Patent Publication Number: US-2009235079-A1

Title: Method and apparatus for watermarking an audio or video signal with watermark data using a spread spectrum

Description:
The invention relates to a method and to an apparatus for watermarking an audio or video signal with watermark data using a spread-spectrum and more than one spreading sequence. 
     BACKGROUND 
     Watermark information (denoted WM) consists of several symbols which are embedded continuously in the carrier content, e.g. in (encoded) audio or video signals, e.g. in order to identify the author of the signals. At decoder site the WM is regained, for example by using correlation of the received signal with a known spreading sequence if spread-spectrum is used as underlying technology. In some watermark technology the watermark information is transmitted asynchronously, i.e. it is continuously tested whether or not WM can be embedded imperceptible within the (encoded) audio or video signals. Only if this is true a WM frame is transmitted. But a WM frame consists of some tens of symbols, each carrying one or more bits which are transmitted synchronously. That means, if the period in which the WM can be embedded is shorter than the frame length, some symbols cannot be recovered at receiver side. 
     Most WM technologies therefore transmit redundancy bits for error correction. But such error correction has a limited capacity only. An error correction can correct some symbols, if one or more symbols cannot be directly recovered at receiver side. But if the capacity of the error correction is exceeded, the WM can not be recovered. 
     Secondly, additional redundancy bits increase the length of the WM frame, which results in a higher probability that the frame is longer than the signal length or section in which the WM frame can be transmitted. Thirdly, error correction is mostly independent of the signal to be watermarked which results, due to the necessary parity bits, in a lower than necessary net bitrate for a ‘good’ signal and still not enough error correction for ‘bad’ signals. A ‘good’ signal can be recovered at decoder side whereas a ‘bad’ signal can not be recovered. 
     WO-A-01/06755 shows an energy level-dependent insertion of watermark data. 
     WO-A-03/103273 describes a system wherein different watermark signals are combined with independent channels of a multimedia signal. 
     Invention 
     Watermarking of audio content can be facilitated by adding a spectrally shaped spread-spectrum signal to the audio signal. One problem is that for some audio signals it is not possible to retrieve and decode the spread spectrum even without an attack between the WM embedder and WM detector. In case it becomes clear at encoder side that the decoder will not be able to decode the current WM due to the current presence of a critical sound signal (e.g. a silent period or pause in a speech signal or a uniform brightness level region in a video signal), the level of the WM could be increased but in such case the WM signal would become audible or visible, respectively. 
     A problem to be solved by the invention is to increase the reliability of the watermarking without making it audible or visible, respectively, and without relying on a watermark signal error correction at decoder side. This problem is solved by the methods disclosed in claims  1  and  2 . An apparatus that utilises this method is disclosed in claims  3  and  4 . 
     According to the invention, not only is the watermark made audio or video signal level dependent, but also the spreading sequence used for the watermark is made audio or video signal level dependent. This means basically that the same WM symbol is encoded by several different spreading sequences. The encoder tests, which one of these WM symbols or sequences can be retrieved best in a decoder, and embeds that selected spreading sequence WM in the audio or video signal to be watermarked. At decoder side all candidate WM spreading sequences are correlated with the received signal and the spreading sequence with the best match is chosen as the correct one. 
     The invention makes watermarking of critical sound or image signals much more robust, which may make the difference between receiving a WM signal and receiving no WM signal at all. The above tests carried out in the encoder cost more processing power since multiple correlations are to be calculated. But advantageously, this does not necessarily increase the complexity and the required processing power at decoder side. 
     The invention is not limited to using spread-spectrum technology. Instead e.g. carrier based technology or echo hiding technology can be used for the watermarking coding and decoding. 
     In principle, the inventive method is suited for watermarking an audio or video signal with watermark data using a spread spectrum, said method including the steps: 
     a) modulating a first candidate encoder spreading sequence by watermark data bits so as to get a modulated watermark signal; 
     b) determining the current masking level of said audio or video signal and performing a corresponding psycho-acoustic or psycho-visual, respectively, shaping of said modulated watermark signal; 
     c) embedding said psycho-acoustically or psycho-visually shaped watermark signal in said audio or video signal; 
     d) spectrally whitening said audio or video signal including said embedded watermark signal; 
     e) de-spreading and demodulating said spectrally whitened audio or video signal including said embedded watermark signal using a correlation so as to get a first candidate watermark signal;
         repeating steps a) to e) one or more times using different candidate encoder spreading sequences;   deciding which one of the correlation results yields the best match and outputting that watermarked audio signal which was watermarked with the corresponding candidate encoder spreading sequence,
 
or is suited for watermarking an audio or video signal with watermark data using a spread spectrum, said method including the steps:
   modulating a first and at least a second candidate encoder spreading sequence by watermark data bits so as to get correspondingly modulated watermark signals;   determining the current masking level of said audio or video signals and performing a corresponding psycho-acoustic or psycho-visual, respectively, shaping of said modulated watermark signals;   embedding said psycho-acoustically or psycho-visually shaped watermark signals in said audio or video signal resulting in a corresponding number of audio or video signals;   spectrally whitening said audio or video signals each one including said corresponding embedded watermark signal;   de-spreading and demodulating said spectrally whitened audio or video signals including said corresponding embedded watermark signal using a correlation so as to get a first and at least a second candidate watermark signal;   deciding which one of the correlation results yields the best match and outputting that watermarked audio signal which was watermarked with the corresponding candidate encoder spreading sequence.       

     In principle the inventive apparatus is suited for watermarking an audio or video signal with watermark data using a spread spectrum, said apparatus including: 
     a) means for modulating a first candidate encoder spreading sequence by watermark data bits so as to get a modulated watermark signal; 
     b) means for determining the current masking level of said audio or video signal and performing a corresponding psycho-acoustic or psycho-visual, respectively, shaping of said modulated watermark signal; 
     c) means for embedding said psycho-acoustically or psycho-visually shaped watermark signal in said audio or video signal; 
     d) means for spectrally whitening said audio or video signal including said embedded watermark signal; 
     e) means for de-spreading and demodulating said spectrally whitened audio or video signal including said embedded watermark signal using a correlation so as to get a first candidate watermark signal, 
     whereby means a) to e) repeat the processing one or more times using different candidate encoder spreading sequences;
         means for deciding which one of the correlation results yields the best match and outputting that watermarked audio or video signal which was watermarked with the corresponding candidate encoder spreading sequence,
 
or is suited for watermarking an audio or video signal with watermark data using a spread spectrum, said apparatus including:
   means for modulating a first and at least a second candidate encoder spreading sequence by watermark data bits so as to get correspondingly modulated watermark signals;   means for determining the current masking level of said audio or video signals and performing a corresponding psycho-acoustic or psycho-visual, respectively, shaping of said modulated watermark signals;   means for embedding said psycho-acoustically or psycho-visually shaped watermark signals in said audio or video signal resulting in a corresponding number of audio or video signals;   means for spectrally whitening said audio or video signals each one including said corresponding embedded watermark signal;   means for de-spreading and demodulating said spectrally whitened audio or video signals including said corresponding embedded watermark signal using a correlation so as to get a first and at least a second candidate watermark signal;   means for deciding which one of the correlation results yields the best match and outputting that watermarked audio or video signal which was watermarked with the corresponding candidate encoder spreading sequence.       

     Advantageous additional embodiments of the invention are disclosed in the respective dependent claims. 
    
    
     
       DRAWINGS 
       Exemplary embodiments of the invention are described with reference to the accompanying drawings, which show in: 
         FIG. 1  known watermark encoder; 
         FIG. 2  watermark decoder; 
         FIG. 3  frame composition; 
         FIG. 4  known WM embedding for a single line in the frequency domain using a first spreading sequence in the encoder; 
         FIG. 5  whitened encoder output signal and decoded watermark in the decoder resulting from the known application of a first spreading sequence; 
         FIG. 6  WM embedding for a single line in the frequency domain using a second spreading sequence in the encoder; 
         FIG. 7  whitened encoder output signal and decoded watermark in the decoder resulting from the application of a second spreading sequence; 
         FIG. 8  watermark encoder according to the invention. 
     
    
    
     EXEMPLARY EMBODIMENTS 
     The smallest self-containing unit of a watermark is called a frame.  FIG. 3  shows three successive frames FR n−1, FR   n  and FR n+1 . A frame consists of a number of synchronisation blocks SYNBL (at least one synchronisation block) which are needed to detect the start of the frame at decoder side, and a number of payload blocks PLBL (at least one valid payload block or symbol) which carry the actual information. Frames are inserted synchronously or asynchronously in the audio or video stream, dependent on the technology. The insertion of the payload blocks is done consecutively, i.e. synchronised after the SYNBL blocks. Each payload block holds one or more bits of information. A payload block is therefore also called a symbol. The payload symbols include the information to be inserted into the WM signal, and optionally contain redundancy information used for error correction. A typical setting is for example 5 synchronisation blocks and 36 payload blocks per frame, each payload block carrying 2 bits, whereby 24 of these 72 bits are used for error correction resulting in a net payload of 48 bits per frame. 
     In the watermarking encoder in  FIG. 1  payload data PLD to be used for watermarking an audio (or video) signal AS is input to an error correction and/or detection encoding stage ECDE which adds redundancy bits facilitating a recovery from erroneously detected symbols in the decoder. In a downstream modulation and spectrum spreading stage MS a modulation and a spreading is carried out. The output signal of stage MS is fed to a psycho-acoustical shaping stage PAS which shapes the WS signal such that the WM is not audible with respect to the current level of audio signal AS, and which feeds its output signal to a signal adder and decision stage SAD and to a decoder stage DEC. The watermark is shaped in stage PAS block-wise according to psycho-acoustic principles, i.e. the ratio between watermark and audio energy may change from symbol to symbol. This shaping represents a multiplication of the watermark signal by the masking level of the audio signal. 
     The decoder stage DEC implements a decoder according to  FIG. 2 . Stages PAS and SAD each receive the audio (or video) stream signal AS and process the WM frames symbol by symbol. Stage SAD determines whether the payload data PLD have been decoded correctly in decoder DEC for a current WM frame FR n . If true, the psycho-acoustical shaped WM symbol is added to the current frame. If not true, the current symbol in the current frame FR n  is skipped. Thereafter the processing continues for the next symbol following the current symbol. After the processing for a WM frame is completed a correspondingly watermarked frame WAS embedded in the audio signal is output. Thereafter the processing continues for the frame FR n+1  following the current frame. 
     In the watermarking decoder in  FIG. 2  a watermarked frame WAS of the audio (or video) signal passes through a spectral whitening stage SPW (which reverses the shaping that was done in stage PAS) and a de-spreading and demodulation stage DSPDM that retrieves the embedded WM symbol data from the signal WAS. The WM symbol is passed to an error correction and/or detection and decoding stage ECDD that outputs the valid payload data PLD. 
     The basic principle for the invention is explained by an example with two watermark sequences being used, where one is exactly the negative version of the other. The output signal r of the encoder is the (vector) sum of the audio signal a and an optionally shaped watermark spreading sequence w: r 1 =a+w. 
     This addition is normally carried out in the time domain, but it is mathematically equivalent to an addition in the frequency domain: r 1 =F −1 (F(a)+F(w)), wherein F( ) denotes a Fourier transform and F −1 ( ) denotes an inverse Fourier transform. 
     At decoder side the watermark is retrieved by correlating the whitened encoder output signal (which in the mean time might be altered by some noise or attack) with the known spreading sequence. A perfect correlation result is achieved if the encoder output signal is the same as the spreading sequence. 
       FIG. 4  shows the addition of a watermark signal vector WM to an audio (or video) signal AS at encoder side for a single spectral line in the frequency domain having an imaginary direction IM and a real direction RE. The encoder output signal EOS has nearly the same angle α as the audio signal AS because the audio signal has a much larger amplitude than the watermark signal. The watermark signal WM is depicted in the drawing in a severely exaggerated fashion. The real magnitude of the watermark signal as resulting from the psycho-acoustic shaping in stage PAS has a level that is about 20 to 70 dB lower than that of the audio signal. 
       FIG. 5  shows the decoding for a single spectral line in the frequency domain. At decoder side the ‘resulting signal’ is received as input signal. Through ‘whitening’ or reverse psycho-acoustic shaping it is normalised to an appropriate magnitude. ‘Whitening’ means multiplying or magnifying the magnitude of each received spectral value of an audio frame such that all audio signal magnitudes (in which the watermark signal is embedded) get the same value in a frame. Thereby the audio signal itself is seriously distorted, but the resulting effect is that the magnitudes of the watermark signal spectral values get a value that basically corresponds to their original magnitude level. 
     In this example the received decoder input signal is reduced in its magnitude. However, because the embedded watermark signal portion is much smaller than the audio signal portion, the reverse shaping or whitening is dependent practically on the audio signal magnitude only, i.e. the magnitude of the ‘input signal’ after whitening or inverse psycho-acoustic shaping, and it is independent from the watermark signal magnitude. 
     Since the angle β between the whitened encoder output signal WEOS and the watermark signal vector WM is nearly ‘π’, i.e. it is closer to ‘π’ than to ‘0’, the correlation in the decoder indicates for this line that a negative or negated spreading sequence has been inserted in the encoder despite that, in fact, a positive spreading sequence was used in the encoder. 
       FIG. 6  shows again the embedding of a watermark signal WM to an audio (or video) signal AS at encoder side for a single spectral line, like in  FIG. 4  but this time with the negative or negated watermark signal or sequence WM of  FIG. 4 . 
       FIG. 7  shows again the decoding for a single spectral line in the frequency domain. Since the angle β between the whitened encoder output signal WEOS and the watermark signal vector WM is closer to ‘0’ than to ‘π’, the correlation in the decoder identifies in this case correctly the negative or negated watermark signal value. 
     In the flowchart of the inventive encoder in  FIG. 8 , which uses signal adaptive spreading sequences, payload data PLD to be used for watermarking an audio or video signal AS is input to an error correction and/or detection encoding stage ECDE. In a downstream modulation and spectrum spreading stage MS a modulation and a spreading is carried out. The output signal of stage MS is fed to a psycho-acoustical or psycho-visual, respectively, shaping stage PAS which shapes the MS output signal such that the WM is not audible, or visible, with respect to the current level of audio or video signal AS, and which feeds its output signal to a signal adder stage SA. The watermark is shaped in stage PAS block-wise according to psycho-acoustic, or psycho-visual, principles, i.e. the ratio between watermark and audio energy may change from symbol to symbol. This shaping represents a multiplication of the watermark signal by the masking level of the audio or video signal. Stages PAS and SAD each receive the audio or video stream signal AS and process the WM frames FR n  symbol by symbol. 
     The watermarked audio or video signal is tested on its correct decodability using the following stages. A candidate watermarked frame CWAS of the audio or video signal passes through a spectral whitening stage SPW (which reverses the shaping that was done in stage PAS) and a de-spreading and demodulation stage DSPDEM that retrieves the embedded candidate WM symbol data from the signal CWAS. The candidate WM symbol is passed to a decision stage DEC. This stage may control the repetition of the processing in stages MS to DSPDEM using the next spreading sequence NSS. As an alternative, the candidate spreading sequences can be used and processed in parallel in stages MS to DSPDEM. After all candidate spreading sequences have been applied on the current frame, stage DEC decides which one of the spreading sequences can be recovered best or correctly in a decoder, i.e. which one gives a clear peak in the correlation. Finally, stage DEC outputs the correspondingly selected valid watermarked audio or video signal frame WAS. 
     Another example is given to explain the invention. Since the phase of an audio signal is easily changed by reverberation or intentional attack, BPSK modulation of the WM signal is not very robust. A better way is to use for example two different m-sequences, one of them for encoding a binary zero (m — 0) and the other one for encoding a binary one (n — 0). The WM decoder correlates the received audio with both m-sequences and chooses that binary value the related correlation result of which gives the best match. 
     The inventive encoder uses in this case for example four different sequences, two for encoding a binary zero (m — 0 and m — 1) and two for encoding a binary one (n — 0 and n — 1). One implementation is to use two different m-sequences (m — 0 and n — 0) and to generate through phase shifting the remaining sequences, i.e. n — 1=(−1)*n — 0 and m — 1=(−1)*m — 0. Another implementation is to use four different m-sequences. If for example a binary zero is to be encoded, a known encoder would use m — 0 only. 
     The inventive encoder, however, adds the shaped version of m — 0 to the audio signal, correlates the sum with m — 0, stores the result of the correlation, also adds the shaped version of m — 1 to the audio, correlates the sum with m — 1, and stores the result of the correlation. A decision algorithm then selects the sequence with the best correlation result. This m-sequence is finally used for the encoding of the current watermark signal frame. 
     Advantageously, the decoder for the improved watermark needs not be changed if only two sequences per value are used and one sequence is the negative or negated version of the other. The correlation simply gives sometimes a negative and sometimes a positive peak for the same binary value. Therefore only the absolute value of the correlation has to be taken into account. 
     Otherwise, the decoder correlates all m-sequences with the received watermarked audio signal. If one of the m_k sequences matches best, a binary zero is detected, otherwise a binary one. 
     The invention makes watermarking much more robust, which may be the difference between receiving no watermark at all and receiving a watermark. Tests have shown that, when using the invention, the peak confidence of the correlation improves by 50% from 32% to 48% (0% meaning no peak in the correlation, 100% meaning a perfect match) when using two different m-sequences per binary value. 
     The costs are the need for more processing power. The encoder has to shape several sequences and to correlate them in order to decide which one is best. But if the same audio signal is watermarked several times with different WM payloads, like for example in watermarking Academy Screeners, the decision which sequence to use can be made once and thereafter stored for use in subsequent encodings. 
     If only two sequences are used per binary value, the decoder needs not be changed at all. Otherwise the decoder has to calculate more correlations. In the academy screener scenario this is irrelevant, since decoding is done very seldom and not in real-time.