United States Patent
`Hartung et al.
`
`A
`
`3,919,462
`[lll
`[45] Nov. 11,1975
`
`[751
`
`METHOD AND APPARATUS FOR
`SCRAMBLING AND UNSCRAMBLING
`COMMUNICATION SIGNALS
`Inventors. Albert F. Hartung. Woodland Hills;
`Frank W. [.ehan. Santa Barbara;
`Charles T. Barooshian. Paci?c
`Palisades; Edward J. Zacharski.
`Malibu. all of Calif.
`Assignee: System Development Corporation.
`Santa Monica. Calif.
`
`Aug. 15. 1973
`Filedi
`Appl. No: 388.439
`
`[SZl US. Cl. .......... .. l78/5.l; l78/DlG. l3; l78/Z2;
`325/34
`[51] Int. Cl.2 .......................................... .. H04N 1/44
`
`[58] Field Of Search . . . .
`
`.
`
`.
`
`.
`
`. . . . .
`
`.
`
`. .. l78/5.l. DlG. l3, 22;
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`325/34
`
`Assistant E.\uminer-—S. C. Buczinski
`Attorney. Agent. or Firm-Fulwider. Patton. Rieber
`Lee <82 Utecht
`
`ABSTRACT
`[57]
`Method and apparatus for scrambling and unscram
`bling television video and audio signals in a subscrip~
`tion television system in which program selections of
`subscribers are known at a central transmitting site.
`and control signals encoded into vertical blanking in
`tervals 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 con
`trol signals are decoded. and. if addressed to the par
`ticular subscriber‘s receiver. operate to enable or dis
`able the unscrambler. or to frequently vary its mode of
`operation. thereby greatly increasing the security of
`the system and deterring viewing of scrambled trans
`missions. Video scrambling and unscrambling are of‘
`fected by inversion of selected horizontal lines of a
`transmitted television picture. and possible modes of
`scrambling and unscrambling include inversion of al
`ternate groups of equal numbers of lines. inversion or
`non-inversion selected on a linc-by-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.
`52 Claims.‘ 16 Drawing Figures
`
`Coll/Ital
`0,474
`
`main/viz E»?
`
`{56]
`
`3.106.604
`3184.537
`3.527.877
`3.538.243
`3.732.355
`
`References Cited
`UNlTED STATES PATENTS
`M71963 Shanahan.
`5/1965
`Court er a.
`W197i) Walker.
`l H1970 Shanahan et al.
`571973
`Harrla el al.
`
`.
`
`..
`
`.. l78/5.l
`ivs/si
`.. l78/S.l
`.......... .. l78/5.l
`l78/5.l
`
`3.757.225
`
`9J'l‘973
`
`Ulicki
`
`..
`
`. . .
`
`.
`
`.
`
`.
`
`. .. [7815.1
`
`3.777.053
`3.789.13l
`
`12/1973
`l/l974
`
`Wiltig et al.
`Harrie} ..
`
`[78/5]
`.
`................. .. l78/S.l
`
`3.790.700
`
`2/1974
`
`Callais et a]. .
`
`.
`
`.
`
`.
`
`. . . . .
`
`.
`
`. . . . . .
`
`. . . . .. l78/5.l
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`3.801.732
`
`4/1974
`
`Reeves. .
`
`. . . .
`
`.
`
`. . .
`
`.
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`.
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`.
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`.
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`.
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`. . . . . . . . .. l78/5.l
`
`W050
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`Page 1
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`Amazon Ex. 1024
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`IPR2014-01532
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`

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`U.S. Patent
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`Nov. 11, 1975
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`U.S. Patent
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`Nov. 11, 1975
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`

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`US. Patent Nov.11, 1975
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`Sheet4of4
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`

`
`1
`
`METHOD AND APPARATUS FOR SCRAMBLING
`AND UNSCRAMBLING COMMUNICATION
`SIGNALS
`
`BACKGROUND OF THE INVENTION
`This invention relates generally to techniques for
`scrambling and unscrambling television signals, and,
`more particularly, to improved scrambling and un
`scrambling techniques applied to a subscription televi
`sion system controllable by a central computer.
`In subscription television, or “pay-TV.” systems,
`subscribers select programs that they wish to view, and
`pay to have those programs transmitted to their televi~
`sion receivers, usually along a coaxial cable. One re
`quirement for such systems is that the transmitted sig
`nals should be unintelligible to non-subscribers or to
`subscribers who have not paid for a particular program.
`Various methods have been suggested for scrambling
`video signals, such as by inserting time delays, or by in
`verting portions of the video signals so that white and
`black images are reversed on portions of the television
`screen.
`The success of a particular scrambling technique de
`pends, ?rst of all, on whether a program is suf?ciently
`scrambled to deter unauthorized viewers from watch
`ing it in a scrambled condition, and secondly, on how
`dif?cult it is for a resourceful viewer to circumvent the
`protection provided by the scrambling techniques.
`Some prior systems provide for limited variation of
`the mode of scrambling and unscrambling, these gener
`ally requiring the insertion of a coded card, or the like.
`to correctly unscramble the signals. However, there has
`long existed a need for a scrambling technique in which
`security can be maximized by rapidly and automatically
`varying the scrambling mode, without the need for
`manual intervention by the subscribers, and which will
`effectively deter viewers of the scrambled television
`picture. The present invention ful?lls this need.
`
`SUMMARY OF THE INVENTION
`The present invention resides in a method and appa
`ratus for scrambling and unscrambling television sig
`nals, wherein the mode of scrambling and unscram
`bling may be varied automatically and continually in
`order to increase the security of the system and to deter
`unauthorized viewers. Brie?y, and in general terms, the
`method of the invention, as it relates to unscrambling at
`a receiver, includes the steps of receiving encoded con
`trol signals and encoded identi?ers of authorized re
`ceivers along with scrambled television signals, decod~
`ing the encoded control signals and identi?ers, compar
`ing the received identi?ers with a unique identi?er as
`sociated with the receiver, and, if the comparison re
`sults in a match, automatically unscrambling the televi
`sion signals in accordance with an unscrambling mode
`contained in the control signals.
`The invention is particularly well suited for use in a
`subscription television system in which the subscribers
`select programs in advance by direct telephone or
`other communication with a central computer. The
`computer controls a transmitter which can address
`control signals to unscrambling equipment at each sub
`scriber’s receiver by means of a unique identi?er asso
`ciated with that equipment. Thus, the unscrambling
`equipment of those subscribers who have selected a
`particular program can be conditioned to unscramble
`the program signals, and to receive subsequent control
`
`3,919,462
`2
`signals relating to changes in the scrambling mode. It
`will be apparent that the transmitted signals have to be
`scrambled according to the same mode as that used in
`unscrambling, and that scrambling and unscrambling
`have to be completely synchronized.
`There is an unscrambling means, or unscrambler, at
`each subscriber’s receiver, and it basically includes de
`coding means, to decode the control signals and en
`coded identi?ers, identi?er comparison means, to ac
`cept only control signals intended for the particular re
`ceiver, and scramble decoder means, to unscramble the
`television signals in accordance with an unscrambling
`mode contained in the control signals. The audio por
`tion of a television signal may also be scrambled, and
`the unscrambler may include means for unscrambling
`these audio signals at the receiver site.
`More speci?cally, in a presently preferred embodi
`ment of this invention, video signals are scrambled by
`the inversion of some of the horizontal lines making up
`a television picture. This has the disconcerting effect of
`reversing the black and white portions of the inverted
`lines, in a black and white picture, or inverting the
`color spectrum in a color picture. The scrambling
`mode at any instant may be such that, for example, the
`inverted lines form patterns of regularly or irregularly
`spaced bars across the picture, and the bars may be
`made to roll up or down. Furthermore, the scrambling
`mode may be changed at a rapid rate, producing an al
`most in?nite variety of moving patterns of inverted
`lines on the screen if the signals are not unscrambled
`prior to video display.
`The scrambled video signals produced by inversion
`of some of the horizontal picture lines are unscrambled
`at each authorized receiver by one of the unscramblers,
`which are functionally complementary to scrambling
`means at the transmitter. In the preferred embodiment,
`receiver identi?ers and control signals are encoded into
`a conventionally formed, composite video and synchro
`nization signal, speci?cally in those portions of the
`video and synchronization signal relating to vertical
`blanking intervals, during which a conventional televi~
`sion picture tube has its electron beam returned to the
`top of the tube after scanning a complete ?eld of the
`picture.
`In the unscrambler at a particular receiver, these
`identi?ers and control signals are decoded, and the
`identi?ers are compared with the unique identi?er as
`sociated with the receiver. If a match is found, the con
`trol signals are further decoded and applied in the un
`scrambler to enable or disable unscrambling, to change
`the mode of unscrambling, or to select a particular pro
`gram on a separate frequency channel. If no match is
`found, the control signals have no special meaning for
`the receiver in question. However, a special “all-call"
`identi?er is available to allow all receivers in the system
`to be controlled, regardless of whether or not they have
`been individually addressed to enable unscrambling.
`Also, certain control signals have meaning for all re
`ceivers which have been previously speci?cally ad
`dressed to enable unscrambling operations. In particu
`lar, an unscrambler synchronization signal, used to syn
`chronize scrambling and unscrambling, is in this latter
`category, and, in one embodiment, the unscrambling
`mode for all enabled unscramblers may be changed by
`control signals not associated with a particular identi
`?er.
`In one of three alternative embodiments of the inven
`tion, the scrambling of unscrambling mode depends on
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`

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`3,919,462
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`3
`the selection of a digit from a plurality of digits in a
`counter used to count horizontal picture lines transmit
`ted or received. The video signal is then inverted. for
`scrambling or unscrambling, only when the selected
`digit is in a particular state.
`In another of the three alternative embodiments.
`each horizontal line of video information is transmitted
`with an encoded signal indicating whether the line is
`inverted or not. The unscrambler decodes this signal
`and accordingly unscrambles the video signals.
`In a third alternative embodiment. the decision
`whether or not to invert a particular line being scram
`bled or unscrambled is derived from the contents of a
`register, which is itself scrambled in a predetermined
`manner while the previous line is being transmitted or
`received. Different scrambling modes may be estab
`lished by storing different starting patterns in the regis—
`ter.
`Scrambling of the television signal may also include
`scrambling the audio portion of the signal, by some
`means. to further deter unauthorized persons from
`watching a scrambled transmission. Conventionally.
`the audio signal is transmitted on a frequency
`modulated carrier spaced from the video carrier by a
`preselected frequency difference. In the presently pre
`ferred embodiment of the invention, the audio carrier
`is shifted away from the video carrier so that the magni
`tude of the difference between the video and audio car»
`riers is increased, thus preventing detection of the
`audio signal in a normally aligned receiver. Unscram»
`bling is effected by a corresponding frequency shift in a
`downward direction.
`It will be appreciated from the foregoing that the
`present invention signi?cantly advances the state of the
`art of scrambling and unscrambling television signals in
`subscription television systems. In particular, since the
`invention is operable to vary the scrambling mode rap
`idly and automatically, it provides greatly increased sc
`curity from unauthorized unscrambling of signals in
`tended only for certain subscribers, without the neces
`sity of subscriber identification by manual means.
`Moreover. the scrambling mode may be selected and
`varied to deter most unauthorized viewers from watch
`ing the scrambled video patterns, especially since the
`audio signal may also be unavailable to the unautho
`rized viewers. Other aspects and advantages of the in
`vention will become apparent from the following more
`detailed description taken in conjunction with the ac
`companying drawings.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a block diagram showing the subsystem
`components of a subscription television system in
`which the invention might be used;
`FIG. 2 is a block diagram showing an unscrambler
`which may be employed in the system of FIG. I, and
`showing how the apparatus of the invention might be
`connected with the system;
`FIG. 3 is a more detailed block diagram of unscram
`bler logic employed in the unscrambler of FIG. 2;
`FIG. 4a is a time-amplitude graph of a conventional,
`composite video and synchronization signal;
`FIG. 4b is a graph similar to that in FIG. 4a, in which
`the video signal portions have been inverted;
`FIG. 5a is a time-amplitude graph of a composite
`video and synchronization signal, showing control sig
`nals encoded into the vertical blanking interval;
`
`4
`FIG. 5bis a time-amplitude graph of a "stretched"
`vertical synchronization pulse derived from the signal
`of FIG. 50;
`FIGS. 6u-e are time-amplitude graphs of various tim
`ing and data signals. and together comprise a timing di
`agram relating to the operation of the unscrambler
`logic of FIG. 3;
`FIGS. 7-9 are block diagrams illustrating three alter
`native embodiments of a scramble decoder which may
`be included in the unscrambler logic of FIG. 3; and
`FIG. 10 illustrates, by way of example, one possible
`form of the accumulator scrambler logic included in
`the alternative embodiment of FIG. 9.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`As shown in the drawings for purposes of illustration,
`the invention is particularly well suited for use in a
`computer controlled subscription television system, the
`principal components of which are shown in FIG. I. In
`such a system. signals from a television program source
`12, such as a television camera or a television network,
`are transmitted to paying subscribers, typically. but not
`necessarily, by means of a coaxial cable 13. In order to
`prevent non-subscribers and subscribers who have not
`paid for a particular program from receiving the trans
`mission, the signals from the television program source
`I2 are processed by a scrambler-encoder 14, which
`modifies the signals in some fashion to make them un
`intelligible to an unauthorized receiver. A modulator
`15 uses the signals from the scrambler-encoder 14 to
`modulate a high frequency carrier for transmission
`along the cable I3. The modulator 15 is conventional
`except that its audio portion is realigned to scramble
`audio signals by shifting the audio carrier and thereby
`increasing the frequency difference between the video
`and audio carriers.
`Each subscriber to the system has a conventional tel
`evision receiver 16, and is supplied with an unscram~
`bler 17 connected between the cable 13 and the re
`ceiver. The unscrambler 17 may have associated with it
`a converter 20 for converting specially assigned carrier
`frequencies, used for transmission over the cable, to a
`frequency corresponding to an unused numbered chan
`nel to which the receiver 16 can be tuned. Although
`only one receiver 16 is shown in FIG. I, it will be appre
`ciated that. in general, a number of separate receivers
`will be connected to the cable 13.
`In the system illustrated, a central computer I8 is
`used to maintain records of available programs and of
`programs selected by the subscribers. Each subscriber
`selects the programs he wishes to view (indicated by
`the block 19), and conveys his selections to the central
`computer 18 by means of a telephone 21. The selec
`tions may be communicated to the computer 18 di
`rectly by means of some digital attachment (not
`shown) acoustically coupled to the telephone 21, or
`may be input to the computer by an operator in voice
`communication with the subscribers. Alternatively,
`there may be a reverse communication path along the
`cable 13 to the computer I8, so that a subscriber may
`select programs by operating switches or buttons (not
`shown) at his television receiver 16. However the pro
`grams are selected is of little consequence so far as the
`present invention is concerned, so long as there is some
`means to determine which subscribers are authorized
`to receive various programs. The scrambler-encoder
`14, which is also connected to the computer 18, typi
`
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`Page 7
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`5
`cally by a telephone line 22, may then be directed to
`encode appropriate unscrambler control signals for
`transmission with the conventional television signals.
`The present invention is principally concerned with
`improved techniques employed in the scrambler
`encoder 14 and the unscrambler 17, and FIG. 2 illus
`trates in block diagram form the unscrambler 17 used
`in a presently preferred embodiment of the invention.
`It will be appreciated, however, that, in systems involv
`ing signal scrambling and unscrambling, the techniques
`used in unscrambling are functionally complementary
`to those used in scrambling. Consequently, although
`only the unscrambler 17 is described in detail herein, it
`will be understood that complementary techniques are
`used in the scrambler-encoder 14, and that these tech
`niques will be readily apparent to those of ordinary skill
`in the art.
`As will be apparent from FIG. 2, many elements of
`the unscrambler 17 are conventional in the television
`electronics art, and these are described herein only for
`the purpose of illustrating the environment in which the
`improvements constituting the invention will operate.
`The scrambled television signals from the cable 13
`(FIG. 1) are input to the unscrambler 17 through an
`input terminal 25 and processed by a conventional
`mixer 26 connected with a conventional oscillator 27
`and channel selector 28. The output from the mixer 26
`is an intermediate frequency (IF) signal, which, as
`shown by the line 29, is connected to conventional cir
`cuitry performing the functions shown in block 31, in
`cluding IF ampli?cation, video detection, automatic
`gain control, and audio IF ampli?cation. As shown in
`FIG. 2, the output from these conventional circuits
`grouped in block 31 includes a composite video and
`synchronization signal, which is still in scrambled form,
`along line 32, and an audio IF signal, along line 33.
`The scrambled video and synchronization signal on
`Iinee 32 is gated through one of two parallel paths 34
`and 35 including an inverting ampli?er 36 and a non‘
`inverting ampli?er 37, respectively, and controlled by
`conventional gating circuits 38 and 39,, respectively.
`When the gate 39 is open and the gate 38 is closed, the
`composite video and synchronization signal is not in
`verted and appears, for example, like the signal shown
`in FIG. 4a. However, when the gate 39 is closed and the
`gate 38 opened for the video portions of the composite
`signal, the video portions are inverted, as shown in FIG.
`4b. The composite video and synchronization signal on
`line 32 is also input over line 41 to a synchronization
`separator 42, which uses techniques well known in the
`art to separate the conventional television synchroniza
`tion signals from the composite signal, and to transmit
`these along lines 43 and 44 to unscrambler logic 45, the
`detail of which is central to this invention, and will be
`discussed herebelow in connection with FIG. 3.
`The unscrambler logic 45 receives control signals en
`coded in the composite video and synchronization sig
`nal along line 46, and operates to generate two basic
`output control signals: and “invert" or “non-invert"
`signal on lines 47 and 48, respectively, connected to
`the gating circuits 38 and 39 to control inversion or
`non-inversion of the video signal, and an “unscramble
`on" or “off " signal on lines 49 and 51, respectively,
`connected to additional gating circuits S2 and 53, re
`spectively, to control audio unscrambling. The audio IF
`signal on line 33 takes one of two parallel paths 54 and
`55 as determined by the gating circuits 52 and 53, the
`path 54 passing through an audio unscrambler 56 be—
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`fore merging with the alternate path 55 and being input
`over line 57 to conventional oscillator and modulator
`circuits 58. The composite video and synchronization
`signal, as unscrambled under the control of the un
`scrambler logic 45, is also input to the oscillator and
`modulator 58, over line 59, and is there used. together
`with the audio signal input over line 57 to modulate a
`high frequency carrier signal in a conventional manner.
`The carrier signal is output from the unscrambler 17 to
`the receiver 16 through an output terminal 61.
`It has been proposed that subscription television sys
`tems be assigned so-called “mid-band” frequencies be
`tween the frequencies assigned to numbered channels
`in the very high frequency (VHF) range. If this were
`the case, and if the receiver 16 (FIG. I) were not
`equipped to receive these mid-band channels, the un
`scrambler illustrated in FIG. 2 would also operate as a
`frequency converter, i.e., it would be tuned to receive
`one of the mid-band frequencies, while the oscillator
`and modulator 58 would be tuned to output a signal at
`a frequency corresponding to an unused numbered
`channel, to which the receiver 16 could be tuned to re
`ceive the mid-band channels.
`The techniques of scrambling and unscrambling tele
`vision signals as thus far generally described with refer
`ence to FIGS. 1 and 2, while not particularly well
`known, are not believed to be novel, but are believed to
`require some emphasis in this speci?cation in order to
`de?ne the environment in which the present invention
`operates, and to convey an appreciation of its novel as
`pects and advantages. The present invention is princi
`pally concerned with improvements in the techniques
`of scrambling and unscrambling as speci?cally embod
`ied in more detailed aspects of the unscrambler logic
`45 (FIG. 2).
`In accordance with the present invention, the mode
`by which the scrambling and unscrambling operations
`are performed may be varied automatically and rapidly
`in order to increase the security of the system and to
`deter unauthorized viewing. In brief, the scrambler~
`encoder 14 (FIG. 1) encodes into the television signal
`control signals addressed to a particular unscrambler
`l7 and directing it to initiate or terminate unscrambling
`operations, to change the mode of unscrambling, or to
`tune to a different incoming program. The unscrambler
`17 (FIG. 1), and more speci?cally, the unscrambler
`logic 45 (FIG. 2), operate to decode the control signals
`and to perform the appropriate control function if it is
`addressed to the unscrambler 17 in question. If a sub
`scriber has not paid or been charged for a particular
`program, the unscrambler 17 will not be directed to un
`scramble the program, which can be viewed, therefore,
`only in scrambled form. Since the scrambler~encoder
`14 can be controlled to select a scrambling mode which
`results in extremely disconcerting patterns on the re
`ceiver 16, most unauthorized viewers are deterred from
`viewing a scrambled program. Furthermore, the system
`has a high degree of security, because the mode of
`scrambling may be rapidly varied in a practically ran
`dom fashion.
`More speci?cally, the scrambler-encoder 14 encodes
`control signals in that portion of the normal composite
`' video and synchronization signal known as the vertical
`blanking interval. As is well known, a television picture
`is conventionally made up of a number of horizontal
`lines traced by an electron beam of varying intensity,
`usually from the top to the bottom of the picture. A full
`frame of the picture typically consists of two interlaced
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`?elds, each tracing alternate lines in the picture. The
`composite signal producing the trace of a ?eld of the
`picture comprises, as can be seen in FIG. 5a. a video
`signal 71 and a succession of horizontal synchroniza
`tion pulses 72 used to control transition of the beam
`from one line to the next. Between successive ?elds of
`the picture, there is a vertical blanking interval during
`which the beam is blanked out and positioned for the
`start of the next ?eld. The vertical blanking interval
`conventionally includes a group of equalizing pulses
`73, some wider vertical synchronization pulses 74, a
`further group of equalizing pulses 75, followed ?nally
`by a number of horizontal synchronization pulses 76
`before the ?rst line of video information in a new ?eld.
`Using a widely known technique, the control signals
`to be transmitted to the unscrambler 17 (FIG. 1) are
`encoded between the horizontal synchronization pulses
`76 which occur towards the end of the vertical blanking
`interval, as shown at 77. The technique is similar, for
`example, to one used by television networks for encod
`ing time-of-day signals into the vertical blanking inter
`val. In the presently preferred embodiment, there are
`three “lines” of control signals, each coded in binary
`digital form as a series of pulses, and each line of signals
`being addressed to a particular unscrambler 17 (FIG.
`1). They may conveniently be thought of as “lines" of
`signals or data, since they appear between horizontal
`synchronization pulses in much the same way as lines of
`video information. However, it will be understood that
`the control signals occur between ?elds of the picture
`and are not normally displayed as video signals. In any
`one vertical blanking interval, control signals may be
`transmitted to up to three separate unscramblers 17,
`using all three “lines”, and, since there are 60 vertical
`blanking intervals per second in television systems in
`the United States, up to 180 separate unscramblers
`may be addressed per second. It will be appreciated
`that a greater number of receivers can be addressed by
`using more “lines" of the vertical blanking interval or
`encoding control signals for more than one unscram
`bler in a single “line”. In the unused line immediately
`following the control signals, an unscrambler synchro
`nization signal 78 is encoded from time to time. This
`signal, as will be subsequently discussed in detail, is re
`quired to synchronize operations of the scrambler
`encoder 14 (FIG. 1) and the unscrambler 17.
`The unscrambler logic 45 (FIG. 2) receives the com
`posite video and synchronization signal over the line
`46, this signal including the control signals encoded as
`illustrated and discussed with respect to FIG. 5a. In de
`coding these control signals, the unscrambler logic 45
`utilizes horizontal and vertical synchronization pulses
`separated from the video signal by the synchronization
`separator 42 and transferred to the unscrambler logic
`45 along the lines 43 and 44 respectively. It will be ap
`preciated from FIG. 50, that there is no single vertical
`synchronization pulse as such, but rather a series of
`pulses during the vertical blanking interval. The verti
`cal synchronization pulse transmitted along the line 44
`is termed a “stretched" vertical synchronization pulse
`and is developed in the synchronization separator 42,
`and illustrated in FIG. 5b. It will be seen that the
`stretched vertical synchronization pulse begins after
`the ?rst group of equalizing pulses 73 in the vertical
`blanking interval, and ends after the final group of
`equalizing pulses 75 and immediately before resump
`tion of the normally spaced horizontal synchronization
`pulses 76.
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`As will be seen, the stretched vertical synchroniza~
`tion pulse 79 is utilized in the unscrambler logic 45 in
`the decoding ofthe control signals 77 (FIG. 5a) and the
`unscrambler synchronization signals 78. It should fur
`ther be noted that the horizontal synchronization signal
`developed in the synchronization separator 42 (FIG.
`2), and transmitted to the unscrambler logic 45 along
`the line 43, is also a “stretched" horizontal synchroni
`zation pulse, including the so-called “front porch‘~ and
`“back porch" portions of the conventional horizontal
`synchronization pulse. as well as a color burst signal in
`cluded in color television transmissions.
`The unscrambler logic 45 (FIG. 2) will now be de
`scribed in greater detail with reference to FIG. 3. Basi
`cally, the logic illustrated in FIG. 3 operates to receive
`control signals encoded in the composite video and
`synchronization signal, at the terminal 85, to decode
`those control signals, utilizing the stretched vertical
`synchronization pulse input at 86 and the stretched
`horizontal synchronization pulse input at 87, and, if the
`control signals are addressed to the unscrambler in
`question, to place the decoded control signals in a con
`trol register 88. A scramble decoder 89, alternative em
`bodiments of which will be discussed with reference to
`FIGS. 7-9, then uses the contents of the control regis
`ter 88, along with other available signals, to generate
`the “invert" or “non-invert” signal, as shown at 91.
`This signal and its inverse, produced by an inverter 90,
`are the signals transmitted over the lines 47 and 48, re
`spectively, in FIG. 2, and as will be recalled from the
`description of FIG. 2, these signals are used to control
`the gating circuits 38 and 39 (FIG. 2) and thereby to
`unscramble the scrambed video signals.
`More speci?cally, the streched horizontal synchroni
`zation pulses input at 87 are fed to a horizontal syn
`chronization pulse counter 92, which is a conventional,
`multi-stage, binary counter, arranged to have all of its
`stages reset to zero by a falling vertical synchronization
`pulse as introduced over line 93. The horizontal syn
`chronization pulse counter 92 has the states of its vari
`ous stages connected, as shown by line 94, to a horizon
`tal synchronization count decoder 95, which uses con
`ventional logic to compare the current setting of the
`ho