Abstract:
Methods, apparatus and computer program products for authenticating the right of a user to descramble and view scrambled television channels are provided. The set top box can use newer and faster technology to tune and descramble television channels and the right of the user to descramble those channels is authenticated by periodically examining all the television channels output by a cable box. If the cable box descrambles a particular channel, the right of the user to descramble that channel has been authenticated or verified and the set top box is enabled to descramble the same channel. As an additional security feature, the time base of the television channel output by the cable box can be compared to the time base of the same channel tuned and descrambled with the set top box. If the time bases match, then the right of the user to descramble and view that particular channel is authenticated. Once the right of the viewer to descramble a particular channel is authenticated, the channel is tuned and descrambled using the newer technology of the set top box. In this manner, the set top box can descramble and tune scrambled television channels without compromising the rights of the company supplying the scrambled television signal.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to descrambling television signals. More specifically, the present invention relates to descrambling scrambled television signals at an electronics device in an entertainment system using a cable box to verify that the viewer has been granted access to the descrambled signals. 
     2. The Prior State of the Art 
     In today&#39;s society, people are spending more time watching television. The demand for more television has led to the creation and development of television channels dedicated to news, sports, movies and special events. A few of these channels, including those affiliated with the major networks, are free and viewers can receive these channels by simply erecting and connecting an antenna to their television. The downside of using an antenna is that only a few channels can be received and the reception is not always clear. In order to receive a larger selection of television channels, the viewer must subscribe to services offered by a cable or satellite television provider. 
     When a viewer subscribes to television services from a cable or satellite operator, the viewer typically selects a television channel package. The least expensive package usually contains a basic set of television channels, while the more expensive packages contain premium television channels, examples of which include movie channels and pay-per-view programming. Premium channels are typically scrambled and consumers pay an additional amount in order to be able to descramble and view those channels. Those consumers that do not choose the more expensive television channel packages receive the premium channels in the scrambled and unviewable format. In some cities, some or all of the basic television channels are scrambled in addition to premium channels. 
     In many instances, a viewer needs a cable box in order to receive cable television channels. The cable box receives signals corresponding to all of the cable television channels, but is configured to descramble only the premium channels that the consumer has purchased. When a viewer subscribes to, modifies, or cancels cable service, the cable operator programs the cable box to descramble the appropriate channels. Cable boxes can be programmed manually by a technician or remotely by sending a conditional access signal from the cable head end to the cable box. 
     While cable boxes perform important functions, such as those described above, they introduce complexity into home entertainment systems, particularly when combined with other consumer electronics devices, such as video tape recorders or set top boxes (STBs) that provide access to the Internet or other services. One problem is related to the act of changing channels. For example, if a cable box is connected to certain types of set top boxes and the consumer desires to change the channel, a code is sent to the STB and then relayed to the cable box. The cable box then tunes the channel and, if necessary, descrambles the channel so that the user may view the selected channel. In other words, every time a person wants to change channels, the signal must be relayed, thereby introducing delay into the process. If a user selects a specific channel, the process of relaying a command signal might be executed repeatedly. For example, if a user selects channel  104 , four codes are relayed to the cable box: the digits “1”, “0”, “4” and the enter command. 
     Adding to this delay is the fact that many cable boxes include hardware or software that is outdated. While newer technologies can change, tune and descramble channels faster than many legacy cable boxes, cable operators often find it infeasible to replace such cable boxes with newer ones. The combination of these factors results in a noticeable delay when a person is rapidly changing channels or channel surfing. Further delay can be introduced by some legacy cable boxes that are incapable of skipping unused channels as the user incrementally advances through the channels in the television signal. 
     Another concern, from the point of view of the cable television provider, is that there are publicly accessible algorithms that permit viewers to descramble many or all scrambled television signals without authorization to do so. The availability of such descrambling algorithms permits viewers to have access to and view scrambled television channels without payment. It would be desirable, therefore, to enable scrambled signals to be descrambled using newer and faster technology without merely replacing legacy cable boxes with new cable boxes. It would be particularly desirable to provide such efficient descrambling capabilities while conforming to access control measures implemented by cable and satellite television providers to allow only authorized subscribers to receive premium channels. 
     SUMMARY OF THE INVENTION 
     The present invention relates to descrambling television signals in a device other than the traditional cable box, while using conditional access settings contained in a viewer&#39;s cable box to authenticate that the viewer has been granted permission to receive the descrambled channels. In this manner, signal descrambling can be conducted using efficient algorithms and new, relatively fast processors without the risk of the viewer gaining access to unauthorized channels. Cable operators do not need to replace legacy cable boxes with new ones, but can continue to use existing cable boxes as authenticators for the right of the viewers to receive the descrambled channels. 
     According to one implementation of the invention, in order to determine whether a viewer is authorized to view descrambled channels, the descrambling device (i.e., the electronics device in which descrambling is to be performed) periodically causes the cable box to cycle through at least some of the scrambled channels in the cable television signal. The descrambling device monitors the output signal of the cable box and determines, for each scrambled channel, whether the cable box has generated a descrambled signal. When the cable box outputs a descrambled signal, the descrambling device determines that the cable box is capable of descrambling the scrambled channel and infers that the cable television provider has granted the viewer access to the channel. Conversely, when the cable box outputs a scrambled channel, the descrambling device determines that the cable box is not capable of descrambling the scramble channel and infers that the cable television provider has not granted the viewer access to the channel. In this manner, the descrambling device uses the existing cable box as an authenticator for the right of the viewer to access particular scrambled channels. Based on the results of the foregoing authentication process, the descrambling device is enabled to descramble the channels to which the viewer has been granted access and is disabled from descrambling the channels to which the viewer has not been granted access. 
     The authentication process can be repeated as desired to remain current with any change in the subscription status of the viewer. For instance, the process can be performed daily during the early morning hours or at another time that is unlikely to interfere with the viewing habits of the user. After the authentication process, when the viewer, by means of a remote control device or otherwise, issues a command to tune to a scrambled channel, the descrambling device rather than the cable box descrambles the channel, assuming that the descrambling device has determined that the viewer is authorized to receive the channel. Thus, scrambled signals are descrambled using the relatively fast descrambling hardware and software of the descrambling device, thereby reducing the delay that might otherwise be present, while preserving the ability of the cable operator to impose conditional access to premium channels. 
     The invention can advantageously include further techniques to prevent viewers from circumventing the authentication process. As described above, the authentication process relies on monitoring the output signal of the cable box to determine whether it includes a scrambled or descrambled signal. Theoretically, a particularly persistent user could tamper with the authentication process by replacing the scrambled output of the cable box with a non-scrambled, albeit different, television signal. Without some further security measures, the descrambling device might falsely interpret the substituted, non-scrambled signal as an indication that the user is authorized to access the scrambled channel. 
     In order to detect such attempts to breach the security features of the invention, the descrambling device can compare the timing of cyclical characteristics of a known instance of a particular descrambled signal with the cyclical characteristics of the output signal of the cable box. For example, the descrambling device descrambles a particular scrambled signal and monitors the timing of the vertical blanking interval or another selected cyclical characteristic of the signal. During the authentication process, the cable box outputs a non-scrambled channel that is supposedly the same as the aforementioned channel that has been descrambled by the descrambling device (but not yet conclusively the same in view of the possibility that the user has tampered with the process). The descrambling device monitors the timing of the vertical blanking interval or another corresponding cyclical characteristic of the non-scrambled output signal. If the two signals are synchronized, it is highly probable that the two signals are the same and that there has been no attempt to circumvent the authentication process, whereas non-synchronization indicates possible user interference. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the manner in which the above-recited and other advantages and objects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a block diagram illustrating one embodiment of a system for authenticating the privilege of a viewer to receive a specified channel and for descrambling and viewing the scrambled signal; 
     FIG. 2 is a block diagram depicting the system of FIG. 1 supplemented with components that compare the synchronization of a signal tuned and descrambled by the cable box with a signal tuned and descrambled by the set top box; and 
     FIG. 3 is a flow diagram illustrating a method whereby a set top box uses the cable box to determine whether a viewer is authorized to access a specified channel. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides an apparatus, method and computer program product for descrambling a scrambled television signal at a descrambling device other than a traditional cable box. The present invention further contemplates monitoring an output signal from a conventional cable box to determine whether the viewer is authorized to access descrambled television channels. Thus, viewers can take advantage of improved tuning and descrambling capabilities without having to obtain a new cable box. Moreover, using the existing cable box as an authenticator for the viewer&#39;s privilege of accessing scrambled channels allows cable operators to retain control over the type and number of channels received by subscribers. 
     The invention is described in terms of diagrams and flow charts. Using the diagrams and flow charts in this manner to present the invention should not be construed as limiting its scope. The embodiments of the present invention may comprise a special purpose or general purpose computer comprising various computer hardware. 
     Embodiments within the scope of the present invention also include computer-readable media having computer-executable instructions or data structures stored thereon. Such computer-readable media can be any available media which can be accessed by a general purpose or special purpose computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired executable instructions or data structures and which can be accessed by a general purpose or special purpose computer, such as one included in a set top box. When information is transferred or provided over a network or other communications connection to a computer, the computer properly views the connection as a computer-readable medium. Thus, such a connection is also properly termed a computer-readable medium. Combinations of the above should also be included within the scope of computer-readable media. Computer-executable instructions comprise, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. The computer-executable instructions and associated data structures represent an example of program code means for executing the steps of the invention disclosed herein. 
     Although not required, the invention can be described in the general context of computer-executable instructions, such as program modules, being executed by one or more processors included in a set top box. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including general purpose computers, personal computers, hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. 
     A. Signal Descrambling 
     FIG. 1 is an example of an apparatus for descrambling a scrambled signal and authenticating the right of a user to access the descrambled signal. Signal  50 , illustrated in FIG. 1, may comprise a single signal or a group of signals, some of which may be scrambled. As described herein, signal  50  is a cable television signal and comprises a group of signals referred to as channels. For convenience, the television signals and the associated hardware are described herein in the context of cable television. However, it is to be understood that the principles disclosed herein are applicable to other systems for delivering television signals to viewers, including direct broadcast satellite and other satellite television systems. Indeed, the term “cable box” is defined to include descrambling boxes conventionally used with satellite dishes or with other sources of scrambled television signals. 
     Some or all of the channels in signal  50  may be scrambled, which means that user  92  is unable to view the content of the scrambled channels on television  90  without a descrambling process. Other channels in signal  50  might not be scrambled and are directly viewable by user  92  on television  90 . Cable box  60  can be any conventional or legacy cable box that is capable of descrambling one or more channels in signal  50 . Indeed, an advantage of one embodiment of the invention is that scrambled channels can be efficiently descrambled without replacing the existing cable box  60 . 
     According to conventional techniques, and in the absence of the present invention, signal  50  would be tuned and, if needed, descrambled by cable box  60 . Cable box  60  receives signal  50  through cable box input  60   a  and outputs the tuned and descrambled signal  50 ′ via cable box output  60   b . Signal  50  may have unused bandwidth that can accommodate future channels. Depending on the nature of cable box  60 , it might attempt tuning to the unused channels, although no meaningful image will be displayed on television  90 . Cable box  60  is capable of descrambling the scrambled channels in signal  50  only if user  92  is authorized to view the channels. For instance, user  92  is authorized to view scrambled premium channels if the user has purchased these channels from the cable television provider. The cable provider typically selectively enables cable box  60  to descramble the scrambled channels by employing a technician who manually configures the cable box or by remotely programming the cable box using a conditional access system. 
     According to one embodiment of the invention, set-top box  70  is included in the entertainment system as illustrated in FIG.  1 . As used herein, the term “set-top box” extends to any electronics device performing the functions or having the structure disclosed herein, regardless of the position of the electronics device with respect to television  90 . Indeed, in one embodiment, set-top box  70  can be integrated with television  90 , while in another embodiment set-top box  70  can be a separate component. A WebTV™ set-top box (available from WebTV Networks, Inc., of Mountain View, Calif.) that has been adapted to perform the methods disclosed herein can be used as set-top box  70 . 
     Set top box  70  improves the response time that would otherwise be needed to change channels in the absence of the present invention. According to the embodiment illustrated in FIG. 1, set top box  70  includes a multiplexor  64  that multiplexes signal  50  and signal  50 ′. Although one of the advantages of this embodiment of the invention is that set top box  70  can tune to and descramble channels without the direct assistance of cable box  60 , multiplexor  64  allows set top box  70  to alternately display channels that have been tuned and descrambled by cable box  60 . Multiplexor  64  is an example of means for receiving a scrambled signal  50  or signal  50 ′. 
     Multiplexor  64  sends the multiplexed signal  66  to tuner  78 . When set top box  70  is used to tune to a selected channel, tuner  78  tunes to the channel. Tuner  78  can receive signal  50  directly from the cable operator and tune to the selected channel without the direct assistance of cable box  60 . Tuner  78  then retransmits tuned signal  68  to descrambler  80 , where the selected channel is descrambled, if needed. The descrambled signal is then transmitted to multiplexor  82 . If set top box  70  is instead used to merely display channels tuned and descrambled by cable box  60 , tuner  78  tunes to the output channel of cable box  60  (e.g., channels  2 ,  3 , or  4 ). In this case, tuner  78  sends signal  68 ′ (which can be substantially similar to signal  50 ′) to multiplexor  82 . Video decoder  84  and frame buffer  86  assist in displaying the channels in signal  50  on television  90 . 
     Tuner  78  and descrambler  80  can be selected to provide tuning and descrambling capabilities at any available, desired processing speed. Because set top box  70  may be used in combination with any legacy cable box  60  in the possession of the user of the system, the tuning and descrambling response time may be significantly greater than that of the cable box. Those skilled in the art, upon learning of the disclosure made herein, will understand the tuning and descrambling hardware and software included in tuner  78  and descrambler  80 . Indeed, tuning and descrambling techniques used in state-of-the-art cable boxes can be used in set top box  70 , if desired. Thus, set top box  70  rather than cable box  60  can be used to tune and descramble television signals when viewer  92  watches television. 
     It can be expected that new scrambling and descrambling algorithms will be introduced over the years. Accordingly, one embodiment of the invention includes the capability for upgrading the descrambling algorithms used by descrambler  80 . For instance, if an existing descrambling algorithm is revised to operate more efficiently, the revised algorithm can be uploaded to set top box  70 . Moreover, if an entirely new scrambling and descrambling technique is developed, the new descrambling algorithm can be transmitted to set top box  70 . 
     Those skilled in the art, upon learning of the disclosure made herein, will recognize that there are many available methods for upgrading software in a set top box. For instance, the software can be transmitted over telephone lines or an other communications link from a remote software repository to set top box  70 . Alternatively, manual upgrading techniques performed by the user are possible. In the case of remote upgrades, the software can be automatically upgraded periodically or as new or revised algorithms become available. Alternatively, new or upgraded descrambling algorithms can be transmitted to set top box  70  on an “as needed” basis. For instance, if set top box  70  encounters a television signal that is descrambled using an unknown algorithm, set top box  70  can initiate communication to a remote software repository using a telephone line or another communications link. In response, algorithms that are appropriate for descrambling the particular television signal can then be uploaded to set top box  70 . 
     The example of FIG. 1 illustrates an embodiment of the invention wherein set top box  70  can tune to and descramble a signal without the direct assistance of cable box  60  and, alternately, can display channels tuned and descrambled by cable box  60 . In an alternative embodiment, set top box  70  can be further simplified so as to be incapable of displaying channels tuned and descrambled by cable box  60 . In either case, however, cable box  60  is retained in the system to provide the authentication features disclosed hereinbelow. 
     B. Authentication of Viewing Privileges 
     While set top box  70  as disclosed above can advantageously tune to and descramble channels from signal  50 , it is typically desirable to enable such tuning and descrambling only for viewers who are authorized to gain access to the descrambled channels when, for example, a viewer has subscribed to a cable package having premium channels. While most cable operators have manual or automated conditional access systems that selectively enable cable box  60  to descramble scrambled channels, such conditional access systems may not be directly compatible with set top box  70 . In other words, cable operators may not have the infrastructure to remotely interact with set top box  70  in order to restrict access to scrambled channels. In view of the desirability to enable only authorized viewers to gain access to scrambled channels, a preferred embodiment of the invention uses existing cable box  60  to authenticate the access privileges of the user with respect to scrambled channels. 
     One embodiment of a method for determining whether a user has a right to receive scrambled channels in descrambled form can be described in reference to FIG.  1 . Set top box  70  verifies the viewing privileges of viewer  92  by determining whether cable box  60  is enabled for descrambling particular scrambled channels of signal  50 . Based on the descrambling capabilities of cable box  60 , set top box  70  descrambles and displays to viewers only those scrambled channels that the viewer is authorized to receive. Because viewing privileges are subject to change when, for instance, the viewer upgrades or cancels cable services, it can be desirable to periodically repeat the process of determining the descrambling capabilities of cable box  60 . For instance, this process can be repeated daily at an early morning hour, or at any other frequency. 
     To learn of the descrambling capabilities of cable box  60 , set top box  70  causes the cable box to cycle through any or all scrambled channels and determines whether the output signal  50 ′ from cable box  60  is descrambled. To illustrate, processor  20  of set top box  70  can control cable box  60  using an infrared (IR) transmitter  62 , causing the cable box to tune to specified channels in signal  50 . In response, cable box  60  tunes to the specified channel and descrambles it if the viewer is authorized to receive that channel. If the viewer is not authorized to receive the channel, cable box  60  tunes to the channel, but is unable to descramble it. Set top box  70  monitors the output signal  50 ′ from cable box  60  to determine whether cable box  60  has descrambled the scrambled channel. The output signal  50 ′ can be monitored as tuner  78  tunes to the cable box output channel associated with signal  50 ′. Signal  68  is transmitted to processor  20  or another component of set top box  70  to determine whether signal  68  is scrambled or descrambled. As described herein, the components of set top box  70  that can be used to cause cable box  60  to cycle through any scrambled channels and determine whether output signal  50 ′ is descrambled represent an example of means for authenticating the right of a user to descramble a scrambled signal. 
     The foregoing process is repeated as cable box  60  cycles through as many as all of the scrambled channels in signal  50 . Subject to further security techniques disclosed hereinbelow, the basic premise on which the authentication features of the invention rest is that set top box  70  is enabled to descramble a particular scrambled channel only if it has determined that cable box  60  has been able to descramble the same channel. Based on the results of the authentication process, processor  20  stored channel authorization information in storage medium  96  for later use in selectively enabling descrambler  80  of set top box  70 . 
     Preferably, storage medium  96  stores the channel authorization information in a persistent form, such that it does not need to be generated each time set top box  70  is powered up. In this manner the persistently stored channel authorization information can be repeatedly used, if necessary, as a viewer turns the set top box  70  on and off. While persistent storage of the channel authorization information has the foregoing advantages, the invention also extends to embodiments wherein the channel authorization information is transiently stored and generated each time the set top box  70  is powered up. 
     After the authentication process has been completed, set top box  70  tunes to a channel requested by viewer  92  using tuner  78 . Based on the channel authorization information contained in storage medium  96 , processor  20  selectively enables descrambler  80  using control signal  98 . If descrambler  80  is thereby enabled, the channel descrambled and displayed to the viewer on television  90 . If, however, descrambler  80  is not enabled, the channel is not descrambled, thereby preventing the viewer from accessing the unauthorized channel. 
     C. Authentication Using Signal Synchronization 
     The foregoing process of determining the viewing privileges of viewer  94  can be successfully used and can adequately prevent viewer  94  from receiving unauthorized access to scrambled channels in most situations. According to one embodiment, however, the invention further employs additional security features for reducing the ability of viewers to tamper with the authentication process. For instance, one could imagine that a viewer might attempt to circumvent the authentication process by inputting to the set top box  70  a non-scrambled, ordinary channel in the place of a scrambled, premium channel during the authentication process as the set top box tests the ability of cable box  60  to descramble the premium channel. Potentially, set top box  70  could falsely interpret the non-scrambled channel as an indication that cable box  60  is capable of descrambling the scrambled premium channel. If this were to occur, the viewer might gain unauthorized access to the premium channel. 
     According to the embodiment illustrated in FIG. 2, set top box  70  measures the timing of the vertical blanking interval or another cyclical, repeating feature of the signal corresponding to a particular channel in order to substantially ensure that the channel being measured by set top box  70  during the authentication process is, in fact, the channel that the set top box intends to test. As used herein, the term “time base” represents the vertical blanking interval or any other cyclical feature of the signal. Because cable TV channels generally originate from independent sources referenced to independent master clocks, it can be assumed that they have independent time bases that drift relative to one another over time. 
     In this embodiment, the authentication process includes a two-pass technique that compares the time base of signal  50 ′ (the “tested or unknown channel”), which supposedly or purportedly comes from cable box  60 , with the time base of the channel received directly from the cable operator (i.e., the “known channel” included in signal  50 ). In other respects, the authentication process can be the same as that described above in reference to FIG.  1 . 
     To measure the time base of the known channel, tuner  78  tunes to the known channel and descrambles the resulting signal  68  at descrambler  80 . A time base detector  100  shown in FIG. 2 measures the timing of the vertical blanking interval or any other time base of signal  68 . This known channel serves as the basis for measuring the synchronization of the time base of the tested channel. 
     To measure the time base of the tested channel, tuner  78  tunes to the output channel associated with cable box  60  as has been described herein in reference to the authentication process of FIG.  1 . The resulting signal  68 ′ is processed by time base detector  100  to measure the time base thereof, as well being analyzed by processor  20  to determine whether the signal is descrambled. The timing of the time base of signal  68 ′ (the tested channel) is compared with the measured time base of the known channel. For example, time base detector  100 , in conjunction with processor  20 , can compare the synchronization of the tested channel and the known channel. 
     If the time bases of the channels are synchronized, it can be reasonably concluded that the tested channel is the same as the known channel, and that the viewer has not interfered with the authentication process. Thus, cable box  60  has been capable of descrambling the tested channel, and it is concluded that the viewer is authorized to receive the descrambled channel. Based on this conclusion, channel authorization information  96  is updated to enable descrambler  80  to descramble the channel. If, on the other hand, the time bases of the tested channel and the known channel are not synchronized, it is assumed that the channels are different, and that the viewer may have attempted to interfere with the authentication process. In this case, the authentication process may be repeated or channel authorization information  96  is updated to disable descrambler  80 . 
     The flow diagram of FIG. 3 summarizes the authentication processes described above. In step  102 , the scrambled channel (i.e., the tested channel) is received at the cable box. The set top box determines, in decision block  104 , whether the cable box is capable of descrambling the tested channel. If not, the viewer is not authorized to receive the channel, and the set top box is disabled from later descrambling this channel as shown in step  106 . 
     If the cable box is capable of descrambling the tested channel, the process advances to decision block  108 , where it is determined whether, as a further security measure, the time base synchronization of the tested channel is to be tested according to the techniques disclosed herein. If time base synchronization testing is not desired, the method proceeds to step  110 , in which descrambling of the channel is enabled at the set top box. 
     If, however, time base synchronization testing is to be conducted, the method advances to step  112 , in which the known channel is directly received at the set top box. The time base of the known channel is measured in step  114  and the time base of the tested channel is measured in step  116 . If, according to decision block  118 , the time bases are synchronized, the viewer is assumed to be authorized to receive the channel. Accordingly, descrambling of the channel is enabled at the set top box in step  110 . If the time bases are not synchronized in decision block  118 , the set top box is disabled from descrambling the channel in step  106 . It is noted that the method of FIG. 3 can be repeated for any number of scrambled channels and can be periodically repeated to respond to any intervening changes in the authorization status of the viewer. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.