EP3001415A1 - Method and apparatus for determining whether a specific watermark symbol out of one or more candidate watermark symbols is embedded in a current section of a received audio signal - Google Patents
Method and apparatus for determining whether a specific watermark symbol out of one or more candidate watermark symbols is embedded in a current section of a received audio signal Download PDFInfo
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- the invention relates to a method and to an apparatus for determining from sets of correlation result values whether a specific watermark symbol out of one or more candidate watermark symbols is embedded in a current section of a received audio signal, or whether no watermark symbol is embedded in the current section of the received audio signal.
- a watermark detector In a watermark detector cross correlations between a received signal and reference patterns are evaluated. Basically, the maximal correlation result value is compared to a threshold in order to determine whether watermark information has been embedded in the received signal. For acoustic path transmission, multiple correlation result peaks are employed for detection, in order to take a multi-path environment into account. Again, an appropriately defined metric aggregating multiple correlation result peaks is compared to a threshold for watermark detection.
- a false positive probability defines the probability that a watermark is detected for unmarked content and is denoted as P fp , which is naturally dependent on the applied watermark detection processing.
- a problem to be solved by the invention is to provide an improved watermark information detection. This problem is solved by the method disclosed in claim 1. An apparatus that utilises this method is disclosed in claim 2. Advantageous additional embodiments of the invention are disclosed in the respective dependent claims.
- order statistics are used for watermark symbol detection from the correlation result values, where the joint probability distribution function (pdf) for one or more peaks of cross correlation values between a current section of the received audio signal and reference patterns is employed directly for watermark detection.
- PDF joint probability distribution function
- Monte Carlo or quasi-Monte Carlo simulations are used for evaluating the false positive probability corresponding to a pdf value threshold.
- a pdf threshold look-up table (LUT) and an associated false positive probability look-up table can be constructed, which both are used for the watermark symbol detection. Using such LUTs significantly simplifies the complexity of watermark detection when taking more correlation result peaks for detection into account. Because the derived false positive probability has intuitive interpretation, it can be used for the design of watermarking systems employing correlation for watermark detection.
- the inventive method is adapted for determining from sets of correlation result values whether a specific watermark symbol out of one or more candidate watermark symbols is embedded in a current section of a received audio signal, or whether no one of said candidate watermark symbols is embedded in said current section of said received audio signal, wherein said current section of said received audio signal was correlated with at least one candidate reference pattern, each one of which representing one of said one or more candidate watermark symbols, said method including:
- the inventive apparatus is adapted for determining from sets of correlation result values whether a specific watermark symbol out of one or more candidate watermark symbols is embedded in a current section of a received audio signal, or whether no one of said candidate watermark symbols is embedded in said current section of said received audio signal, wherein said current section of said received audio signal was correlated with at least one candidate reference pattern, each one of which representing one of said one or more candidate watermark symbols, said apparatus including means configured to:
- a watermark detector In a watermark detector, cross correlations between a received signal and reference patterns are evaluated. Usually, the maximal correlation result value is compared to a threshold in order to determine whether a watermark is embedded in the received signal.
- multiple correlation result value peaks are employed for watermark detection, in order to take a resulting multi-path environment due to echoes and reverberation into account.
- An appropriately defined metric aggregating multiple correlation result value peaks is compared to a threshold for watermark detection.
- a false positive probability defines the probability that a watermark is detected for unmarked content and is denoted as P fp . It is naturally dependent on the applied detection method.
- the smallest P fp among all watermark symbols is compared to a threshold in order to decide whether watermark information is present in the received signal. If the smallest P fp is smaller than the threshold, a watermark is assumed to be present. The symbol associated with the smallest P fp is taken as the embedded watermark symbol. Otherwise, if the smallest P fp is higher than the threshold, it is declared that no watermark data is present.
- P fp is defined as the probability that n p or more correlation result values for a random correlation array subject to Gaussian distribution are larger than or equal to the actual n p peaks under consideration.
- detection is based on comparison of multiple peaks.
- the number of disjoint complementary cases exponentially increases with increased n p , which limits its application, especially for environments with severe reflections and/or reverberations. Another issue is associated with the interpretation of the defined P fp .
- watermark detection is carried out by comparing evaluated P fp values. Given a P fp threshold, it is not straightforward to determine the probability of evaluated P fp values for unmarked content watermark being smaller than the given P fp threshold. For single peak cases, the probability of evaluated P fp values for unmarked content watermark being smaller than the given P fp threshold is equal to the given P fp threshold. However, for n p > 1 that is not the case, as illustrated in Fig.
- P fp values delivered from the detector described in WO 2011/141292 A1 and PCT/EP2014/066063 are compared to P fp thresholds shown on the x-axis.
- the probability for P fp values being lower than a threshold is estimated by dividing the number of P fp values lower than the threshold by the total number of delivered P fp values.
- the probability of evaluated P fp values for unmarked content watermark being smaller than the given P fp threshold is higher than the given P fp threshold for n p > 1. With increased n p the deviation between both becomes larger.
- order statistics are used for watermark detection.
- two look-up tables are employed for P fp function evaluation.
- order statistics as decision metric provides a nice interpretation of evaluated P fp function values, namely, the probability of evaluated P fp function values for unmarked content watermark being smaller than the given P fp threshold is exactly equal to the given P fp threshold for any n p value.
- the probability distribution function denoted pdf of peaks resulting from unmarked content can be employed for watermark detection.
- the decision criterion is to minimise the likelihood pdf. That is, the higher the pdf value for multiple peaks, the more likely it is that these peaks are generated from unmarked content. Conversely, the lower the pdf value, the more likely it is that these peaks are generated from marked content.
- the constraint v 0 ⁇ v 1 ⁇ ⁇ ⁇ v n p -1 is referred to as peak constraint.
- watermark detection can be carried out by comparing the pdf values of normalised peak vectors in correlation arrays corresponding to different watermark symbols, and the symbol resulting in the smallest pdf value is selected as embedded watermark symbol.
- a threshold should be used to avoid a high false positive probability, or in other words, the resulting P fp using that threshold should be below the target P fp . That is, only when the smallest pdf value g ( w ) is sufficiently low, it is decided that a watermark is present in the received signal. Otherwise, if the threshold is not low enough, for unmarked content, a watermark will be detected with a high probability. Consequently, the corresponding P fp becomes high. Therefore it is necessary to evaluate P fp for a specific threshold for pdf values g ( w ).
- the pdf values g ( w ) for different watermark symbols are compared to a threshold in order to decide whether or not a watermark is present. If g ( w ) is smaller than the threshold, it is decided that a watermark is present. And the watermark symbol resulting in the smallest pdf value is taken as the embedded one. If none of evaluated pdf values is smaller than the threshold, it is assumed that no watermark information data is present.
- g ( z ) is interpreted as a threshold for determining the presence of watermark.
- M a larger number of length- L correlation arrays, say M , can be generated according to the Gaussian distribution. Normalised peak vectors of these correlation arrays are denoted as ⁇ w ( i ) , 1 ⁇ i ⁇ M ⁇ and are used for evaluating g ( w ( i ) ).
- the pdf values for extremely small or extremely large peak values are extremely small.
- P fp is represented by the area below the distribution function where pdf values are smaller than the threshold. Therefore, the evaluation of P fp can be interpreted as one-dimensional integration for the single-peak case. For multi-peaks, it is a multi-dimensional integration.
- Fig. 2 indicates that an increase of the threshold th also increases P fp .
- a look-up table with K entries for pdf thresholds is defined, for example, linearly on the log-scale in the range [ ⁇ g max , ⁇ g max ], ⁇ ⁇ 1, ⁇ ⁇ 1, where g max denotes the maximal pdf value for all possible normalised peak vectors.
- the false positive probability is determined numerically.
- the evaluation of the false positive probability can be interpreted as multi-dimensional integration.
- the convergence of Monte Carlo or quasi-Monte Carlo simulations is independent of dimension, while linear-grid based methods do depend on dimension and therefore do not converge well with increased dimension. Therefore the Monte Carlo simulation is used for the numerical evaluation of the false positive probability, whereby the Monte Carlo simulation is carried out according to the Monte Carlo method.
- LUTs lookup tables
- One LUT stores values of probability distribution function (pdf) for normalised peaks of correlation between non-watermarked content and reference patterns, and the other one stores values of false positive probability corresponding to entries in the pdf LUT. That is, each entry in the pdf LUT corresponds to a unique entry in the LUT for false positive probability. Different correlation lengths and different number of peaks result in different LUTs.
- determined LUTs are stored in the memory unit of watermark detector, which is accessed during watermark detection. As mentioned above, for watermark detection, the detector performs correlation between received audio section and reference patterns corresponding to watermark symbols. Correlation values are sorted to find peaks, which are normalised by standard deviation.
- the standard deviation is estimated either individually for each set of correlation result values corresponding to individual candidate watermark symbol, or by averaging over sets of correlation result values. Afterwards, the probability distribution function is evaluated for the normalised peaks. And the LUT for probability distribution is accessed to find the entry index which is nearest to the evaluated pdf value from the normalised peaks. This entry index is then used to access the second LUT for the false positive probability. And the false positive probability corresponding to the peaks found is then evaluated by means of interpolation or extrapolation.
- n p -dimensional hypercube l w min w max n p is used for Monte Carlo simulation.
- all volume outside the hypercube is ignored for the P fp evaluation.
- the inventors have found that, by a careful choice of w min ,w max , the influence on the evaluated P fp values is negligible for relevant P fp values in practical applications.
- Fig. 3 depicts the determination of w min , w max for the single-peak case.
- the false positive probability can be reformulated as (see the definition in equation (2)):
- a g ⁇ w d w were denotes the hyper-region subject to the peak constraint w 0 ⁇ w 1 ⁇ ... ⁇ w n p -1 and with pdf values g ( w ) less than the threshold th.
- g '( w ) g ( w ) for w ⁇
- g '( w ) 0 for w ⁇ and w ⁇
- 1 A can be interpreted as the distribution function for an n p -dimensional random vector w uniformly distributed in the hypercube .
- equation (3) is the expectation of g '( w ) with respect to a uniformly distributed random vector w :
- the pdf value g ( w ) is evaluated and compared to the pdf threshold th. If g ( w ) ⁇ th, g ( w ) values are accumulated. The final result of accumulation is scaled by A M , which delivers an estimated false positive probability. Consequently, for each entry in the pdf threshold LUT, the corresponding false positive probability is determined numerically according to equation (4).
- Fig. 4 shows a flow diagram for the generation of the pdf threshold LUT and P fp LUT, which are used for the watermark detection.
- step 41 the aim is stated to construct a pdf threshold LUT with K entries, given a pdf range [ p min , p max ].
- Normalised peak vectors are generated in step 43: Generate M times normalised peak vectors using Monte Carlo or quasi-Monte Carlo processing.
- Monte Carlo a random generator is used to generate normalised peak vectors uniformly distributed in the hyper-cube [ w min , w max ] n p , where [ w min , w max ] defines the range of generated random normalised peak values.
- a low-discrepancy sequence like Sobol sequence is generated as normalised peak values, which also approximate the uniform distribution.
- each generated normalised peak vector is sorted such that the peak constraint is fulfilled: w 0 ⁇ w 1 ⁇ ⁇ ⁇ w n p -1 , which is used to calculate pdf value g ( w ( m ) ) in step 45.
- the calculated values g ( w ( m ) ) are compared with threshold entries th i in the pdf threshold LUT.
- step 47 all P fp,i entries in the P fp LUT having a corresponding pdf threshold greater than g ( w ( m ) ) are increased by g ( w ( m ) ).
- i is incremented and, as long as i ⁇ K in step 48, the i loop continues with step 46.
- m is incremented and, as long as m ⁇ M in step 49, the m loop continues with step 43.
- the final P fp values are estimated in step 40 by scaling the P fp LUT entries by w max - w min n p n p ! M .
- FIG. 5 A first flow diagram for watermark detection is shown in Fig. 5 . There are nSymbols watermark symbols in the watermark symbol alphabet. Watermark detection is carried out as follows:
- FIG. 7 A second flow diagram for watermark detection is shown in Fig. 7 . There are nSymbols watermark symbols in the watermark symbol alphabet. Watermark detection is carried out as follows:
- step 77 the minimal P f p * value for all candidate watermark symbols is then compared to a second threshold T max > T min . If the minimal P f p * value is smaller than T max , the symbol resulting in the minimal P f p * value is determined to be the embedded one and is output in step 79. If the minimal P f p * value is not smaller than T max , it is decided in step 78 that no watermark is present in the received current signal section.
- a received watermarked signal is re-sampled in an acquisition or receiving section step or stage 61, and thereafter may pass through a spectral shaping and/or whitening step or stage 62.
- correlation step or stage 63 it is correlated section by section with the nSymbols reference patterns.
- a symbol detection or decision step or stage 64 determines, whether or not a corresponding watermark symbol is present in the current signal section.
- a secret key was used to generate pseudo-random phases, from which related reference pattern bit sequences or symbols were generated and used for watermarking the audio signal.
- these pseudo-random phases are generated in the same way in a corresponding step or stage 65, based on the same secret key.
- related candidate reference patterns or symbols are generated in a reference pattern generation step or stage 66 and are used in step/stage 63 for checking whether or not a related watermark symbol is present in the current signal section of the received audio signal.
- a look-up table 67 for probability distribution function values and a look-up table 68 for false positive probabilities are used for the embedded watermark symbol determination as described above.
- the described processing can be carried out by a single processor or electronic circuit, or by several processors or electronic circuits operating in parallel and/or operating on different parts of the complete processing.
- the instructions for operating the processor or the processors according to the described processing can be stored in one or more memories.
- the at least one processor is configured to carry out these instructions.
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Abstract
From sets of correlation result values it is determined whether a specific watermark symbol out of one or more candidate watermark symbols is embedded in a received audio signal. For all candidate watermark symbols, from each corresponding set of correlation result values, a group (np ) of maximal values together form a peak vector (v i ). From the normalised peak values (w i ) a probability distribution function (pdf, g(w i )) and a false positive probability function (Pfp (w i )) are calculated. If the values of the false positive probability function are smaller than a first threshold value (Tmin), the current candidate watermark symbol is taken as a true watermark symbol. If not yet all candidate watermark symbols have been processed, the next candidate watermark symbol is selected. Otherwise, a minimal value of the false positive probability functions for all candidate watermark symbols is determined (76) and is compared (77) with a second threshold value (Tmax). If it is smaller than the second threshold value, the current candidate watermark symbol is selected. Otherwise, it is determined (78) that no watermark symbol is present.
Description
- The invention relates to a method and to an apparatus for determining from sets of correlation result values whether a specific watermark symbol out of one or more candidate watermark symbols is embedded in a current section of a received audio signal, or whether no watermark symbol is embedded in the current section of the received audio signal.
- In a watermark detector cross correlations between a received signal and reference patterns are evaluated. Basically, the maximal correlation result value is compared to a threshold in order to determine whether watermark information has been embedded in the received signal. For acoustic path transmission, multiple correlation result peaks are employed for detection, in order to take a multi-path environment into account. Again, an appropriately defined metric aggregating multiple correlation result peaks is compared to a threshold for watermark detection.
- A false positive probability defines the probability that a watermark is detected for unmarked content and is denoted as Pfp, which is naturally dependent on the applied watermark detection processing.
- A problem to be solved by the invention is to provide an improved watermark information detection. This problem is solved by the method disclosed in
claim 1. An apparatus that utilises this method is disclosed inclaim 2.
Advantageous additional embodiments of the invention are disclosed in the respective dependent claims. - According to the invention, order statistics are used for watermark symbol detection from the correlation result values, where the joint probability distribution function (pdf) for one or more peaks of cross correlation values between a current section of the received audio signal and reference patterns is employed directly for watermark detection. In addition, Monte Carlo or quasi-Monte Carlo simulations are used for evaluating the false positive probability corresponding to a pdf value threshold. A pdf threshold look-up table (LUT) and an associated false positive probability look-up table can be constructed, which both are used for the watermark symbol detection.
Using such LUTs significantly simplifies the complexity of watermark detection when taking more correlation result peaks for detection into account.
Because the derived false positive probability has intuitive interpretation, it can be used for the design of watermarking systems employing correlation for watermark detection. - In principle, the inventive method is adapted for determining from sets of correlation result values whether a specific watermark symbol out of one or more candidate watermark symbols is embedded in a current section of a received audio signal, or whether no one of said candidate watermark symbols is embedded in said current section of said received audio signal, wherein said current section of said received audio signal was correlated with at least one candidate reference pattern, each one of which representing one of said one or more candidate watermark symbols, said method including:
- a) taking from the current set of correlation result values a group of maximal values which together form a peak vector;
- obtaining from the values of said peak vector a value of a probability distribution function;
- obtaining from said probability distribution function value a false positive probability value, which false positive probability value represents a probability that peaks resulting from correlation between a candidate reference pattern and non-watermarked audio signal content have a smaller pdf value than said probability distribution function value;
- determining whether said false positive probability value is smaller than a first threshold value and, if true, determining that the current candidate watermark symbol is the watermark symbol present in said current section of said received audio signal;
- if not yet all candidate watermark symbols have been processed, select the next candidate watermark symbol as said current candidate watermark symbol and go to a);
- otherwise, determining a minimal value of said false positive probability function values for all candidate watermark symbols;
- comparing said minimal value with a second threshold value that is greater than said first threshold value;
- if said minimal value is smaller than said second threshold value, determining that the current candidate watermark symbol is the watermark symbol present in said current section of said received audio signal;
- otherwise, determining that one of said candidate watermark symbols is present in said current section of said received audio signal.
- In principle the inventive apparatus is adapted for determining from sets of correlation result values whether a specific watermark symbol out of one or more candidate watermark symbols is embedded in a current section of a received audio signal, or whether no one of said candidate watermark symbols is embedded in said current section of said received audio signal, wherein said current section of said received audio signal was correlated with at least one candidate reference pattern, each one of which representing one of said one or more candidate watermark symbols, said apparatus including means configured to:
- a) take from the current set of correlation result values a group of maximal values which together form a peak vector;
- obtain from the values of said peak vector a value of a probability distribution function;
- obtain from said probability distribution function value a false positive probability value, which false positive probability value represents a probability that peaks resulting from correlation between a candidate reference pattern and non-watermarked audio signal content have a smaller pdf value than said probability distribution function value;
- determine whether said false positive probability value is smaller than a first threshold value and, if true, determine that the current candidate watermark symbol is the watermark symbol present in said current section of said received audio signal;
- if not yet all candidate watermark symbols have been processed, select the next candidate watermark symbol as said current candidate watermark symbol and go to a);
- otherwise, determine a minimal value of said false positive probability function values for all candidate watermark symbols;
- compare said minimal value with a second threshold value that is greater than said first threshold value;
- if said minimal value is smaller than said second threshold value, determine that the current candidate watermark symbol is the watermark symbol present in said current section of said received audio signal;
- otherwise, determine that one of said candidate watermark symbols is present in said current section of said received audio signal.
- Exemplary embodiments of the invention are described with reference to the accompanying drawings, which show in:
- Fig. 1
- probability of the calculated Pfp values for unmarked content being smaller than the given Pfp threshold;
- Fig. 2
- probability distribution for single peak case;
- Fig. 3
- determination of wmin ,wmax for the single-peak case;
- Fig. 4
- flow diagram for generation of pdf threshold LUT and Pfp LUT, which are used for watermark detection;
- Fig. 5
- first flow diagram for watermark detection based on order statistics;
- Fig. 6
- block diagram for watermark information detection in a received audio signal;
- Fig. 7
- second flow diagram for watermark detection based on order statistics.
- Even if not explicitly described, the following embodiments may be employed in any combination or sub-combination.
- In a watermark detector, cross correlations between a received signal and reference patterns are evaluated. Usually, the maximal correlation result value is compared to a threshold in order to determine whether a watermark is embedded in the received signal.
However, when receiving a watermarked audio signal that has passed an acoustic path, multiple correlation result value peaks are employed for watermark detection, in order to take a resulting multi-path environment due to echoes and reverberation into account. An appropriately defined metric aggregating multiple correlation result value peaks is compared to a threshold for watermark detection. A false positive probability defines the probability that a watermark is detected for unmarked content and is denoted as Pfp . It is naturally dependent on the applied detection method. - For the audio watermarking system proposed in M. Arnold, X.M. Chen, P. Baum, U. Gries, G. Doerr, "A phase-based audio watermarking system robust to acoustic path propagation", IEEE Transactions on Information Forensics and Security, vol.9, no.3, pp.411-425, March 2014, it has been verified that cross correlation values between unmarked content and a reference pattern are Gaussian distributed with zero mean for a sufficiently large correlation length.
The inventors have found that statistical methods can be employed for watermark detection. That is, if Pfp can be evaluated for individual correlation arrays, Pfp itself may be used for watermark detection. Specifically, Pfp values are evaluated for different symbols in the watermark symbol alphabet. The smallest Pfp among all watermark symbols is compared to a threshold in order to decide whether watermark information is present in the received signal. If the smallest Pfp is smaller than the threshold, a watermark is assumed to be present. The symbol associated with the smallest Pfp is taken as the embedded watermark symbol. Otherwise, if the smallest Pfp is higher than the threshold, it is declared that no watermark data is present. - In
WO 2011/141292 A1 andPCT/EP2014/066063
Another issue is associated with the interpretation of the defined Pfp . InWO 2011/141292 A1 andPCT/EP2014/066063
However, for np > 1 that is not the case, as illustrated inFig. 1 , in which Pfp values delivered from the detector described inWO 2011/141292 A1 andPCT/EP2014/066063 Fig. 1 , the probability of evaluated Pfp values for unmarked content watermark being smaller than the given Pfp threshold is higher than the given Pfp threshold for np > 1. With increased np the deviation between both becomes larger. - Therefore, in the improved processing, order statistics are used for watermark detection. To resolve the related complexity issue, two look-up tables are employed for Pfp function evaluation. Moreover, using order statistics as decision metric provides a nice interpretation of evaluated Pfp function values, namely, the probability of evaluated Pfp function values for unmarked content watermark being smaller than the given Pfp threshold is exactly equal to the given Pfp threshold for any np value.
- From the viewpoint of maximum likelihood detection, the probability distribution function denoted pdf of peaks resulting from unmarked content can be employed for watermark detection. The decision criterion is to minimise the likelihood pdf. That is, the higher the pdf value for multiple peaks, the more likely it is that these peaks are generated from unmarked content. Conversely, the lower the pdf value, the more likely it is that these peaks are generated from marked content.
- Under the assumption that correlation result values are independent, i.e. identically distributed random variables, the pdf of multiple peaks occurring in the correlation result can be evaluated based on order statistics, see H.A. David and H.N. Nagaraja, "Order statistics", John Wiley & Sons, 3rd edition, 2003.
Specifically for the audio watermarking system under consideration, let {r 0 , r 1,..., r L-1} denote L absolute correlation result values subject to the following zero mean folded Gaussian distribution (ri ≥ 0) :
where σ denotes the standard deviation. Order statistics deal with sorted random variables r (0) ≥ r (1) ≥ ... ≥ r (L-1), which are obtained by sorting the original correlation result values {r 0,r 1,...,r L-1}.
Let v = [v 0 = r (0), v 1 = r (1), ..., v np -1 = r (np -1)] denote the peak vector containing np largest peaks in a correlation array. Tests have shown that np can be in a range from 3 to 12. The constraint v 0 ≥ v 1 ≥ ··· ≥ v np -1 is referred to as peak constraint. The joint pdf of these peaks can be derived as (see the above mentioned David/Nagaraja book):
where
For convenience, a normalised peak vector is defined as
In principle, watermark detection can be carried out by comparing the pdf values of normalised peak vectors in correlation arrays corresponding to different watermark symbols, and the symbol resulting in the smallest pdf value is selected as embedded watermark symbol. On the other hand, a threshold should be used to avoid a high false positive probability, or in other words, the resulting Pfp using that threshold should be below the target Pfp . That is, only when the smallest pdf value g(w) is sufficiently low, it is decided that a watermark is present in the received signal. Otherwise, if the threshold is not low enough, for unmarked content, a watermark will be detected with a high probability. Consequently, the corresponding Pfp becomes high. Therefore it is necessary to evaluate Pfp for a specific threshold for pdf values g(w). - For watermark detection, the pdf values g(w) for different watermark symbols are compared to a threshold in order to decide whether or not a watermark is present. If g(w) is smaller than the threshold, it is decided that a watermark is present. And the watermark symbol resulting in the smallest pdf value is taken as the embedded one. If none of evaluated pdf values is smaller than the threshold, it is assumed that no watermark information data is present.
For a specific normalised peak vector z, the false positive probability is defined as the probability that a random normalised peak vector w has a pdf value lower than the pdf value of z: Pfp (z) = Pr{g(w) < g(z)}. (2) In this context, g(z) is interpreted as a threshold for determining the presence of watermark.
Experimentally, a larger number of length-L correlation arrays, say M, can be generated according to the Gaussian distribution. Normalised peak vectors of these correlation arrays are denoted as {w (i), 1 ≤ i ≤ M} and are used for evaluating g(w (i)). Let m denote the number of generated correlation arrays fulfilling g(w (i)) < g(z), then -
Fig. 2 illustrates the distribution function g(w) for the single-peak case, where the correlation length is L =16384. The pdf values for extremely small or extremely large peak values are extremely small. For a given threshold th, Pfp is represented by the area below the distribution function where pdf values are smaller than the threshold. Therefore, the evaluation of Pfp can be interpreted as one-dimensional integration for the single-peak case. For multi-peaks, it is a multi-dimensional integration.
Moreover,Fig. 2 indicates that an increase of the threshold th also increases Pfp . Therefore in the above experiment g(w (i)) < g(z) also indicates that Pfp (w (i)) < Pfp (z), where g(z) and g(w (i)) are interpreted as two thresholds. Consequently, if calculating Pfp for each correlation array, there are m values lower than Pfp (z) out of M calculated Pfp values, i.e.
In other words, on average, for every ten calculated Pfp values there is one value smaller than 0.1, for every 100 calculated Pfp values there is one value smaller than 0.01. This kind of interpretation is important for the design of a watermarking system. When setting a threshold for Pfp values, or equivalently for pdf values, the probability that watermark data is detected in unmarked content is equal to the Pfp threshold. - Since theoretical evaluation of false positive probabilities defined in equation (2) is difficult for np > 1, they are determined numerically. Specifically, a look-up table (LUT) with K entries for pdf thresholds is defined, for example, linearly on the log-scale in the range [αgmax ,βgmax ],α << 1, β ≤ 1, where gmax denotes the maximal pdf value for all possible normalised peak vectors. According to equation (1), for a multi-dimensional point w* resulting in gmax , it is obvious that
Given L and np , it is straightforward to determine w* and consequently the maximal pdf value gmax. For L =16384, the following table lists gmax values for 1 ≤ np ≤ 8 :np = 1 np = 2 np = 3 np = 4 np = 5 np = 6 np = 7 np = 8 4.0216 3.8563 3.7566 3.6277 3.5808 3.5406 3.5055 3.4744 - For each entry in the pdf threshold LUT the false positive probability is determined numerically. The evaluation of the false positive probability can be interpreted as multi-dimensional integration. However, for multi-dimensional integration the convergence of Monte Carlo or quasi-Monte Carlo simulations (see R.E. Caflisch, "Monte Carlo and quasi-Monte Carlo methods", Acta Numerica, vol.7, pp.1-49, January 1998) is independent of dimension, while linear-grid based methods do depend on dimension and therefore do not converge well with increased dimension. Therefore the Monte Carlo simulation is used for the numerical evaluation of the false positive probability, whereby the Monte Carlo simulation is carried out according to the Monte Carlo method.
- At the watermark detector, there are two lookup tables (LUTs) used for watermark detection. One LUT stores values of probability distribution function (pdf) for normalised peaks of correlation between non-watermarked content and reference patterns, and the other one stores values of false positive probability corresponding to entries in the pdf LUT. That is, each entry in the pdf LUT corresponds to a unique entry in the LUT for false positive probability. Different correlation lengths and different number of peaks result in different LUTs. And determined LUTs are stored in the memory unit of watermark detector, which is accessed during watermark detection.
As mentioned above, for watermark detection, the detector performs correlation between received audio section and reference patterns corresponding to watermark symbols. Correlation values are sorted to find peaks, which are normalised by standard deviation. The standard deviation is estimated either individually for each set of correlation result values corresponding to individual candidate watermark symbol, or by averaging over sets of correlation result values. Afterwards, the probability distribution function is evaluated for the normalised peaks. And the LUT for probability distribution is accessed to find the entry index which is nearest to the evaluated pdf value from the normalised peaks. This entry index is then used to access the second LUT for the false positive probability. And the false positive probability corresponding to the peaks found is then evaluated by means of interpolation or extrapolation. - For convenience, an np -dimensional hypercube
-
Fig. 3 depicts the determination of wmin ,wmax for the single-peak case. The volume of the hypercube is A=(wmax -wmin ) np . - The false positive probability can be reformulated as (see the definition in equation (2)):
were denotes the hyper-region subject to the peak constraint w 0 ≥ w 1 ≥ ... ≥ w np -1 and with pdf values g(w) less than the threshold th. The function g'(w) is defined as g'(w) = g(w) for w ∈ , and g'(w) = 0 for w ∈ and w ∉ .
Moreover,
That is, np random variables [w 0,w 1,..,w np -1] uniformly distributed in are generated M times. For these cases fulfilling the peak condition w 0 ≥ w 1 ≥ ... ≥ w np -1, the pdf value g(w) is evaluated and compared to the pdf threshold th. If g(w) < th, g(w) values are accumulated. The final result of accumulation is scaled by - Although Monte Carlo or quasi-Monte Carlo simulations solve the problem associated with linear grid based numerical integration for increased dimension, the number of generated multi-dimensional points fulfilling the peak constraint decreases exponentially with increased dimension, however.
More precisely, only
If for example np =2, two values [a 0,a 1] are generated randomly according to uniform distribution: - If a 0≥a 1 , w 0 = a 0 and w 1 = a 1 are assigned, where a 0 is interpreted as the largest peak, and g(w) is evaluated and compared to threshold th. Potentially, g(w) is added to the accumulation;
- If a1 > a 0 , w 0 = a 1 and w 1 = a 0, where a 1 instead of a 0 is now interpreted as the largest peak. Again, g(w) is evaluated and compared to threshold th. Potentially, g(w) is also added to the accumulation.
-
Fig. 4 shows a flow diagram for the generation of the pdf threshold LUT and Pfp LUT, which are used for the watermark detection.
Instep 41, the aim is stated to construct a pdf threshold LUT with K entries, given a pdf range [pmin ,pmax ]. For example, K entries linear on a log10-scale are defined as
Step 42 is the initialisation of the Pfp LUT: K entries in the Pfp LUT are initialised to zero: Pfp,i = 0, 1 ≤ i ≤ K. A loop over m starts with m = 1. Normalised peak vectors are generated in step 43: Generate M times normalised peak vectors using Monte Carlo or quasi-Monte Carlo processing. In the Monte Carlo case, a random generator is used to generate normalised peak vectors uniformly distributed in the hyper-cube [wmin ,wmax ] np , where [wmin ,wmax ] defines the range of generated random normalised peak values. For quasi-Monte Carlo simulations, a low-discrepancy sequence like Sobol sequence is generated as normalised peak values, which also approximate the uniform distribution.
Instep 44 each generated normalised peak vector is sorted such that the peak constraint is fulfilled: w 0 ≥ w 1 ≥ ··· ≥ w np -1, which is used to calculate pdf value g(w (m)) instep 45. A loop over i starts with i=1.
In step 46, the calculated values g(w (m)) are compared with threshold entries thi in the pdf threshold LUT. If g(w (m)) < thi, the Pfp LUT entries are updated in step 47: all Pfp,i entries in the Pfp LUT having a corresponding pdf threshold greater than g(w (m)) are increased by g(w (m)). Thereafter i is incremented and, as long as i ≤ K instep 48, the i loop continues with step 46.
Thereafter m is incremented and, as long as m ≤ M instep 49, the m loop continues withstep 43. After generating all M times normalised peak vectors and corresponding updating of the Pfp LUT entries, the final Pfp values are estimated instep 40 by scaling the Pfp LUT entries by - A first flow diagram for watermark detection is shown in
Fig. 5 . There are nSymbols watermark symbols in the watermark symbol alphabet. Watermark detection is carried out as follows: - In
step 50, a result values array or block r of a cross correlation between a current section of the received signal and reference patterns is provided, for example by means of fast Fourier transform and inverse fast Fourier transform. - A second flow diagram for watermark detection is shown in
Fig. 7 . There are nSymbols watermark symbols in the watermark symbol alphabet. Watermark detection is carried out as follows: - In
step 70, a result values array or block r of a cross correlation between a current section of the received signal and reference patterns is provided, for example by means of fast Fourier transform and inverse fast Fourier transform. - Otherwise, in
step 76, i* is defined as i* = arg min i {Pfp (w i)} andstep 77 the minimalstep 79. If the minimalstep 78 that no watermark is present in the received current signal section. - In the watermark decoder processing in
Fig. 6 , a received watermarked signal is re-sampled in an acquisition or receiving section step or stage 61, and thereafter may pass through a spectral shaping and/or whitening step orstage 62. In the following correlation step orstage 63 it is correlated section by section with the nSymbols reference patterns. A symbol detection or decision step orstage 64 determines, whether or not a corresponding watermark symbol is present in the current signal section.
At watermark encoder side, a secret key was used to generate pseudo-random phases, from which related reference pattern bit sequences or symbols were generated and used for watermarking the audio signal. At watermark decoder side, these pseudo-random phases are generated in the same way in a corresponding step orstage 65, based on the same secret key. From the pseudo-random phases, related candidate reference patterns or symbols are generated in a reference pattern generation step orstage 66 and are used in step/stage 63 for checking whether or not a related watermark symbol is present in the current signal section of the received audio signal.
In the symbol detection or decision step orstage 64, a look-up table 67 for probability distribution function values and a look-up table 68 for false positive probabilities are used for the embedded watermark symbol determination as described above. - The described processing can be carried out by a single processor or electronic circuit, or by several processors or electronic circuits operating in parallel and/or operating on different parts of the complete processing.
The instructions for operating the processor or the processors according to the described processing can be stored in one or more memories. The at least one processor is configured to carry out these instructions.
After estimating Pfp for each entry in the pdf threshold LUT, another LUT for Pfp is constructed. Based on interpolation/extrapolation, mapping from pdf values to false positive probabilities is established. More specifically, given a normalised peak vector w, the corresponding pdf g(w) is evaluated according to equation (1). The entry in the pdf threshold LUT is found which is nearest to g(w). If the entry is not at the boundary of the pdf threshold LUT, the corresponding entry in the Pfp LUT and its neighbors are used to evaluate the false positive probability corresponding to g(w) by means of interpolation. If the entry in the pdf threshold LUT nearest to g(w) is at the threshold LUT boundary, extrapolation may be necessary to calculate the corresponding false positive probability for g(w).
In
A second loop over i starts with i=1. In
In
Otherwise, in
In this description, "no watermark is present/detected" means that none of the candidate watermarks is present or detected.
The thresholds Tmin and Tmax can be adapted to the current application. Tests have shown that Tmin = 10-8 and Tmax = 10-2 are suitable values. For security applications the thresholds should be selected smaller and for metadata transmission the thresholds should be selected higher.
In
In
In
Claims (8)
- Method for determining (64) from sets of correlation result values whether a specific watermark symbol out of one or more candidate watermark symbols is embedded in a current section of a received audio signal, or whether no one of said candidate watermark symbols is embedded in said current section of said received audio signal, wherein said current section of said received audio signal was correlated (63) with at least one candidate reference pattern (66), each one of which representing one of said one or more candidate watermark symbols, said method including:a) taking (711, 712) from the current set of correlation result values a group (np ) of maximal values which together form a peak vector (v i ,w i );- obtaining (72) from the values (w i ) of said peak vector a value of a probability distribution function (pdf, g(w i ));- obtaining (73) from said probability distribution function value a false positive probability value (Pfp (w i )), which false positive probability value represents a probability that peaks resulting from correlation between a candidate reference pattern and non-watermarked audio signal content have a smaller pdf value than said probability distribution function value;- determining (74) whether said false positive probability value (Pfp (w i )) is smaller than a first threshold value (Tmin) and, if true, determining (79, 64) that the current candidate watermark symbol is the watermark symbol present in said current section of said received audio signal;- if not yet all candidate watermark symbols have been processed, select (75) the next candidate watermark symbol as said current candidate watermark symbol and go to a);- otherwise, determining (76) a minimal value- comparing (77) said minimal value- if said minimal value- otherwise, determining (78) that one of said candidate watermark symbols is present in said current section of said received audio signal.
- Apparatus for determining (64) from sets of correlation result values whether a specific watermark symbol out of one or more candidate watermark symbols is embedded in a current section of a received audio signal, or whether no one of said candidate watermark symbols is embedded in said current section of said received audio signal, wherein said current section of said received audio signal was correlated (63) with at least one candidate reference pattern (66), each one of which representing one of said one or more candidate watermark symbols, said apparatus including means (64, 67, 68) configured to:a) take (711, 712) from the current set of correlation result values a group (np ) of maximal values which together form a peak vector (v i ,w i );- obtain (72) from the values (w i ) of said peak vector a value of a probability distribution function (pdf, g(w i ));- obtain (73) from said probability distribution function value a false positive probability value (P fp ( w i )), which false positive probability value represents a probability that peaks resulting from correlation between a candidate reference pattern and non-watermarked audio signal content have a smaller pdf value than said probability distribution function value;- determine (74) whether said false positive probability value (Pfp (w i )) is smaller than a first threshold value (Tmin) and, if true, determine (79, 64) that the current candidate watermark symbol is the watermark symbol present in said current section of said received audio signal;- if not yet all candidate watermark symbols have been processed, select (75) the next candidate watermark symbol as said current candidate watermark symbol and go to a);- otherwise, determine (76) a minimal value- compare (77) said minimal value- if said minimal value- otherwise, determine (78) that one of said candidate watermark symbols is present in said current section of said received audio signal.
- Method according to claim 1, or apparatus according to claim 2, wherein said obtaining (72) of a probability distribution function value (pdf, g(w i )) and said obtaining (73) of a false positive probability function value (Pfp (w i )) is a corresponding calculation, or is a fetching of corresponding values from related look-up tables (67, 68).
- Method according to claim 1 or 3, or apparatus according to claim 2 or 3, wherein said false positive probability function (Pfp (w i )) is determined using a Monte Carlo simulation or a quasi-Monte Carlo simulation.
- Method according to the method of one of claims 1, 3 and 4, or apparatus according to the apparatus of one of claims 2 to 4, wherein the correlation result values of said peak vector (vi ) are normalised (712, w i ) by a standard deviation value which is estimated either individually for each set of candidate watermark symbol correlation result values, or by averaging over sets of correlation result values,
and wherein said probability distribution function (pdf, g(wi)) is defined as
where L is the correlation length and w = [w0,w1 ,...,wnp -1]. - Method according to the method of one of claims 3 to 5, or apparatus according to the apparatus of one of claims 3 to 5, wherein said probability distribution function values pdf and said false positive probability function values (Pfp ) for said related look-up tables (67, 68) are calculated as follows:- initialising (41) a pdf threshold value look-up table with K entries and a given pdf range [pmin,pmax ] by- carrying out the following steps for m = 1,...,M, M being a number of randomly generated vectors:-- generating (43) normalised peak vectors uniformly distributed in a hyper-cube [w min, wmax ] n
p using Monte Carlo or quasi-Monte Carlo processing, where [wmin ,wmax ] defines the range of generated random normalised peak values;-- sorting (44) said normalised peak vectors so as to get a sorted peak vector w (m);-- calculating (45) pdf values g(w (m));-- carrying out the following steps for i = 1,...,K:--- comparing (46) said calculated values g(w (m)) with said threshold values thi in said pdf threshold value look-up table;--- if g(w (m))<thi , increasing (47) corresponding Pfp,i entries in said false positive probability function Pfp look-up table by g(w(m)); - Method according to the method of one of claims 1 and 3 to 6, or apparatus according to the apparatus of one of claims 2 to 6, wherein said received audio signal is an audio signal received after passing an acoustic path.
- Computer program product comprising instructions which, when carried out on a computer, perform the method according to one of claims 1 and 3 to 7.
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