Embedding a watermark in an information signal
First Claim
1. A method of embedding a watermark in an information signal, comprising the steps of:
- generating a series of watermark samples representing the watermark;
dividing the information signal into frames of a given length;
Fourier transforming the frames into series of coefficients;
modifying the magnitudes of said coefficients as a function of the watermark samples, while leaving the phase of the coefficients substantially unchanged; and
inverse transforming the series of modified coefficients into modified signal frames.
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Abstract
Disclosed is a method and an arrangement for embedding a watermark in an information signal, in particular an audio signal. The method is based on modification of the magnitude (not the phase) of Fourier coefficients and does not require the original signal for detection. The embedder divides (10) the signal into frames of a given length, and subjects each frame to a Fast Fourier Transform (11). The Fourier coefficients X(k) are modified (20,21) as a function of a predetermined secret watermark W. A payload (P) is encoded in the embedded watermark by cyclically shifting (41) the watermark W by a number (v) of samples representing said payload.
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Citations
12 Claims
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1. A method of embedding a watermark in an information signal, comprising the steps of:
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generating a series of watermark samples representing the watermark;
dividing the information signal into frames of a given length;
Fourier transforming the frames into series of coefficients;
modifying the magnitudes of said coefficients as a function of the watermark samples, while leaving the phase of the coefficients substantially unchanged; and
inverse transforming the series of modified coefficients into modified signal frames. - View Dependent Claims (2, 3, 4, 5)
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6. A method of detecting a watermark in an information signal, comprising the steps of:
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generating a watermark as a series of watermark samples;
dividing the information signal samples into frames of a given length;
Fourier transforming the frames into series of coefficients;
calculating the magnitude of each coefficient;
determining the correlation between a series of coefficient magnitudes and the series of watermark samples;
generating an indication signal if said correlation exceeds a predetermined threshold. - View Dependent Claims (7, 8)
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9. An arrangement for embedding a watermark in an information signal, comprising:
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means for generating a series of watermark samples representing the watermark;
means for dividing the information signal into frames of a given length;
means for Fourier transforming the frames into series of coefficients;
means for modifying the magnitudes of said coefficients as a function of the watermark samples, while leaving the phase of the coefficients substantially unchanged; and
means for inverse transforming the series of modified coefficients into modified signal frames.
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10. An arrangement for detecting a watermark in an information signal, comprising:
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means for generating a watermark as a series of watermark samples;
means for dividing the information signal samples into frames of a given length;
means for Fourier transforming the frames into series of coefficients;
means for calculating the magnitude of each coefficient;
means for determining the correlation between a series of coefficient magnitudes and the series of watermark samples;
means for generating an indication signal if said correlation exceeds a predetermined threshold.
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- 11. An information signal having an embedded watermark, characterized in that the information signal has been divided into frames of a given length, the magnitudes of the Fourier coefficients of the series have been modified as a function of a watermark while leaving the phase of the coefficients substantially unchanged, and the series of modified coefficients have been inverse transformed into modified signal frames.
Specification