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Description  |
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FIELD OF THE INVENTION
This application relates to improvements in subscription television systems. More particularly, the invention relates to a communications system and format for a subscription television system which permits communication of individually
addressable text messages from a central location to individual subscribers.
BACKGROUND OF THE INVENTION
The sophistication of communications systems involving subscription television is continually increasing. The systems being installed at this time have greatly improved communications flexibility as compared to those previously in use, while the
quality of the transmitted signal is continually being improved, as is flexibility in the business arrangements possible between the subscriber and the broadcaster. For example, in U.S. Pat. No. 4,484,217 to Block et al., there is disclosed a
subscription television system in which all billing calculations are carried out at the decoder, that is, at the individual subscriber's location. When a subscriber makes a payment, e.g., by simply mailing a check to the broadcaster, the broadcaster
then sends a message which is detected by that particular decoder and used by it to update the user's credit and/or account balance information. The decoder then controls whether the user is to have access to a particular program or not in accordance
with this billing information, possibly also in accordance with other sorts of access controls transmitted by the broadcaster. For example, each program transmitted may include an indication that it belongs to a particular "tier" or class of programs.
The decoder then permits access to the program if programs of that tier had previously been authorized.
The Block et al. patent also discloses that teletext data, that is, textual information relating to stock market reports and the like, could also be transmitted using this system. However, the teletext data discussed in the Block et al. patent
appears to be system-wide data, that is, data relevant to all subscribers of the system more or less equally. The Block et al. patent does not teach any way in which, for example, an individual broadcaster may send a chosen subscriber a personal message
other than billing update data, as above.
"Specification of Standards for Broadcast Teletext Signals", a document published by the British Broadcasting Corporation et al. in September, 1976, describes the teletext broadcasting system in effect in the United Kingdom. According to this
system, teletext information such as closed-captioned programming is available for the hearing impaired. According to this system, as understood by the applicants, the subscriber simply depresses a button on a key pad comprised by the decoder, and the
decoder then selects alphanumeric data which is transmitted during the vertical blanking interval of the standard television signal. The decoder can then add this closed-captioned textual material directly to the video screen, so that it can be seen by
the hearing impaired. Again, however, this material is transmitted system-wide, and there is no provision made for transmission of individual messages to individual subscribers.
Those skilled in the art will recognize that there are a number of constraints on the operation of any successful subscription television system. For example, there must be an extremely reliable and trustworthy method of computing charges to be
billed to the subscriber and for communication of credit status updates. It is also desirable that the subscriber be permitted to prepay for certain classes of service, for example, to have access to a particular broadcaster's offerings. On the other
hand, it is also desirable that an "impulse" purchase system be provided such that the viewer could purchase a particular program on a "pay-per-view" basis when desired. Similarly, it is desired that a running list of programs purchased by the
subscriber be available, such that he can confirm that he is indeed being billed correctly. It is furthermore desired that means be provided for securing the system against improper use, which includes both prevention of viewing of inappropriate subject
matter and purchase of fee-bearing material by persons not authorized to do so.
The requirement that the credit status be updateable further requires that communication be possible in both directions between the decoder and the transmitter. However, it would be inefficient to provide individual uplink transmission
facilities at each decoder in a satellite transmission system, for example. Therefore, it is desirable that a two-way communications facility be provided connecting each decoder with the transmitter, but in which the two data links are not made using
the same medium. Therefore, the communication facility which carries subscription television signals from the transmitter to the decoder is not the same as that used to carry payment status information, queries or the like from the decoder to the
transmitter.
As indicated above, the prior art suggests that individual decoders can be addressed for transmission of billing information. It would also be desirable if individual user messages could be transmitted to individual users such as to provide an
additional communications facility not previously made available. If this is to be done, there are several criteria which must be met. Paramount among these is security; preservation of the secrecy of the message and its availability only to the
particular addressee is of utmost importance. At the same time, it is desired to be able to address messages to classes of subscribers to transmit important civic data, emergency messages, or the like.
Another complexity in providing a secure subscription television system is the fact that the decoder cannot be considered to be in "friendly hands". That is, unlike an encrypted data channel as used by financial institutions or in military
applications, in which both transmitter and receiver are in friendly hands, an additional security exposure which must be faced in connection with a television system is analysis of the transmitted signal. In a subscription television system, the
transmitted signal must, of course, be protected, as a very important threat is posed by viewers who have purchased a decoder and attempted to modify it to bypass the security mechanisms, the billing arrangements or the like. A further problem is posed
by those who would make copies of, for example, television movies using videotape recorders or the like for subsequent viewing or resale.
It is, therefore, desirable that a system be provided which provides a very high level of security, both as to the transmitted signal and as to the operation of the decoder, and one in which some sort of "fingerprint" identifying the source of an
unauthorized copy can be made.
It is therefore a broad object of the invention to provide a communications format and capability for a subscription television system such that individual text messages may be directed to individual subscribers.
It is a further object of the invention to provide a communication format for a subscription television system in which individual text messages can be transmitted to individual subscribers, in which the message can only be received by the proper
receiver and in which the text is very heavily protected against improper receipt, that is, encrypted, such that it can only be received and decrypted at the correct decoder, thus providing an additional level of security to the message handling system.
It is a further object of the invention to provide a communication format for subscription television systems such that an individual message can be sent to individual users, but in which the individual decoder is enabled to receive and decrypt
the specified message only upon activation by the proper individual.
A further object of the invention is to provide a communications format for a subscription television system in which individual text messages can be received by individual subscribers in response to queries by the subscriber, in response to the
desire of the broadcaster, or upon initiation by the decoder itself.
SUMMARY OF THE INVENTION
The above needs of the art and objects of the invention are met by the present invention, which comprises a communication format, system and method for a subscription television system. According to the invention, a number of distinct "lines" of
information are transmitted during each vertical blanking interval of the video signal. One line of the information includes "system data" relevant to operations of the entire system. Another group of the lines are referred to as "addressed packets",
which are directed to individual subscribers, while another group of lines includes teletext information. The addressed packets include identification of individual subscribers which are transmitted in clear text (that is, are not encrypted) such that
the individual decoders recognize their own numbers. The addressed packets include certain cipher information, used in decrypting other portions of the information contained in the blanking interval and in the video signal. The addressed packet also
indicates that a particular "page" of teletext information is being transmitted which is directed to a particular subscriber. The decoder, having detected the existence of a message addressed to it responsive to the information contained in the
addressed packet, then selects subsequently transmitted teletext "lines" included in subsequent vertical blanking intervals, and uses these to assemble a teletext "picture" which can then be displayed on the user's television screen.
In a preferred embodiment, the broadcaster continually sends teletext information which is useful in subsequently assembling messages. The teletext information repetitively transmitted includes "templates". For example, should the subscriber
seek to ascertain his billing status, he will so indicate by pressing a button or buttons on a user key pad comprised by his decoder. A microprocessor comprised in the decoder will note this request and will access its own internal memory to determine
the number of a template suitable for displaying this information. When that page of text is subsequently transmitted, as part of the regular sequence of teletext messages, the microprocessor selects it, and then completes the template by filling in
various dollar amounts and the like unique to the particular subscriber. This completes assembly of the billing report requested. The microprocessor then can display the complete billing report. In this way, the teletext information comprised in the
template, that is, the information needed for the text message which is not user specific, such as column headings, current dates, and the broadcaster's name and address, which are common to all subscribers, need not be stored at the user location. This
greatly reduces the amount of memory which must be provided for each decoder. In the same way, the templates can be varied by the broadcaster, e.g., to update the listing of the services available or the like.
The decoder itself can also generate various reports, for example, to indicate to the subscriber that his credit is running low. The microprocessor can select a template page from the sequence of template pages continually being sent by the
broadcaster and can complete it with user-specific billing information, all as generally discussed above.
In this way, it will be observed that there are three different ways of providing a teletext message, that is, user-initiated, broadcaster-initiated, and decoder-initiated.
The system data transmitted according to the invention includes data which is relevant in the operation of the entire system. For example, the system data will include an identification of the program currently being transmitted. This
identification can be a number keyed to a catalog, for example, such that if the number is stored at the decoder, the user can later employ it to determine whether he is being correctly charged for a particular program. The program identifier may also
include a program tier number indicating that a particular program belongs to a tier of programs. The program tier number is used by the decoder to determine whether the user is entitled to have access to the particular program. Examples of program
tiers include special sports events, X-rated movies, various pay TV channels and the like. The system data also includes a decryption key which is used by the decoder to decrypt encoded video, audio and teletext material.
In the preferred embodiment, the system data is extensively protected by means of additional error correction characters and by duplication, such that it is very reliably received. Due to this duplication, more system data is required for
operation of the system than can be transmitted in a single line in a single field. Therefore, the system data is transmitted over a number of fields; in a preferred embodiment, this number is three. The system data transmitted in a given field is also
duplicated a predetermined number of times, five in a preferred embodiment, such that majority logic can readily be used to ensure correct receipt of the system data.
The system data includes in addition to the data described above an encryption key which is used in the decoder to decrypt portions of the video, audio and teletext information. By virtue of the very high degree of security provided by the
duplicative transmission and error correction facility, system data is received with high reliability. The key is changed at the completion of each "cryptocycle", that is, upon complete transmission of the system data, to further protect against its
illegal duplication.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood if reference is made to the accompanying drawings, in which:
FIG. 1 shows an overall view of a communication system according to the invention;
FIG. 2 shows an overall view of the signal format according to the invention, and explains certain nomenclature used for understanding the system of the invention;
FIG. 3 shows in broad outline the format of the horizontal blanking interval;
FIG. 4 shows additional details of the format of the horizontal blanking interval;
FIG. 5 shows an overview of the material carried in the 16 lines of the vertical blanking interval in a 525 line embodiment of the invention;
FIG. 6 shows in some additional detail the clock recovery data carried in line 1 of the vertical blanking interval;
FIG. 7 shows in additional detail the framing recovery data carried in line 2 of the vertical blanking interval;
FIG. 8 shows in schematic form the sequence of transmission of the three packets of data making up the system data;
FIG. 9 shows schematically the arrangement of line 3 of the vertical blanking interval, during which the system data is transmitted;
FIG. 10 shows in detail the data carried in the first packet, packet A, of the system data;
FIG. 11 shows in detail the data carried during the second packet, packet B, of the system data;
FIG. 12 shows in detail the data carried in the third packet, packet C, of the system data;
FIG. 13 shows the general arrangement and contents of four different types of addressed packets which can be transmitted in lines 4-8 of the vertical blanking interval;
FIG. 14 shows the outline of a teletext header line which can be transmitted in any one of lines 9-13 of the vertical blanking interval;
FIG. 15 shows a text line, that is a line of teletext which may be transmitted during any one of lines 9-13 of the vertical blanking interval;
FIG. 16 shows in flow chart form the steps taken which result in display of a teletext message on a subscriber's television screen;
FIG. 17 shows the hardware of the decoder used for display of teletext information;
FIG. 18 details the relative arrangement of the microprocessor and teletext support chip (MATS), its connection and relationship to the microprocessor, and the functions performed by each;
FIG. 19 shows the processing of the system data by the MATS and the format in which it is supplied to the microprocessor;
FIG. 20, comprising FIGS. 20a-20c, shows in schematic form a flow chart of the processing of the system control data transmitted by the broadcaster for control of each of the decoders of the system;
FIG. 21 shows in schematic form the use of the information transmitted to the decoders by way of the addressed packet; and
FIG. 22 shows generally the data decryption scheme according to the system of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As mentioned above, it is an important object of the present invention to provide a communications format for a subscription television system which has the capability of transmitting individual text messages. This is to be accomplished without
adding unduly to the complexity of the system or its cost, and without requiring any sort of uplink communication between the decoder and the transmitter. Moreover, the system is to be provided with unprecedented flexibility in choice of program by both
the broadcaster and the user, and the subscriber billing is to be substantially automated and performed at the decoder. Finally, the whole system is to be menu-driven, that is, the subscriber is to be directed in operation of the system by messages
displayed on his television screen.
These objects of the invention are achieved by the present system, which is described in the following text, of which the following is a Table of Contents, and in which reference is made to the accompanying drawings.
A. System Overview
FIG. 1 shows in highly schematic form a communication system suitable for achieving the objects of the invention listed above. At a first transmitter location, indicated generally by the numeral 10, the teletext message sequence is first
assembled as indicated at 12. This may include standard items such as templates, that is, data which provides a format within which user-specific information can be added by a decoder to personalize a message; personal messages directed to individual
users; billing status updates, that is, billing information generated in response to a telephone call or letter from the subscriber to the broadcaster; and broadcast messages, that is, messages which are out of the ordinary course of business, but which
are transmitted to substantially all or a predetermined subcategory of subscribers (for example, that there is some general danger of which all should be aware).
At 14, the teletext information from item 12 is assembled together with video information, and in a preferred embodiment of the system of the invention, with multichannel audio. As will be detailed below in connection with FIGS. 3 and 4, six
channels of digital audio are provided in the system of the invention. These are transmitted during the horizontal blanking interval and may be provided on a pay-per-listen basis, whereby the broadcaster can generate additional revenue from audio
transmission.
At 16, the assembled teletext, video and audio signal is encrypted. The signal transmitted includes what are referred to as addressed packets, which among other functions alert an individual subscriber's decoder that a message has been sent to
it, and teletext information. The subscriber identification portion of the addressed packet is transmitted in clear text, that is, is not encrypted, so that it can be detected without decryption. The addressed packet further contains certain cipher key
information used in decrypting system control data. In turn the system control data is required for decryption of encrypted user messages. Thus, a double level of security is provided, as only the individual decoder can even receive the message, and
only that decoder can use the system data which it also must receive to decode the individual message, which is subsequently sent. The composite, encrypted signal is transmitted as indicated at 18. It may travel via a satellite 20, by a landline or a
combination of both to receiving antenna 22. It is there passed to a decoder 24 which comprises means for detecting the signal, decrypting the encrypted portions of it, completing any messages requiring any subscriber-specific data, such as billing
status information and the like, and display of the received messages. Of course, the decoder also permits user selection of the decrypted audio and video information, and includes means for maintaining the billing history information related to a
specific user, for restricting access of individuals to the video signal, prevention of impulse purchase of programs by those not authorized to do so, and so on, all as detailed below.
The system of the invention is completed by means permitting the user to communicate with the broadcaster. Ordinarily, the user will only need to communicate with the broadcaster sporadically, for example, to request addition of a service, or to
pay a bill or the like. Conventional communications facilities such as the telephone system or the mails are suited for this function, as indicated schematically at 8. In this way, no uplink facility at the user's station need be provided.
B. The Communications Format Employed
1. Overview
FIG. 2 shows the overall transmission format of the system according to the invention. As is conventional in television, 30 "frames" each comprising a still image are transmitted per second as indicated. Each frame includes two "fields" as also
shown. In a preferred embodiment of the invention, the video encoding scheme employed is that referred to generally as "B-MAC". This is an acronym for type B format, Multiplexed Analog Component system. "Type B" refers to the fact that data is carried
integral to the video signal. See generally Lowry, "B-MAC: An Optimum Format for Satellite Television Transmission", SMPTE Journal, November 1984, pp. 1034-1043, which discusses in detail the B-MAC format and explains why it was chosen over various
competing systems.
According to the present invention, the vertical blanking interval (VBI) of each field contains certain "system data" necessary for operation of a subscription television system according to the invention, as well as addressed packets and
teletext lines used to carry data needed for the operation of individual decoders and for transmission of messages to individual subscribers. In the preferred embodiment of the present invention, the vertical blanking intervals of 16 total fields are
used for complete transmission of all system data required, which includes an encryption key which is changed every 16 fields, that is, on the order of three times per second. As also shown in FIG. 2, each line also includes a horizontal blanking
interval (HBI). During the HBI are transmitted six channels of high quality digitally-encoded audio information, with error correction, such that the decoder can also be used to supply a high quality audio signal. This can be used to provide the audio
component of the corresponding video signal (or several versions thereof, in different languages) or an additional audio signal, such that subscription audio is also made available according to the system of the invention.
2. The Horizontal Blanking Interval
FIG. 3 shows the format of the horizontal blanking interval (HBI), which begins each line of each field. In the preferred embodiment, the HBI consists of 78 total bits of four-level frequency-shift-keyed (FSK) data. The HBI is interposed
between the vertical blanking interval or video information from a previous line and that of the present line. A typical horizontal blanking interval as shown begins with a two-bit guard band 30, followed by 45 bits of audio and utility data 32, a
second two-bit guard band 34, twenty bits of color burst information 36, a further guard band 38, six more bits of data 40 and a final guard band 42, after which the VBI or the video signal of the particular line commences. The position of the color
burst 36 within the HBI varies, to provide signal scrambling; descrambling involves the use of a repetitively-transmitted key, described below in connection with FIGS. 10-12. The number of bits of data in the HBI of each line can be varied
pseudorandomly as well; the average number is 51.
FIG. 4 shows some additional details of the horizontal blanking interval data 32 and 40 shown in FIG. 3. In the example shown, fifty-one total bits of data are provided in each line of the HBI, and each bit is four-level FSK encoded, such that
each bit period includes transmission of two bits. One bit can be referred to as sign and the other as magnitude as indicated on FIG. 4. As shown, the first seventy-eight bits are digital audio; thus each frame provides a thirteen-bit digital
representation of a sample of each of six audio channels. High quality transmission of audio frequencies up to approximately 15 kHz is thus provided. Following the audio information are six bits of stepsize and bandwidth information. The stepsize bits
indicate the size of the steps numbered by the thirteen bits of information preceding, and the bandwidth information relates to the amount of emphasis or de-emphasis of the signal employed. Alternate fields carry the stepsize and bandwidth data. Both
these terms are used as conventional in the "Dolby delta modulation" scheme, which is employed in the preferred embodiment of this invention for transmission of the audio. Following are twelve bits of error correction code (ECC) for correction of the
audio, indicated at 48. Four utility bits follow at 50, and the last bits 52 of the data are parity check bits for checking the parity of the error correction bits 48.
3. The Vertical Blanking Interval
FIG. 5 shows the arrangement of the lines which make up the vertical blanking interval (VBI). The VBI includes 16 lines in the 525-line NTSC version of | | |
