The invention relates to a method for distributing audiovisual sequences.
In order to protect an audiovisual sequence against hacking, it is known to tattoo the audiovisual sequence in a visible or invisible way, so as to identify the holder of a pirated copy. It is also known to encipher the audiovisual sequence during a transmission in order to prevent the illegal playing of the sequence. Of course, it is possible to combine the two protection methods by enciphering the audiovisual sequence at the level of an enciphering module on the transmitting side and by tattooing the sequence after the deciphering at the level of a tattooing module on the receiving side. However, such a method would not prevent a fraudor from retrieving the sequence at the output of the deciphering module prior to the passage thereof at the tattooing module. Such a fraudor could then freely use the non tattooed audiovisual sequence if he or she could decipher the sequence.
In order to solve this general problem, a method is known for distributing a marked audiovisual sequence from a nominal audiovisual sequence to a receiving item of equipment, said nominal audiovisual sequence having a nominal content, the method including steps wherein:                a first modified flow having a modified content different from the nominal content is generated, and        a second marked complementary flow including marked complementary digital information is generated;        said first modified flow and said marked complementary information are transmitted to the receiving item of equipment so as to allow the restoration of said marked audiovisual sequence at the receiving item of equipment.        
Such a method is known from the application WO 2004/062281. In one embodiment of this application, the complementary flow includes marking instructions intended to insert an invisible and customized mark into the marked audiovisual sequence. In a preferred embodiment of the application WO 2004/062281, these instructions more particularly make it possible to reverse the LSB of some visual coefficients, such as the DC coefficients. The absence or not of a reversion on an LSB will make it possible to determine the first mark inserted into the audiovisual sequence during an identification step. With such instructions being inserted into the complementary flow, a fraudor could not have access to the audiovisual content prior to the application of the marking instructions and thus prior to the insertion of the first customized mark.
However, this type of method cannot be used in combination with known tattooing or marking devices. As a matter of fact, in the above-mentioned application, the elements contained in the marked complementary information enabling the marking of said audiovisual sequence contain instructions which are specific to the marking operation: reversion of the LSB of some visual coefficients. Marking instructions are thus predefined so that it is not possible to mark the audiovisual sequence using a standard marking device thus generating a priori unknown marking information.
Following the above-mentioned document, a problem that the invention intends to solve is thus to facilitate the marking of the audiovisual sequence. Another problem which the invention intends to remedy consists in allowing the marking of an audiovisual sequence while allowing the utilization of any type of marking device or without knowing the marking information beforehand. Such problems are solved by the invention which relates to a method such as described hereabove, wherein the step during which the second mark complementary flow is generated, includes steps wherein:                an operation of marking of said nominal audiovisual sequence is carried out so as to determine a marked audiovisual sequence having a marked content;        a difference between the marked content, on the one hand, and said modified content or said nominal content, on the other hand, is determined; said marked complementary digital information depending on said difference.        
Thus, according to the invention, using the difference between, on the one hand, the marked content and the modified content or the nominal content, on the other hand, it is possible to determine what mark has been applied to the nominal audiovisual sequence during the marking operation, while transmitting data enabling the restoration of the marked audiovisual sequence in a secure way. Thus, it is possible to transmit one securely marked audiovisual sequence without knowing a priori the marking information and thus from any tattooing device.
According to one embodiment of the invention, the step consisting in determining a difference between said nominal content and said modified content includes steps wherein:                a second modified complementary flow comprising complementary digital information capable of allowing the restoration of the nominal content from the modified content is generated,        at least a piece of marking information is determined as a function of the bit differences between the marked content and the nominal content;        said marked complementary digital information are determined as a function of said complementary information and said marking information.This embodiment has the advantage of being implemented on a known protection module. In this embodiment, said marking information, said complementary information and said marked complementary information can have an identical format. This more particularly makes it possible to make the transmission method even safer.        
Besides, in one embodiment, the step consisting in generating said second marked complementary flow includes steps wherein:                a second modified complementary flow comprising complementary digital information capable of allowing the restoration of the nominal content from the modified content is generated,        marking information are determined so as to enable the restoration of said marked audiovisual sequence from said nominal audiovisual sequence, said marking information being determined further to the operation of marking said nominal audiovisual sequence;        said marked complementary digital information are determined as a function of said complementary information and said marking information, wherein said marking information, said complementary information and said marked complementary information have an identical format.According to another embodiment of the invention, the step consisting in determining a difference between said marked content and said modified content includes steps wherein:        said marked content and said modified content are compared at the bit level so as to determine said difference.This more particularly makes it possible to easily obtain the difference between the marked content and the modified content.        
In order to obtain a marked audiovisual sequence which is also customized, said marked complementary information may include a customization identifier. This customization identifier can include a single identifier of said receiving item of equipment and/or a single identifier of a user of said receiving item of equipment, and/or a single identifier of said marking operation. This makes it a customization of the marked content possible as a function of the selected identifier and by possibly using an identifier database. The marking according to the invention can thus include a tattooing and a customization.
In order to improve the protection of the audiovisual sequence against possible fraudors, said marked complementary digital information can include information relating to digital rights associated with the nominal audiovisual sequence. Said marked complementary information include a tattoo so that said marked content is visually and aurally identical to the nominal content, so that possible fraudors will not be able to detect the marking and so that an authorized user will not be disturbed in the consumption of said marked audiovisual sequence. Said nominal audiovisual sequence has a nominal format wherein said modified content has a format identical with said nominal format, so that a user can have access to some information of the nominal audiovisual sequence without being able to consume this sequence in a satisfactory way without a particular authorization.
The invention also relates to a system for distributing a marked audiovisual sequence from a nominal audiovisual sequence to a receiving item of equipment, said nominal audiovisual sequence having a nominal content,
the system including:
                means capable of generating a first modified flow having a modified content different from the nominal content, and        means capable of generating a second marked complementary flow including marked complementary digital information;        means capable of transmitting to the receiving item of equipment said first modified flow and the marked complementary information, so as to allow the restoration of said marked audiovisual sequence at the level of the receiving item of equipment;the system being characterised in that the means for generating a second marked complementary flow include:        means capable of carrying out an operation of marking said nominal audiovisual sequence so as to determine the marked audiovisual sequence having a marked content;        means capable of determining a difference between, on the one hand, said marked content and on the other hand, said modified content or said nominal content;        means capable of generating said marked complementary digital information as a function of said difference.        
In the Figures, identical references refer to similar technical elements except otherwise indicated hereinunder.