Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The present invention relates to a decoder for a pay-TV
system wherein security of transmission is increased.
In a pay-TV system or similar systems, the subscriber
selects the programs of interest and he pays for receiving these
programs which are transmitted in a scrambled or encoded mode in
order not to be recognizable for the non subscribers or for the
subscribers which have not chosen nor paid for a particular
program. The signals may be transmitted by cable, antenna or
satellite. The sophisticated systems of pay-TV further comprise
the possibility called "pay-per-view" which permits to purchase
programs either immediately before the beginning of the emission
or during the emission itself and this by means of an
unidirectional transmission system. In this case, it becomes
necessary to memorize a certain credit at the location of the
user, credit which is at disposal for permitting to purchase and
receive an emission pay-per-view that the user is interested to
look at. If the memorized credit is sufficient, the amount is
debited from the credit and the signal received from the
transmitting station is unscrambled or decoded for permitting a
comfortable reception thereof.
The document WO 81/02499 relates to a method and a
system for scrambling the transmission of video information, more
particularly for pay-TV. Parts of the video signal, e.g. parts or
segments of the lines or of the frames are transmitted in a
sequence different from the natural one. The manner of
rearranging the sequence of the segments of lines for the
transmission may be controlled by a code generator which changes
the sequence at each frame
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or even as a function of another appropriate time base. The
code which can be enciphered changes at random and it may
be transmitted completely or partially with the scrambled
video signal.
The Patent US 4 484 217 relates to a method and a
system for billing at a distance a program of pay-TV. The
credit at disposal is memorized at the location of the
subscriber and a cost signal is transmitted by the trans-
mitter. Pay-per-view is made possible by the fact that
there is foreseen an impulse purchase. A unique code may
accompany the transmitted signal for identification purpo-
se. If the decoder of the subscriber recognize the program
as a program purchased by impulses, the program cost is
displayed. In order to visualize the program, the subscri-
ber enters an adequate demand in the decoder and the
latter compares automatically the cost of the program with
the credit at disposal. If the cost does not exceed the
credit, the program is unscrambled and the cost is deducted
from the credit. However, such a system is not foreseen for
permitting the display in form of teletext transmitted by
the video signal of informations relating to the forecasted
emissions in their chronological order nor for displaying
information in form of instructions for the manipulations
to be executed by the user on the decoder, in order to
facilitate the dialog between the latter and the user and
for avoiding manipulation errors.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to
facilitate and to improve the dialog between a subscriber
to a pay-TV system and a decoder of the signals received.
Moreover, the decoder must permit an easy and rapid rechar-
ging of the credit by the subscriber without the necessity
to consult the administration or management center.
To solve this problem, the method according to the
present invention for facilitating the dialog betweeen a
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subscriber to a system of pay-TV and a decoder of the signals
received is characterized in that one memorizes informations
transmitted by teletext regarding programs of emissions, in
that one selects by menu with a keyboard of the decoder the
memorized informations and in that one uses the memorized
informations for controlling a text generator which permits to
display the information on the screen of the television
receiver, the display of the said information facilitating the
dialog between the subscriber and the decoder.
The decoder for facilitating the dialog between
itself and a subscriber to a system of pay-TV is characterized
in that it comprises a receiver of data of teletext which
permits to extract these informations from the transmitted
video signal, a microprocessor with at least one memory for
the extracted informations delivered by the receiver of data
or teletext, a text generator controlled by the microprocessor
and capable to display on the screen of a television receiver
said memorized information selected by menu by means of a
keyboard connected to the microprocessor.
Due to the fact that one memorizes in the decoder of
the subscriber the informations transmitted by teletext
regarding the programs of forecasted emissions, like title of
the emission, price, date of transmission, code number, etc.
and that these informations can be selected by menu by means
of the keyboard of the decoder and displayed by a text
generator on the screen of the television receiver, there
exists a possibility of dialog between the subscriber and the
decoder, the latter being a user's guide for all operations or
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manipulations to be executed.
The invention may be summarized, according to one
broad aspect, as a decoder for a pay-TV system comprising a
memory arranged to receive a video signal scrambled by line
permutation, in which each line is segmented and digitalized
into a number of samples, and means for descrambling the
memorized video signal according to a variable descrambling
code, characterized in that said memory is constituted of a
number of buffer lines, for example 32 lines, wherein said
samples are sequentially introduced into successive cells, each
buffer line being constituted of as many cells as are necessary
to store the number of samples of one line of the video signal,
the entry of each sample in a cell of the buffer causing out-
putting of the sample previously memorized in the same cell,
such that the memory always remains filled with its number of
lines, and each buffer line is always filled with the number
of samples that it can contain, the order of extraction of the
memorized buffer lines to carry out descrambling being
determined by the selection, from the buffer lines, of those
buffer lines which permit depermutaticn of the lines, said
selection being controlled by the descrambling code.
According to another broad apsect, the present
invention provides a pay-TV system comprising, at the emission,
means for scrambling a video signal by line permutation,
wherein each line is sampled and digitalized into a number of
samples, and means for transmitting the scrambled video signal
in real time with a variable transmission key, and at the
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reception, a decoder according to one of the above-described
aspects, the decoder including means for receiving said
transmission key and deriving therefrom said descrambling code.
BRIEF DESCRIPTION O~ THE DRAWINGS
The invention will be described further by means of
an example of execution illustrated in the drawings.
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Figure 1 shows the fundamental principle of a system of
pay-TV,
Figure 2 shows schematically the principle of entry and
extracting of a line of the video signal in a
buffer of 32 lines,
Figure 3 shows a block diagram of the decoder according to
the invention,
1 0
Figure 4 shows the principle of scrambling and unscrambling
of the video signals transmitted,
Figure 5 shows the rotation of a line of the video signal,
Figure 6 shows the inversion of a line of the video signal,
Figure 7 shows a mirror symmetry of a line of the video
signal,
Figure 8 is a block diagram of the CPTV-card,
Figure 9 shows the physical structure of a line of the vi-
deo signal where data is inserted,
Figure 10 shows the structure of the physical data in the
Didon-operating system, and
Figure 11 shows the structure of the data in the Didon-
Operating system.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows a microcomputer 1 provided for genera-
ting data according to a format of teletext regarding the
program of the forecasted emissions, like title, price,
date or period of transmission, etc. as well as other
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informations useful for the user. This data is introduced
or superimposed at 3 through an interface 2 in the video
signal delivered by a known television camera 4 or any
other element capable of generating a video signal. The
lines corresponding to the return of the vertical scanning
of the video signal which carry normally no informations
are used in the system according to the invention as a
support for data of teletext so that this data is trans-
mitted with the video signal. Figure 9 shows the structure
of a return line of the vertical scanning of the video
signal. One sees that the line comprises 32 bytes of data
of teletext besides instructions of synchronization and
addresses in Hamming code. The 32 bytes are protected by 2
bytes of CRC-16 error detection code. The video signal with
the data of teletext is then delivered to a scrambler 5
which scrambles the image, e.g. by changing the sequence
according to which the lines are normally transmitted or
by other means. The scrambled signal is transmitted wire-
less, by video cable or by satellite to the receiver 6 of
the subscriber. The receiver comprises a circuit 7 for
.- unscrambling which delivers the unscrambled signal to the
television receiver 8.
The scrambled signal is fully compatible with the
SECAM and PAL(NTSC) norms. The scrambling may be executed
in normal version or in deep version. In the normal ver-
sion, the lines of the video signal are permuted between
themselves, the active part only of the line being permu-
ted. In the SECAM system, an even line is permuted with an
even line and an odd line is permuted with an odd line. In
the PAL(NTSC) system , the burst is left unchanged. Each
line is sampled and digitalized on 8 bits with a sampling
frequency f = 3 x fburst or f = 4 x fburst synchronous
to the burst so that in the PAL(NTSC) system the phase
of the color is preserved. Each line is thus divided in
principle in 3 (or 4) x 256 segments of 8 bits each.
Figure 4 shows the principle of scrambling and
unscrambling of the video signals transmitted. At the
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emission side, a random generator 25 generates code words in real
time. An information 26 relating to the identification of the
emission to be transmitted is delivered with a transmission key
and the code word to an enciphering system 27 e.g. the system DES
which delivers in real time the enciphered data 28 to be
transmitted. The transmission key may be transmitted in
enciphered form. At the receiving side, the transmitted signal
together with the transmission key is delivered to the deciphering
system 29 (e.g. of the card CPTV) which delivers the deciphered
code word (provided however that the emission in progress has been
purchased) and the information of identification 26. The code
word controls a pseudo-random generator 30 which in its turn
delivers input pointers 31 for a table 32 of 256 codes of
permutation. At each line of the video signal, the table selects
amongst 33 buffers the one which permits the de-permutation of the
lines. It is possible to enter and to extract from the buffers
the lines in any desired order. Figure 2 shows schematically a
memory buffer 18 which memorizes the 3 x 256 segments 19 of 8 bits
forming one line of the video signal. In this memory, the
segments or samples are entered sequentially in successive
locations or positions of the memory. Figure 2 shows that the
introduction of a new segment 20 in one of the positions of the
memory liberates the segment 21 which was previously memorized at
this position so that the memory is always filled with 32 lines
and that a buffer is always filled with 3 x 256 segments. The
scrambling/unscrambling as indicated above provides an excellent
safety against piracy because of the following reasons:
- The number of possible permutations of the lines is
very great so that it is difficult to find the "right
combination", either at random or by correlation.
- The code word is transmitted in real time so that
even if a pirate finds the right combination, this one is only
valid for a very short instant of time, e.g. one second.
- The code word is transmitted in enciphered form,
e.g. according to the system DES but not exclusively, this system
DES being practically unbreakable.
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- The deciphering of the code word takes place as
will be seen later in a "smart" card (card pay television or card
CPTV) comprising a microprocessor. It will be seen that this card
guarantees a complete safety. As an alternative, the
microprocessor of the card may be one of the elements of the
decoder.
- The CPTV-cards are reprogrammable which permits to
change periodically the keys of encoding.
in the version of deep scrambling, one effects the
following scramblings in addition to the permutation of the lines
as mentioned above:
- In accordance with Figure 5, a rotation of the
active part of the line about itself, which means that the active
part of the line starts e.g. in the middle of the true line and is
followed, after the end of the true line, by the beginning of this
same line.
- In accordance with Figure 6, an inversion of the
polarity of the active line with respect to the level
corresponding to 50 IRE. This operation permits to effect an
automatic adjustment of the level of luminosity for maintaining it
constant. This permits to suppress the possibility of image
recognition by changing the ambient lighting.
- In accordance with Figure 7, a mirror symmetry of
the video line according to which the active part of the video
line is submitted to an axial symmetry around an axis
perpendicular to the black level of the video signal.
It is remembered that the scrambling affects only the
active part of the line, this active part being permuted as well
as the lines corresponding to the return of the vertical scanning
which permits the scrambling of the data of teletext, but in all
cases the horizontal synchronizing
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pulses remain unchanged. At the output of the buffer, the
data is converted in an analog signal by a digital/analog
converter of 8 bits. One obtains thus an unscrambled SECAM
or PAL(NTSC) signal for being displayed on the screen of
the television receiver. Let us examine now the principle
of the decoder and its possibilities of applications.
Figure 3 shows that the video signal from the trans-
mitting station is delivered to the buffer 9 of 32 lines
which permits to de-permute the lines as indicated above,
and to a receiver of data of teletext 10. It has been
shown that the data in format of teletext is superimposed
on the video signal so that it is transmitted at the same
time and through the same channel than the video signal.
This data is transmitted by the lines corresponding to the
return of the vertical scanning of the video signal. A
microprocessor 11 is the central element of the decoder. It
comprises dead and live memories for all informations which
may be displayed on the screen of the television receiver,
these informations which are in format of teletext being
partly transmitted by the transmitting station or head
station and memorized in a live storage unit of the micro-
processor and partly memorized in a dead storage unit of
the microprocessor 11. The microprocessor is associated to
a safety card called CPTV-card (card pay television) 12
normally inserted in the decoder and to a keyboard 13 of
the decoder. The microprocessor delivers, in response to an
actuation of the keyboard, the data of teletext in an
adequate numerical form to a text generator 14 which
controls the display on the screen of the television
receiver. The microprocessor 11 controls in cooperation
with the CPTV-card 12 the buffer 9 for de-permuting the
lines as mentioned previously. The unscrambled video
signal as well as the signal of the text generator are at
disposal at 15 for being displayed by the television
receiver. Further, the microprocessor is also connected to
a modem 16 coupled itself to a telephone line 17.
Let us examine now in more details the possibilities
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of the above described decoder.
The CPTV-card is an active card comprising a
microprocessor with a non volatile memory (RAM with battery, NVRAM
or E2PROM) which is illegible from the outside (safety), and a
clock. In accordance with Figure 8, the card comprises two
electronic parts: a security device with a CPU and the associated
memories like ROM, E2PROM, RAM as well as a memory interface and a
separate E2PROM memory of 2 Kbytes. The card is in principle in
form of a credit card. It comprises the four following functions:
- Decoding of the code word which is enciphered at
the transmission side e.g. according to the system DES. The card
receives from the microprocessor 11 the enciphered key and
returns, when the emission has been purchased, to the
microprocessor the deciphered key permitting to depermute the
lines of the video signal.
- Memorizing of the list of informations of
identification of the emissions which have been purchased as well
as of the subscriptions to a particular sort of emissions (sport,
culture, cinema, cooking, etc). This memorization of the list of
the emissions which have been purchased permits the CPTV-card to
return to the microprocessor the deciphered key only if the
emission has really been purchased. Further, and for statistical
purposes as well as for invoicing, it is possible to have a
knowledge of the emissions purchased.
- Management of the credit. The CPTV-card memorizes
an information representative of the credit which is at disposal
for purchasing emissions or subscriptions. In order to purchase
an emission, the user visualizes by action on the keyboard 13 the
list of the forecasted emissions which have been received
previously from the head station and are memorized in the
microprocessor 11 of the decoder. These memorized emissions are
selected by menu and displayed on the screen of the television
receiver 8. The television receiver displays in addition to the
title of the emission, its number, its price, the date of emission
and other useful information if necessary. Then, the user selects
the number of the desired emission or subscription by actuating
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the corresponding keys of the keyboard. If the credit at disposal
which is memorized in the card is sufficient for purchasing the
desired emission or subscription, the latter is purchased and its
number is immediately memorized in the card on the list of the
emissions purchased and the corresponding amount is deducted from
the memorized credit at disposal. When the purchased emission is
broadcasted, it is identified by the card by comparison with its
information of identification registered on the list of the
purchased emissions and the card delivers to the microprocessor
the key for unscrambling.
- The fourth function of the CPTV-card is a function
of measurement of time by means of its clock. This function is
used in relation with certain emissions, e.g. emissions of
interactive games between the user and the head station as
described later on.
One sees that the card distinguishes over the cards from
the prior art in that it memorizes all confidential or secret
informations which are necessary to unscramble the video signal
with the data of teletext, like the key for unscrambling and the
credit at disposal. The card manages the credit, memorizes the
emissions purchased and permits the unscrambling of the purchased
emissions. It may be reset and the whole memorized data may be
changed. It is thus possible to modify the keys for unscrambling
and to reuse the card after it has been newly programmed.
For all operations to be effected, the user is guided by
the decoder itself which displays on the screen of the television
receiver by means of the text generator 14 texts and instructions
for actuating the keyboard. To this purpose, part of the
information which is capable to be displayed in the form of text
is memorized permanently in a ROM of the microprocessor 11, this
part of the information corresponding in principle to the
instructions of actuation of the keyboard and the other part of
the information is temporarily memorized in a RAM of the
microprocessor 11, this second part of the information being
transmitted by the head station and concerning more particularly
the forecasted programs. The following possibilities are offered
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to the user:
- Display of the titles, prices, etc. of the
forecasted programs,
- Guide for the purchase of the forecasted emissions,
- Guide for the purchase of subscriptions
(windows/channels),
- Guide for the auxiliary functions of the decoder,
- Dialogue with the decoder, clearly intelligible for
the user,
- Reception of messages (private, general),
- Reception of standard teletext.
The preceding shows that the dialogue between the user
and the decoder is enormously facilitated by the decoder according
to the preset invention.
- Further, the decoder is particularly well adapted for
the purchase of an emission by the user, directly from its home or
location. The purchase of this emission being effected by the
user in a dialogue with the decoder (pay-per-view) without any
transmission of information to the head station or administration
center, which means that all purchasing operations are effected in
the CPTV-card, the decoder being capable to understand the orders
given by the user and to transmit them to the CPTV-card. To this
purpose, the user selects simply by means of the keyboard the
title of the desired emission by typing the number of this
emission and by actuating a key "validate". If the credit
contained in the CPTV-card is sufficient, the number or the code
of the selected emission or subscription is memorized in the list
of emissions purchased, the corresponding amount is deducted from
the credit at disposal in the card and, when the emission
purchased is broadcasted, the card delivers to the microprocessor
the code for unscrambling the emission. This principle permits to
avoid an
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overload of the telephone lines and permits the user to
purchase at any time an emission, even if it is already in
progress (pay-per-view).
If the credit at disposal in the card is null or
insufficient for permitting the purchase of a desired
emission or subscription, a new credit or an increase of
the credit still at disposal may be recharged, resp. added
in accordance with the following possibilities:
- Purchase by the user at the administration center
of a code (e.g. of 9 ciphers) corresponding to the amount
of the credit to be reloaded and reload of the credit by
inserting this code in the decoder, the new credit being
memorized in the CPTV-card.
- By postal exchange of the card at regular intervals
of time which permits the head station to know which
programs have beeen viewed (statistics).
- Reload of the credit in a sales point (video club,
bank) which also permits to know which programs have been
vlewed .
20- By remote control; credit may be reloaded in the
card wireless or by cable upon order of the head station.
Possible applications of the decoder according to
the invention are as follows:
- Display of menus for the purchase of subscription
programs with possibility of knowing at all time the pro-
grams purchased, these programs being designated by a
particular sign (letter, cipher), display of the credit
still at disposal, of the date of expiration of the sub-
scriptions, of the total offer from the head station, that
is updating of the programs which will be transmitted at
a later date, modifications of the conditions by groups of
subscribers, etc. All of these informations which are
delivered by the receiver of data of teletext (10) are
memorized in the microprocessor 11 and displayed upon
request on the screen of the television receiver by the
text generator 14.
- Interactive games: This concerns as an example a
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game of lotto with the head station, the user taking part
to the game by staking a number and by inserting the
corresponding amount by means of the keyboard. The number
and the amount are memorized in the CPTV-card. The winning
number drawn at the transmission side is transmitted by the
video signal in enciphered form as data of teletext and if
it corresponds to the number staked by the user, the latter
is authenticated by the CPTV-card by means of its internal
clock which determines exactly the time which has elapsed
between the stake and the control of the winnings in order
to avoid fraud, being well understood that the stake must
precede the drawing at the emission side. If the player is
a winner, the decoder calls the station of emission by
means of the modem 16 and the winnings is gratified in
form of an increase of the credit in the CPTV-card, this
increase being controlled by the head station (see under).
- Reception of private messages transmitted by the
head station. These messages are broadcasted in a format of
teletext and the decoder verifies that it is itself the
right one which is entitled to receive the message. To
this purpose, each CPTV-card comprises a memorized parti-
cular identification number which is transmitted by the
head station with each message. If the decoder is the one
to which the message is addressed, the CPTV-card permits
the display of the message on the screen of the television
recelver .
- Safety code. It is a code provided for avoiding
that non authorized persons, e.g. children, purchase
emissions or subscriptions the amount of which exceeding a
predetermined amount, or that these children take part to
games which are not intended for them. In this case, it is
necessary to introduce by means of the keyboard a safety
code in the decoder as a pass word. If the latter is not
introduced or if it is not correct, the decoder will not
execute the orders which are given to it.
- Connection by modem. Thanks to the modem 16 connec-
ted to the microprocessor 11 and to a telephone line 17,
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it is possible for the decoder in which the CPTV-card is
inserted, to call automatically the head station, e.g.
during the night (due to the presence of the clock of the
CPTV-card) in order to be recharged by a new credit if the
5former one is already exhausted. At the same time, the
decoder transmits to the head station the set of the
emissions which have been viewed. If desired, the decoder
may call the head station periodically, at fixed dates.
The decoder may be used as a receiver of standard
10teletext.
As an example of the organization of the data in
the lines of the video signal, Figure 10 shows the struc-
ture of the physical data in the case of the data trans-
mission system~Didon (French). One line L0, L1, L2, L3,
15or L4 is transmitted at each frame of the video signal.
Thus a group of 5 lines (Lo to L4) is transmited every 5
frames. One sees more particularly that the lines L2 and
L3 transmit pages of data of 32 Bytes each. The addressings
B0 to B5 permit to select one group of subscribers amongst
20224 groups each of 240 subscribers. The addressings com-
prise the number of the group, the action to be executed,
e.g. the recharging of credit, and the list of the subs-
cribers which are concerned by the particular action. The
synchronization is ensured by a rest of CRC-16 as shown.
25Further, each line is protected by a CRC-16 which permits a
detection of error and each group of 5 lines is protected
by an EXCLUSIVE OR which permits to correct one erroneous
line. The advantage of an organization of the data accor-
ding to Figure 10 is that it facilitates enormously the
30retrieval of the information in the decoder.
As a further example, Figure 11 shows the structure
of the data in the case of the Di-oS (Didon operating sys-)
tem. One sees that the data is organized in 65536 pages
of 32 bytes each. The four first pages (0 to 3) of the
35structure of the data comprise a table which delivers for
each channel capable to be transmitted the address of the
first page of a bloc of pages corresponding to this channel
*Trademark _ 14 _
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as well as the length of the bloc which is identical with
the number of pages of this bloc and the number of the
revisions or modifications which have been made to the
bloc. The advantage of the organization according to Figure
11 is that it leads to a versatile structure for the
transmission of the data and that it permits an versatile
future extension.
The preceding shows that the decoder according to the
invention offers a very great number of possibilities to
the user and that it facilitates more particularly the
dialog with the user due to the display on the screen of
the television receiver of the programs of the forecasted
emissions and of the instructions which have to be executed
on the keyboard for the purchase of these emissions or for
reloading of credit. Moreover, the CPTV-card makes the
decoder extremely safe against piracy.
