Watermark embedding and detection

ES 2 282 151 T3

DESCRIPTION

Insertion and detection of watermarks.

Field of the invention

The invention relates to a method and an arrangement for watermarking an information signal, for example an audio or video signal. The invention also relates to a method and arrangement for detecting a watermark in such an information signal.

Background of the invention

A known method of watermarking a video signal is disclosed in international patent application WO-A-99/45705. In this method, a watermark pattern is added to the video signal. A watermark detector correlates the same pattern with the suspect signal. If the correlation exceeds a given threshold, the pattern is said to be present. The presence or absence of the pattern represents a single bit of information. The embedded watermark can also carry a multi-bit payload. In the system disclosed in WO-A-99/45705, the payload is represented by a combination of one or more basic patterns and spatially displaced versions thereof. The payload is encoded in the respective displacement vectors. The watermark detector correlates each basic pattern with the suspect signal, and determines the spatial positions of the basic patterns relative to each other. The detector also checks if these positions constitute a valid payload.

The document HANGSHENG XU ET AL: “Applications of digital watermarking technology in audio signals” J. AUDIO ENG. SOC, JOURNAL OF THE AUDIO ENGINEERING SOCIETY, vol. 47, no. 10, October 1999 (199910), pages 805-812, XP000928475 USA discloses a method for inserting a watermark, in which a watermark is made based on the content of the signal in which the watermark will be inserted. The original audio is divided into different types of segments according to their characteristics in the time domain. For each incoming audio segment, frequency characteristic analysis is used to characterize the content of the audio segment. Using the frequency characteristics, the audio segment is classified into one of these predefined types. Based on the number of types, an insert scheme is selected. Only a basic watermark pattern is used.

The process of correlating watermark patterns to the suspect signal requires the watermark detector to present locally stored versions of those patterns. In light of this, it is desired that the watermarking system employ only a few different patterns. The patterns that are being used are kept secret from the outside world. However, even without knowledge of the patterns, a hacker can compromise the system if he has the relevant inserter at his disposal. You can feed such an inserter with an arbitrary input signal and subtract the signal from its watermarked version. The difference signal obtained in this way resembles the watermark of any other watermarked signal, depending on the perception pattern used in the available watermark inserter. If the difference signal is combined with (for example, added to or subtracted from) a watermarked signal, the embedded watermark will be basically removed or at least no longer represent a valid payload. In any case, the inserted watermark has become ineffective.

Object and summary of the invention

It is an object of the invention to provide a more secure method and arrangement for inserting and detecting a watermark in an information signal, even if a hacker has a watermark inserting device at his disposal.

To this end, the method according to the invention comprises the steps of analyzing a given property of the information signal and determining a real value of said property, associating different watermarks with different values ​​of said property, and inserting the associated watermark with that real value. The corresponding watermark detection method comprises the steps of analyzing a given property of the information signal and determining a real value of said property, associating different watermarks with different values ​​of said property, and detecting the associated watermark. with that real value.

The invention achieves that the pattern of the inserted watermark changes from time to time, depending on the content of the information signal. Feeding the inserter with an arbitrary signal to produce a signal that resembles the watermark, as described above, no longer works because the arbitrary signal has different properties. A significant advantage of the invention is that the number of different watermark patterns that the detector must store can be much less. This number is a result of balancing the complexity of the detector with respect to safety.

There are numerous examples of information token properties that can be used to select the watermark pattern to be inserted. The only requirement that must be fulfilled is its robustness or invariability with respect to the inserted watermark. Advantageous examples of properties are different distributions of luminance values ​​of a video signal or different forms of the frequency spectrum of an audio signal.

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Other aspects of the invention will become apparent and elucidated with reference to the embodiments described below. The examples relate to the insertion and detection of watermarks of video signals, but it will be appreciated that the invention applies equally to audio signals or to any type of multimedia signal.

Brief description of the drawings

Figure 1 schematically shows a diagram of a watermark inserting device according to the invention.

Figure 2 schematically shows a diagram of a watermark detector according to the invention.

Figure 3 shows an arrangement to illustrate the operation of the inserter and the watermark detector.

Figures 4 and 5 show other embodiments of the watermark insertion device according to the invention.

Figure 6 shows another embodiment of the watermark detector according to the invention.

Description of preferred embodiments

Figure 1 schematically shows a diagram of an embodiment of a watermark inserting device 1 according to the invention. It will be assumed here that the inserted watermark represents a 1 bit payload. For example, the absence of a watermark indicates that the video signal can be freely copied, while the presence of a predetermined watermark indicates that making a copy of the signal is prohibited.

The inserter receives an input video signal I in the form of a sequence of images, and comprises an adder 11 that adds a watermark pattern W, to each image. The inserter further comprises an image analyzer 12, a selector 13, and a read-only memory 14 in which a plurality of different W patterns are stored.₁ to W_N watermark. The analyzer 12 receives the video signal and analyzes a given property P of the video signal as a function of time. The actual value of property P found by parser 12 is applied to selector 13. In response to it, the selector selects one of the patterns W₁ to W_n watermarks stored for adder 11 to insert.

The analyzer 12 can take a number of forms. Some examples will be given to provide sufficient illustration to enable one of skill to design suitable alternative embodiments. The property being analyzed can be the distribution of luminance values ​​along the image (spatial distribution) or through a sequence of images (temporal distribution). In a first example, the analyzer divides each image into sub-images, and determines which of said sub-images has the highest average luminance. The number of relevant subpictures is the actual value of property P. In a second example, the analyzer assigns "0" to each subpicture that exhibits low average luminance and a "1" to each subpicture that exhibits high average luminance. Each video image is now characterized by an n-bit code, where n is the number of sub-images. The relevant n-bit code is the actual value of property P. The property being parsed can also be local image activity. Such an analysis can be carried out in the frequency domain.

Figure 2 schematically shows a diagram of a preferred embodiment of the watermark detector 2 according to the invention. The detector receives a suspect video signal J and comprises an image analyzer 22, a selector 23 and a read-only memory 24 that are identical to their corresponding counterparts in the inserter 1. Therefore, the analyzer 22 analyzes the same property P of the video signal, and the selector 23 selects the same watermark pattern W, from among the patterns W₁ to W_n stored, than the inserter.

The detector further comprises a correlation circuit 21 that calculates the correlation between each image of the suspect video signal and the applied watermark pattern W,. If the correlation exceeds a predetermined threshold, the selected watermark pattern W, is said to be present (D = 1), otherwise, it is said to be absent (D = 0).

The correlation circuit 21 is preferably of a type that performs the correlation for all possible spatial positions of the applied watermark with respect to the image. Such a correlation circuit is disclosed in WO-A-99/45705. Generates a correlation pattern that shows a peak for each spatial position of the watermark. WO-A-99/45705 describes that multiple peak positions can represent a payload. However, as mentioned above, the payload in this example is a 1-bit control signal. Detection circuit 2 will consider the presence of 2 or more peaks as an invalid payload (D = 0).

It is assumed that the patterns W₁ to W_n The watermarks are secret and cannot be retrieved by interrogating the inserter or detector circuits. As will be explained below with reference to Figure 3, the invention prevents a hacker from compromising the system when he has an insertion device at his disposal. In Fig. 3, a potential hacker receives a video signal V 'having a watermark embedded by an embedding device 1a. The V 'signal may be a recorded signal, in which case the actual insert

ES 2 282 151 T3 took place a long time ago: the insertion device 1a is of a type as described above with reference to figure 1.

The hacker has an identical inserter 1b at his disposal. An arbitrary video signal X is applied to said inserter 1b to locally generate a watermarked video signal X '. An adder 3 subtracts the arbitrary signal X from its watermarked version X '. The difference signal (which closely resembles the inserted watermark pattern) is then combined (added to or subtracted from) the watermarked signal V 'by an additional adder 4. The suspect signal V "thus processed is applied to the watermark detector 2 as described above with reference to FIG. 2.

Without the provisions of the invention, both insertion devices 1a and 1b insert the same watermark on the respective input signals. This results in either the removal of the watermark on the suspect V ”signal or an invalid payload due to multiple occurrences of the watermark pattern W at different positions. In both cases, the detector generates an output signal D = 0 and the pirate attack is successful.

With the provisions of the invention, the water mark W, (i = 1 ... n) in the signal V 'will generally differ from the water mark Wj (j = 1 ... n) of the signal X', because the contents of the video signals V and X are different. The properties analysis algorithm of detector 2 responds to the content of the signal V ”, which is basically equal to the content of V. Consequently, the watermark pattern that is being checked by the detector 2 is the watermark pattern W, which has been inserted by the inserter 1a. The detector ignores the additional presence of a different pattern Wj, and the pirate attack therefore fails.

A possible alternative attempt is to feed the inserter 1b with the watermarked signal V 'instead of an arbitrary signal X, to force the inserter 1b to select the same watermark W, as the watermarked device 1a. insertion. To avoid this, the devices 1a and 1b are preferably of a type that prevent the insertion of a watermark into a signal that is already watermarked. Figure 4 shows a schematic diagram of such an insertion device. It comprises the same adder 11, image analyzer 12, selector 13 and ROM 14 as the insertion device shown in figure 1. It also comprises the correlation circuit 21 of the detector shown in figure 2. The correlation circuit 21 detects if the input signal I already includes the watermark pattern W¡ to be inserted. If so (D = 1), a switch 15 is controlled to prevent the watermark pattern W, from being inserted multiple times.

Figure 5 shows a schematic diagram of a watermark inserter for inserting a multi-bit payload into the video signal. The insertion device comprises the same adder 11, image analyzer 12, selector 13 and ROM 14 as described above with reference to figure

1. ROM 14 now stores a plurality of sets of watermark patterns. The inserter further includes an encoding circuit 16 that receives a selected set i of patterns W_i4, Wi, 2 ..., basic watermark and encodes a multi-bit payload at the relative positions of those patterns. The basic patterns are relatively small in size (for example, 128x128 pixels). The watermark pattern generated by the encoder 16 is then superimposed on the image by an overlay circuit 17. ROM 14 stores different sets of basic patterns for different values ​​of the signal property P. The actual set of basic patterns that are applied to the encoder 16 is controlled by the actual value of the property P and changes as a function of time.

Figure 6 shows the corresponding watermark detector. The detector comprises a folding circuit 25 for folding and storing 128x128 pixel image segments in a buffer prior to correlation. The detector further comprises the same correlation circuit 21, image analyzer 22, selector 23 and read-only memory 24, as described above with reference to FIG. 2. ROM 24 stores different sets of basic patterns for different values ​​of the signal property P. The actual set of basic patterns that are applied to the correlation circuit 21 is controlled by the actual value of the property P.

It should be noted that the invention is not limited to the watermarking systems described in the embodiments. For example, a watermarking system is known that uses n different watermark patterns, each pattern corresponding to one bit of an n-bit payload. According to this invention, the inserter and detector of such a system include different sets of n patterns. A particular set is then selected in response to the actual value of a property of the signal.

In summary, a method and an arrangement for inserting and detecting a watermark in an information signal is disclosed. The inserted water mark (W¡) is selected (13) from a plurality of marks (W¡ ... W_N) of water depending on a property P of the signal. An example of such a property is the distribution of luminance values ​​of the current video image calculated by an analysis circuit (12). The corresponding watermark detector performs the same operation: the watermark that is being searched for depends on the same property of the signal. The invention achieves that the inserted watermark changes from time to time depending on the content of the information signal, so that it cannot be easily hacked.