Classifications

• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
  • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
  • G10L19/018—Audio watermarking, i.e. embedding inaudible data in the audio signal

Definitions

• the invention relates to a method and to an apparatus for transmitting or regaining watermark data embedded in an audio signal by using modifications of the phase of said audio signal .
• Watermarking of audio signals intends to manipulate the audio signal in a way that the changes in the audio content cannot be recognised by the human auditory system.
• Most audio watermarking technologies add to the original audio signal a spread spectrum signal covering the whole frequency spectrum of the audio signal, or insert into the original audio signal one or more carriers which are modulated with a spread spectrum signal.
• the currently most prominent technology uses a psycho-acoustically shaped spread spectrum, see for instance WO-A-97/33391 and US-A-6061793. This technology of- fers a good compromise between audibility and robustness, although its robustness is not optimum.
• the encoded data i.e. the watermark
• phase coding W. Bender, D. Gruhl, N. Morimoto, A. Lu, "Techniques for Data Hiding", IBM Systems Journal 35, Nos.3&4, 1996, pp. 313-336.
• phase modulation A further technology is phase modulation:
• phase coding technique A disadvantage of the above phase coding technique is that it is neither robust against cropping nor achieves an acceptable data rate, and both phase related techniques need the original audio signal for decoding and therefore the de- tector works in a non-blind manner.
• the problem to be solved by the invention is to increase the watermark detection reliability at decoder side and to improve the robustness of the watermark signal, thereby still allowing blind detector operation in the decoder.
• This problem is solved by the methods disclosed in claims 1 and 3. Apparatuses that utilise these methods are disclosed in claims 2 and 4.
• the invention uses phase modification of the audio signal for embedding the watermark signal data.
• a blind detection at decoder side is feasible, i.e. the original audio signal is not required for decoding the watermark signal.
• the phase of the audio signal can be ma- nipulated by the phase of a reference phase sequence (e.g. a spread spectrum sequence or an m-sequence or a pseudo-random distribution of phase values between and including ⁇ - ⁇ ' and
• This may include splitting the audio signal in overlapping blocks, transforming these blocks with the Fourier or any other time-to-frequency domain transform and changing the original phase based on pseudo-random numbers of a ref- erence phase sequence and a model of the human auditory system, inversely (Fourier) transforming the phase-changed spectrum back into the time domain and carrying out an overlap/add on the blocks.
• the resulting changed audio signal sounds like the original one.
• a strong (e.g. - ⁇ /+ ⁇ ) phase manipulation is carried out only within one or more small frequency ranges which are located in the higher frequencies and/or in noisy audio signal sections, the corresponding frequency ranges being determined according to psycho- acoustic principles.
• phase values in the remaining frequency ranges can be changed, too, the allowable extent of the phase changes being controlled according to psycho- acoustic principles.
• amplitude of (less audible) spectral bins can be changed according to psycho- acoustic principles in order to allow even greater (non- audible) phase changes.
• the watermarked audio signal is decoded at decoder side by correlating the received audio signal with corresponding inversely (Fourier) transformed candidate reference phase sequence which had been used in the encoding, or by using a matched filter instead of correlation.
• the invention achieves a good compromise between robustness and audibility, achieves a high data rate, facilitates a real-time processing and is suitable for embedded systems.
• the inventive method is suited for watermarking data embedded in an audio signal by using modifications of the phase of said audio signal, said method including the steps : - controlling by the value of a current bit of said watermark data the selection or the generation of a corresponding reference data sequence; modifying, according to said corresponding reference data sequence, phase values in a current time-to-frequency domain converted block of said audio signal, whereby within said current block the allowable frequency range or ranges for said phase value modification by a pre-determined maximum amount are determined by psycho-acoustic related calculations; frequency-to-time domain converting the modified version of said current block of said audio signal; outputting the corresponding section of the watermarked audio signal.
• the inventive apparatus is suited for water- marking data embedded in an audio signal by using modifications of the phase of said audio signal, said apparatus including: means being adapted for controlling by the value of a current bit of said watermark data the selection or the gen- eration of a corresponding reference data sequence; means being adapted for modifying, according to said corresponding reference data sequence, phase values in a current time-to-frequency domain converted block of said audio signal, whereby within said current block the allowable fre- quency range or ranges for said phase value modification by a pre-determined maximum amount are determined by psycho- acoustic related calculations; means being adapted for frequency-to-time domain converting the modified version of said current block of said audio signal, and for outputting the corresponding section of the watermarked audio signal.
• the inventive watermark decoding is suited for regaining watermark data that were embedded in an audio sig- nal by using modifications of the phase of said audio signal, wherein the value of a current bit of said watermark data was controlled by the selection or the generation of a corresponding reference data sequence and, according to said corresponding reference data sequence, phase values in a current time-to-frequency domain converted block of said au- dio signal were modified, whereby within said current block the allowable frequency range or ranges for said phase value modification by a pre-determined maximum amount was determined by psycho-acoustic related calculations, and the modified version of said current block of said audio signal was frequency-to-time domain converted so as to form a corresponding section of the watermarked audio signal, said method including the steps: correlating or matching a current block of said watermarked audio signal with a frequency-to-time domain con- verted version of candidates of said reference data sequences; determining from the correlation or matching result a bit value of said watermark data.
• the inventive watermark decoding apparatus is suited for regaining watermark data that were embedded in an audio signal by using modifications of the phase of said audio signal, wherein the value of a current bit of said watermark data was controlled by the selection or the genera- tion of a corresponding reference data sequence and, according to said corresponding reference data sequence, phase values in a current time-to-frequency domain converted block of said audio signal were modified, whereby within said current block the allowable frequency range or ranges for said phase value modification by a pre-determined maximum amount was determined by psycho-acoustic related calculations, and the modified version of said current block of said audio signal was frequency-to-time domain converted so as to form a corresponding section of the watermarked audio signal, said apparatus including: means being adapted for generating or storing frequency- to-time domain converted versions of candidates of said reference data sequences; means being adapted for correlating or matching a current block of said watermarked audio signal with a frequency-to- time domain converted version of candidates of said reference data sequences, and for determining from the correlation or
• Fig. 1 simplified block diagram of an inventive watermark encoder and decoder
• Fig. 2 more detailed watermark encoder block diagram
• Fig. 4 watermark decoder block diagram
• Fig. 5 correlation result Fig. 6 yes/no phase changes in specific areas of the audio signal spectrum
• Fig. 7 additional psycho-acoustically controlled phase changes in other areas of the audio signal spectrum
• Fig. 8 increased phase changes in the audio signal spectrum based on amplitude changes in the audio signal spectrum.
• an original audio input signal AUI is fed (framewise or blockwise) to a phase change module PHCHM and to a psycho-acoustic calculator PSYA in which the current psycho-acoustic properties of the audio input signal are determined and which controls in which frequency range or ranges and/or at which time instants stage PHCHM is allowed to assign watermark information to the phase of the audio signal.
• the phase modifications in stage PHCHM are carried out in the frequency domain and the modified audio signal is converted back to the time domain before it is output. These conversions into frequency domain and into time domain can be performed by using an FFT and an inverse FFT, respectively.
• the corresponding phase sections of the audio signal are manipulated in stage PHCHM according to the phase of a spread spectrum sequence (e.g. an m-sequence) stored or generated in a spreading sequence stage SPRSEQ.
• a spread spectrum sequence e.g. an m-sequence
• the watermark information i.e. the payload data PD
• a bit value modulation stage BVMOD that controls stage SPRSEQ correspondingly.
• a current bit value of the PD data is used to modulate the encoder pseudo-noise sequence in stage SPRSEQ. For example, if the current bit value is ' 1 ' , the encoder pseudo-noise sequence is left unchanged whereas, if the current bit value corresponds to '0', the encoder pseudo-noise sequence is inverted. That sequence consists of a 'random' distribution of values and preferably has a length corresponding to that of the audio signal frames.
• the current frequency range or ranges which are used for the phase changes depend on the current audio signal AUI and are dynamically determined by the psycho-acoustic model.
• the phase manipulation can be carried out at different frequency ranges in order to prevent a cut-off of these areas. It is also possible to additionally add a 'normal' spread spectrum watermark signal to the amplitude of the audio signal in the time or frequency domain.
• the phase change module PHCHM outputs a corresponding watermarked audio signal WMAU.
• the watermarked audio signal WMAU passes (framewise or blockwise) through a correlator CORR in which its phase is correlated with one or more frequency-to-time domain converted versions of the candidate decoder spreading sequences or pseudo-noise sequences (one of which was used in the encoder) stored or generated in a decoder spreading sequence stage DSPRSEQ.
• the correlator provides a bit value of the corresponding watermark output signal WMO.
• the correlation output at decoder side con- tains always a meaningful peak (corresponding to a watermark information bit), which is often not the case if a (shaped) spreading sequence was added to the audio signal amplitude. It is not possible to remove this kind of watermarking from the audio signal without destroying the quality of the audio signal drastically. The robustness of the watermarking is therefore increased.
• phase vectors p_0 and p_l are created, each one comprising 513 pseudo random numbers between - ⁇ and ⁇ (in practise, the first and the last value is never used, but for the sake of simplicity this fact is omitted here) .
• the audio input signal AUI is cut into blocks or frames of length 1024 samples in a windowing stage WND.
• the first block is transformed in Fourier transformer FTR into spectral domain using FFT, which results in a vector s (amplitude, phase) of length 513.
• FFT Fourier transformer
• PHLC phase limit calculator
• This modified audio signal sounds like the original signal, but contains a watermarking data bit.
• Blocking artefacts can be reduced in an overlap-and-add stage OADD by overlapping blocks for example with a well- known sine window.
• Fig. 3 shows an example plot of the original phase of a block of signal s and the modified phase marked by ⁇ o' of that signal block, whereby a very crude psycho-acoustic model was used that allows at maximum a 10-degree phase shift at each frequency bin.
• Fig. 4 shows the data flow in the inventive watermark de- coder.
• the watermarked audio signal WMAU passes (framewise or blockwise) through an optional shaping stage SHP to a correlator CORR.
• the shaping amplifies or attenuates the received audio signal such that its amplitude level becomes flat, or gets value ⁇ l'.
• ⁇ l' IFFT(eiP_ 0 )
• w_l IFFT (eiP. 1 ).
• a correlation of a watermarked audio signal with a sequence w_0 or w_l that has the same phase vector like the embedded watermark data bit will show a peak PK in the correlation result, whereas a correlation of that watermarked audio sig- nal with the other sequence w_l or w_0 , respectively, shows only noise in the correlation result.
• the correlator assigns the corresponding bit values and provides the thereby resulting watermark output signal WMO.
• Fig. 5 shows the correlation result for the example phase signal of Fig. 3. "CPH” marks part of the correct phase signal whereas "WPH” marks part of the wrong phase signal.
• the correlator CORR can be replaced by an appropriate matched filter, leading to the same result.
• the correlator CORR can be replaced by an appropriate matched filter, leading to the same result.
• experiments have shown that the processing is much more robust if two different phase vectors are used.
• the basic technology of the inventive processing can be combined with features known from spread spectrum watermarking: - splitting the payload in independent frames which start with synchronisation blocks followed by payload bits that are protected by error correction;
• a further improvement can be achieved by not only consider- ing the phase, but also the amplitude of the audio signal.
• the psycho- acoustic module PSYA or PHLC determines that at a certain frequency bin a phase shift of 10 degree is not audible.
• An improved psycho-acoustic module will determine that the 10 degree phase shift is not audible only with the given current amplitude, but if a current amplitude were half a 15 degree phase shift would be permissible still without being audible.
• the amplitude value or values of the original spectrum would be halved and their corresponding phase values would be changed by 15°.
• Figures 6 to 8 illustrate three embodiments of the invention .
• Fig. 6 shows in a power P/frequency f presentation the original audio spectrum amplitude ASA in a current audio block.
• the phase values are set to a predetermined maximum audio signal phase change value ASPH.
• the scale at the right border shows the relative phase change RPH.
• Fig. 7 there are additional phase changes ASPH in other frequency ranges of the audio signal spectrum, the amount of which phase changes is determined according to psycho- acoustics.
• the phase of the audio signal is modified adaptively using psycho-acoustic calculations by an amount that is smaller than the maximum amount.
• Fig. 8 shows still further increased phase changes in the audio signal spectrum based on amplitude changes ASPH in the audio signal spectrum, in response to an audio signal changed amplitude ASCHA (the amount of which is exaggerated in the drawing) .
• the most right scale shows the amplitude change ACH.

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• Engineering & Computer Science (AREA)
• Computational Linguistics (AREA)
• Signal Processing (AREA)
• Health & Medical Sciences (AREA)
• Audiology, Speech & Language Pathology (AREA)
• Human Computer Interaction (AREA)
• Physics & Mathematics (AREA)
• Acoustics & Sound (AREA)
• Multimedia (AREA)
• Compression, Expansion, Code Conversion, And Decoders (AREA)
• Signal Processing For Digital Recording And Reproducing (AREA)
• Soundproofing, Sound Blocking, And Sound Damping (AREA)

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• 2005
  • 2005-09-16 EP EP05090261A patent/EP1764780A1/en not_active Withdrawn
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