block diagram of parts of a pay TV
`system which include a video broadcast centre VBC, a
`subscriber’s home terminal HT, and a control and data
`collection centre DCC; and
`FIGS. 2, 3, and 4 are diagrams illustrating the manner
`in which scrambling and unscrambling are controlled in
`the pay TV system, these figures relating to the DCC,
`VBC, and HT respectively.
`Referring to FIG. 1, there is shown therein parts of a
`pay TV system in which video signals from a program
`source 10 are scrambled in a scrambler 11, the resultant
`scrambled video signals and audio signals from the pro-
`gram source being supplied to a transmitter 12 for
`broadcasting to subscribers of the pay TV system. As
`shown, the audio signals are not scrambled, but they
`could also be scrambled in known manner if desired.
`The broadcast signals are illustratively supplied to a
`subscriber’s home via a cable 13, but they could alterna-
`tively be supplied by electromagnetic radiation, directly
`or via a satellite link. The units 10, 11, and 12 constitute
`a video broadcast centre VBC.
`
`IO
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`20
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`25
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`4
`The subscriber’s home includes a conventional chan-
`nel converter 14, television receiver 15, and telephone
`16, the latter being connected via a coventional tele-
`phone line 17 to a telephone central office 18. An un-
`scrambler 19 is coupled between the output of the chan-
`nel converter 14 and the input of the television receiver
`15, and is also coupled via an interface circuit 20 to the
`telephone line 17. The interface circuit 20 is preferably
`of the type described in a co-pending patent application
`Ser. No. 247,229 filed on Mar. 25, 1981 by T. H. Murto
`and S. D. Alvey, entitled “Interface Circuits for Con-
`nection to Non-Dedicated Telephone Lines”, the entire
`disclosure of which is hereby incorporated herein by
`reference. The unscrambler 19 and interface circuit 20
`constitute a home terminal HT.
`The pay TV system also includes a control and data
`collection centre (DDC) 21, which can be located at the
`program source 10, scrambler 11, and transmitter 12, or
`at the telephone central office 18, or separately from
`both as is assumed here. In any event the centre 21 is
`coupled via an appropriate interface 22 to the telephone
`central office 18 for communicating recurrently with
`each unscrambler 19 via the relevant telephone line 17,
`and is coupled, either permanently or when required via
`a telephone or data transmission line, to the scrambler
`11 for supplying encryption and program data thereto.
`As an alternative, which is not further described here
`but which will be readily understood by those skilled in
`the art, the program data may instead originate in the
`program source 10, and be supplied from there to both
`the scrambler 11 and the centre 21.
`The centre 21 recurrently, for example monthly,
`supplies data to the scrambler 11 as described below to
`control the scrambling of Video signals from the pro-
`gram source 10. The scrambling and unscrambling can
`be effected in any of numerous different known ways,
`but are preferably effected in the manner described in
`the copending application by J. A. Bond et al. already
`referred to. As also described in that application, data
`relating to the scrambling and to the program material
`is separately transmitted simultaneously with the scram-
`bled video signal. The centre 21 also recurrently, for
`example monthly, supplies via the interface 22, the tele-
`phone central office 18, and the interface 20 information
`which as described below is used by the unscrambler 19
`to enable unscrambling of the video signals for repro-
`duction thereof on the television receiver 15.
`The scrambling and unscrambling are effected under
`the control of a first variable DK, a second variable PD,
`and a third variable ICK. The first and third variables
`DK and ICK change recurrently with relatively long
`periods between changes. For example, these variables
`are changed monthly. The second variable PD changes
`recurrently at a much higher rate; for example this
`variable changes from one field of the video signal to
`the next. In order to identify uniquely each unscrambler
`19, each home terminal HT is assigned a subscriber
`number SN in dependence upon which all information
`transmitted between the DCC and the respective HT is
`encoded. Similarly,
`information transmitted between
`the DCC and the VBC is encoded in dependence upon
`a number VN.
`Referring to FIG. 2, the DCC includes a true random
`number generator 201 from the output of which the first
`variable DK and the third variable ICK are derived and
`securely stored in a store 202. These variables are thus
`produced completely randomly. A different first vari-
`able DK is produced and stored for each of a plurality
`
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`4,388,643
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`5
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`5
`6
`of groups SG of television programs, so that subscribers
`the variable DK for each group SG of television pro-
`grams to which the relevant subscriber subscribes, are
`can subscribe selectively to different groups or types of
`television programs. A generator 203 provides data SG
`_ supplied from the store 202, encoded by the encoder
`" 209 and decoded by the decoder 403 in accordance with
`identifying each such group, and program, billing, and
`the subscriber number SN supplied from the stores 204
`control data BCD relating to the respective programs.
`and 401 respectively, and are stored in the store 401. In
`A store 204 stores the number VN, which is supplied
`this manner, information is transmitted in both direc-
`to an encoder 205 which encodes the data BCD and SG
`tions between the DCC and the HT, encoded in accor-
`from the generator 203 and the variables DK and ICK
`dance with the subscriber number SN to make it secure.
`in dependence upon this number for recurrent transmis-
`For unscrambling a scrambled video signal, the infor-
`sion of this data to the VBC. In the VBC, as shown in 10
`mation BCD, SG, PD, and IC transmitted from the
`FIG. 3, this encoded information is decoded by a de-
`VBC for each field of the video signal is stored for each
`coder 301 which is supplied with the number VN from
`field in a register 405 in the HT. The data BCD is sup-
`a store 302. The decoded data BCD and SG are stored
`plied from the register 405 and stored in the store 404 to
`in respective parts of a register 303, and the decoded
`variables DK and ICK are stored in the store 302. In 15 provide billing information for later transfer to the bill-
`this manner, the variables DK and ICK are supplied
`ing data store 211 as described above. In addition, a
`securely from the DCC to the VBC.
`logic circuit 406 is supplied with the variable PD and
`A pseudo-random number generator 304 in the VBC
`the data BCD and SG from the register 405, and com-
`generates a pseudo-random number which constitutes
`bines and encodes these in accordance with the variable
`the second variable PD, which is stored in a further part 20 ICK supplied from the store 401 to produce further data
`of the register 303. A logic circuit 305 is supplied with
`for checking the integrity of the data in the register 405.
`the variable PD and the data BCD and SG which are
`This further data is checked against the data IC from
`stored in the register 303, and combines and encodes
`the register 405 in a checking circuit 407.‘For example,
`these in accordance with the variable ICK supplied
`the logic circuit 406 can be identical to the logic circuit
`from the store 302 to produce integrity check data IC, 25 305 in the VBC so that this further data is the data IC,
`which is stored in a further part of the register 303. The
`in which case the checking circuit 407 can be a data
`data stored in the register 303 is transmitted each field
`comparator. Alternatively, the circuit 407 could pro-
`of the video signal to each home terminal HT.
`duce the complement of IC and the circuit 407 could
`The variable PD, delayed by the duration of one
`comprise a gating circuit.
`video field in a delay unit 306 to allow time for process- 30
`If the checking ciruit 407 establishes the proper integ-
`ing the information in the register 303, is supplied to a
`rity of the data in the register 405, then via a lead 408 it
`control logic unit 307. The unit 307 is also supplied with
`enables a control logic unit 409. The unit 409 is supplied
`the relevant variable DK for the type of video signal
`with the variable PD from the register 405, delayed in a
`(television program group) which is to be scrambled,
`delay unit 410 by the duration of one field of the video
`which is supplied from the store 302 which in turn is 35 signal to allow time for processing the data in the regis-
`addressed by the data SG stored in the register 303. The
`ter 405, and with the relevant variable DK which is
`unit 307 produces from the variables DK and PD 21
`read out from the store 401 which is addressed by the
`scrambling control signal on a line 308, in accordance
`data SG in the register 405. When enabled, the unit 409
`with which the video signal is scrambled. The scram-
`produces on a path 411 an appropriate unscrambling
`bling, and also the data transmission, are preferably 40 control signal for correct unscrambling of the relevant
`effected in the manner described in the copending appli-
`video signal field.
`cation by J. A. Bond et al. alreadly referred to.
`If the checking circuit 407 detects an error in the data
`The store 204 in the DDC also stores the subscriber
`for a particular field, then via a line 412 it triggers a
`number SN of each HT. Recurrently, for example
`pseudo-random number generator 413 to supply to the
`monthly, a polling control unit 206 in the DDC estab- 45 delay unit 410 a correct version of the variable PD,
`lishes communication with each HT via an I-IT selection
`rather than the variable PD from the register 405. To
`path 207, and via the telephone central office 18 as
`this end, the generator 413 is identical in form to the
`described with reference to FIG. 1. The polling control
`corresponding generator 304 in the VBC, and is also
`unit 206 also addresses the store 204 via a path 208 to
`synchronized by the variable PD being supplied thereto
`read out to an encoder 209 and a decoder 210 the sub- 50 from the register 405 so that the generator 413 can at
`scriber number SN of the HT with which communica-
`any time produce the correct variable PD. In addition,
`tion is established. Correspondingly, the number SN in
`the checking circuit 407 supplies via a line 414 an indi-
`the relevant HT (FIG. 4) is supplied from a store 401, in
`cation to the store 404 that an error has occurred; such
`which it is permanently stored, to an encoder 402 and a
`indications are accumulated in the store 404, an error
`decoder 403.
`55 count being incremented in response to each error indi-
`During communication between the DCC and the
`cation, and the accumulated error count is read and
`HT, billing information from a store 404 in the HT is
`reset by the DCC simultaneously with the transfer of
`encoded by the encoder 402 in dependence upon the
`billing information from the store 404 to the store 211.
`number SN from the store 401,
`is transmitted to the
`In the event that the error count reaches a predeter-
`DCC, and is decoded by the decoder 210 in dependence 60 mined value, indicating a fault or interference with the
`upon the same number SN from the store 204. The
`equipment, the store 404 provides a signal viaaline 415
`decoded billing information is stored in a billing data
`to inhibit the control logic unit 409 from further pro-
`store 211, which via a path 212 informs the polling
`duction of the proper unscrambling control signal. This
`control unit 206 of this. The billing data store 211 can be
`signal on the line 415 can also be provided in response to
`accessed by means not shown, as required for billing the 65 an indication which can be entered into the store 404 in
`subscriber for pay TV programs which have been un-
`the event that the unscrambling equipment is tampered
`scrambled. Furthermore, during the communication
`with in any way, for example if an equipment cover is
`between the DCC and the HT, the variable ICK, and
`opened.
`
`PMC Exhibit 213
`
`Apple v. PM
`|PR2016-0075
`
`Page
`
`PMC Exhibit 2134
`Apple v. PMC
`IPR2016-00755
`Page 8
`
`

`
`7
`It will be seen from the foregoing description that the
`invention provides a particularly secure manner of con-
`trolling scrambling and unscrambling in a pay TV sys-
`tem. The security of the system may, however, be fur-
`ther enhanced by protecting the contents of the stores in
`the DCC from unauthorized access, and by changing
`the variables DK and ICK more frequently. The size
`(e.g. number of bits) of these variables and the nature of
`the logic functions 305, 307, 406, and 409, as well as the
`size of the numbers VN and SN and the nature of the
`encoding and decoding functions, can obviously be
`selected arbitrarily to suit particular requirements.
`Whilst the use of the variable ICK and the resultant
`production of the data IC to check the integrity of the
`data transmitted by the VBC has been described above,
`it should be appreciated that these features could be
`dispensed with without departing from the broadest
`aspects of this invention as claimed. Furthermore, the
`variables DK and ICK could obviously be stored in the
`VBC and/or the HT in their encoded forms if desired,
`and be decoded when required. The various logic cir-
`cuits and encoding and decoding functions can be im-
`plemented using either hardware or software. Numer-
`ous other modifications, variations, and adaptations
`may be made to the described embodiment without
`departing from the scope of the invention as defined in
`the claims.
`I claim:
`
`1. A method of controlling scrambling and unscram-
`bling 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 sub-
`scriber 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 sig-
`nal 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 genera-
`tor.
`
`5. A method of controlling scrambling and unscram-
`bling 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;
`
`10
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`15
`
`20
`
`25
`
`30
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`35
`
`45
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`50
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`55
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`60
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`65
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`4,388,643
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`8
`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 vari-
`ables;
`transmitting the second variable and the check data
`to the subscriber station simultaneously with trans-
`mission 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 depen-
`dence 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 genera-
`tor.
`
`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, synchroniz-
`ing the production of said second variable at the sub-
`scriber station with said transmitted second variable,
`and unscrambling the video signal in dependence upon
`the decoded stored first variable and the second vari-
`able 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 sub-
`scriber 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.
`11. A method asclaimed in claim 1 or S and compris-
`ing 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 repre-
`senting the type of television program to which the
`scrambled video signal relates; and
`at the subscriber station, selecting the respective de-
`coded stored first variable in dependence upon said
`data representing the type of television program
`for unscrambling the video signal.
`*
`Ill
`4*
`IR
`It
`
`PMC Exhibit 213
`
`Apple v. PM
`IPR2016-0075
`
`Page
`
`PMC Exhibit 2134
`Apple v. PMC
`IPR2016-00755
`Page 9