Method for scrambling and unscrambling a video signal

A scrambling method wherein each image line (23-309) of a frame having been produced in unscrambled form before being scrambled, is given the pseudorandomly determined address (B) of the memory line in the storage memory (8) in which said image line is to be stored on reception; and wherein the moment (or the running order) for broadcasting said image line is determined so that on reception, the image line causes, because it is stored in the memory line to which it was addressed, the image line previously stored at the same address to be output to the television set at the right moment (or in the right order) so that an unscrambled picture is reconstructed.

The present invention relates to a method for scrambling and unscrambling a 
video signal comprising steps of, at an emitting site, scrambling the 
sequences of the video signal in accordance with a predetermined 
scrambling function which can be different for each sequence, emitting 
each sequence of such scrambled video signal, then, at a receiving site, 
unscrambling said sequences of emitted video signal. 
The main application of the present invention is in pay-television (pay-TV) 
systems in which it is essential to scramble the video emission and 
unscramble or decode the program at the subscriber's location by means of 
a suitable decoder possessed by a subscriber who is authorized to view the 
emitted program. 
Some methods for scrambling and a corresponding unscrambling are well known 
and, in such methods, it is aimed to make the emitted image and/or sound 
incomprehensible or their vision and/or audio uncomfortable. 
A method for scrambling consisting of permuting a constant predetermined 
number of lines of the video frame (or video image) in accordance with a 
predetermined permuting function is well known. In this case, the 
unscrambling consists in carrying out the reverse function from the 
function used during the emission to permute such predetermined number of 
image lines. As it is not technically easy to thus permute a great number 
of image lines, the limit is now about 32 permuted image lines. 
This well known scrambling method, sometimes called the fixed block 
inversion method, has a drawback which consists of requiring, at the 
receiving site, a memory of great size because it is necessary to have, 
just after sending to the TV set a sequence of 32 lines, several next 
memorized lines for being capable to send immediately the next image to 
the TV set. This method is relatively simple to carry out, at an emitting 
site, but there are problems of security, reliability and the cost at the 
reception site because of the great number of decoders. On the other hand, 
this method is not reliable enough relative to some kinds of pirated 
viewing. In fact, it is possible to recover the permutation order, even if 
this order frequently varies during the emitting phase. 
It can be seen in the example mentioned above that the unscrambling 
function is almost the same as the scrambling function since both 
functions have the same nature and are the reciprocal. Further, this 
function is simple to pirate because it is unconnected with other 
parameters or informations. 
The present invention avoids the above drawbacks. 
For this purpose, the method according to the invention is characterised in 
that, at the transmitting site, a first video sequence is scrambled in 
accordance with a first scrambling function and another video signal 
sequence is scrambled in accordance with another scrambling function and, 
at the receiving site, said other video signal sequence which is scrambled 
in accordance with said other scrambling function is stored in place of 
said first video signal sequence which is scrambled in accordance with 
said first scrambling function, replacing it, and, during this 
replacement, said other video signal sequence is stored in a scrambled 
manner in accordance with an actual scrambling function which is equal to 
a predetermined combination of the first scrambling function and the other 
scrambling function. 
According to an embodiment of the invention, each scrambling function 
consists in emitting a group of lines of the video image in an order 
different than a normal order which corresponds to the intelligible image, 
providing at the receiving site a storage memory comprising a 
predetermined number of memory lines, each memory line being able to store 
one received image line, the storage of the one received image line 
causing the sending of a said previously stored line in this memory line 
to the TV set, indicating for each emitted image line the address in the 
memory line in which this emitted image line has to be stored, and 
determining the emitting time of this image line in such a way that it 
replaces in the memory a previous image line just at the time when this 
previous image line has to be sent to the TV set in the normal order of 
the image lines for generating the intelligible image. 
According to an embodiment of the invention, at the emitting site, the 
scrambling method consists of: allocating to each image line of at least 
one intelligibly produced image field before its scrambling, the 
pseudo-random address of the memory line of the storage memory in which 
this image line will be stored at the receiving site; and determining the 
emitting time (or emitting order) of this image line to cause, at the 
receiving site, in response to storage of this image line in the memory 
line corresponding to its address, the sending of the image line which was 
previously stored at the same address to the TV set in a sequence of video 
lines in the normal order for generating the intelligible (or clear) 
image. 
According to an embodiment of the invention, at the transmitting site, the 
method of scrambling further comprises: permanently maintaining a 
correspondence table between, on the one hand, the identification number 
of each image line of at least one frame or field of the intelligibly 
produced emission before it is scrambled and, on the other hand, its 
storage address in the storage memory at the receiving site; eventually 
eliminating from this table said image lines when they are finally sent to 
the TV set; arranging a number of stacks equal to the number of memory 
lines of the storage memory used at the receiving site, each stack 
corresponding to one memory line; successively stacking in each stack the 
identification number of each image line which will be addressed to the 
memory line corresponding to that stack during the reception of one or 
several frames or fields, beginning this procedure with the last line of 
the field(s) and stacking until the first line of the first frame or 
field; then, when all the stacks for the field(s) are stacked, carrying 
out an unstacking of each stack beginning with the bottom of the stack and 
determining, for each image line thus extracted from the stack by said 
unstacking, its emitting time (or its emitting order), this emitting time 
corresponding to the time at which the image line, which is in the same 
stack just above the image line which has just been unstacked, has to be 
sent to the TV set for generating the intelligible or clear image.

A digital image (frame or field) line conventionally includes a great 
number, for example, 256 or 512, of digital samples but only eight samples 
"a, b, c, d, e, f, g, h" are shown in the drawings for purposes of 
simplification. 
Referring to FIG. 1, an addressing and storage mode of the scrambled image 
lines received at the subscriber's decoder is illustrated. A storage 
memory (or buffer) 8 conventionally called a buffer, including 32 memory 
lines with each memory line being capable of storing the digital 
information of one full image line, can be seen. 
According to the method of the invention, an emitted line Z is sent to the 
buffer 8 and replaces a previously stored line X in the same memory line n 
of the buffer 8. The storage of the image line Z in the memory line n of 
the buffer 8 causes the previously stored image line X to be output and 
sent to the TV set, possibly after reconditioning or canceling a rotation. 
Accordingly, the image line emission order depends on the address n of 
each of the stored lines since each line Z will be emitted only when it 
replaces a previously stored line X in the buffer memory line 
corresponding to the Z line address, and only at the time when the 
previously stored line X is to be sent to the TV set. 
It can be seen that the order for emitting the lines (line emission order) 
is not merely and arbitrarily permuted, but the permutation order is the 
result of a combination with each image line address in the buffer 
situated at the receiving site. Thus, the scrambling function by image 
line permutation is a relative function and not an absolute function. 
According to an embodiment of the invention and with reference to FIG. 2, 
the scrambling method consists of: 
a) permanently maintaining a correspondence table between, on the one hand, 
the identification number L of each image line of at least one frame or 
field of the intelligibly produced emission before it is scrambled and, on 
the other hand, its storage address B in the storage memory (or buffer) 8 
at the receiving site; 
b) eliminating from this table the image lines when they are finally sent 
to the TV set to be able to constitute a next table; 
c) providing a number B (in this example 32) of stacks P (e.g. P1 to P32) 
equal to the number B of memory lines of the storage memory (or buffer) 8 
used at the receiving site and giving a reference to each stack which is 
identical to the memory line which corresponds thereto; 
d) successively stacking in each stack P the identification number (309, 
308, 307, 306, 305, . . . ) of each image line that will be addressed 
during the receiving phase to the line of buffer 8 corresponding to this 
stack, beginning with the last line (in this example line 309) and 
stacking until the first line is stacked (in this example, the first line 
is line 23--in a video field, the active lines are conventionally 
designated by indexes 23 to 309); 
e) then, when all of the stacks are thus constituted for the frame(s) or 
field(s), carrying out an unstacking of each stack (from P1 to P32) by 
beginning with the bottom of the stack (e.g., for the first stack P1, the 
extraction begins with 308 then 306, etc . . . ) and determining for each 
image line whose identification number is thus extracted from a stack by 
said unstacking (e.g., for the line 308) its emitting time, that time 
being the time which corresponds to the time where the image line in the 
same stack just above the image line which is unstacked must be sent to 
the TV set to constitute the intelligible image (in this example, for line 
308 the emission time of this line must be the instant when the line 306 
is to be sent to the TV set to form the intelligible image; 
f) hence, all the image lines of the field(s) to be emitted can be arranged 
in a memory of great size in accordance with the time where each one will 
be emitted, and when all the lines of the field(s) are so arranged, it is 
sufficient to emit the image lines in accordance with their own 
arrangement in this memory. 
An example of device for carrying out this scrambling method, with 
referenced to FIG. 2, is shown schematically in FIG. 3. In this figure, a 
microprocessor (.mu.P) 14, a RAM 16 and a ROM 15 are shown. The ROM 16 
includes a logic processing program for carrying out the logic steps 
executed in the microprocessor 14, which correspond to the method 
described above with reference to FIG. 2. The RAM 16 stores variable data 
during execution of the program. Such a disposition is conventional. This 
device outputs, via a dual port RAM 17, the series 18 of the image line 
identification numbers (in the present example the series 23, 28, 305, . . 
. 29) corresponding to the series of image lines as they have to be 
successively emitted so as to restitute at the receiving site a clear 
image in a decoder simultaneously receiving a corresponding unscrambling 
code. 
FIG. 4 shows an embodiment of the remaining portion of the device for 
carrying out the method of the invention. In this portion, an entry path 
S1 inputs the intelligible image to the emitting station. This image is 
digitalized through an A/D converter 10 and is stored in successive lines 
(e.g. the field lines 23-309) in a buffer 11. During this storing, another 
identical buffer 12, in which has been stored a previous field, is read 
with a reading order of the lines (23-309) determined according to the 
scrambling method of the present invention (with the device of FIG. 3), 
and each extracted line passes through a D/A converter 13 and is then 
transmitted, this transmission $2 being scrambled as the lines are not 
output in natural order (which is lines Nos. 23, 24, . . . , 309). 
Three buffers can be used in place of the two buffers 11 and 12 in order to 
have time during the transition from one buffer to another.