SECURE WATERMARK METHOD AND APPARATUS FOR DIGITAL SIGNALS

Watermark data (116) is encoded in a digitized signal (110) by forming a noise threshold spectrum (210) which represents a maximum amount of imperceptible noise, spread-spectrum chipping (206) the noise threshold spectrum with a relatively endless stream of pseudo-random bits (204) to form a basis signal (112), dividing the basis signal into segments, and filtering the segments to smooth segment boundaries. The data encoded in the watermark signal is precoded to make the watermark data inversion robust and is convolutional encoded to further increase the likelihood that the watermark data will subsequently be retrievable notwithstanding lossy processing of the watermarked signal. To produce the endless pseudo-random bit stream, subsequent bits of the sequence are generated in a pseudo-random manner from previous bits of the sequence. The pseudo-random bits are appended to the stream of pseudo-random bits and, additionally, replace a number of bits of the state. The remaining bits are shifted such that, in effect, the least recently generated bits of the state are discarded. Thus, after production of a number of pseudo-random bits of the stream, the state changes significantly. Accordingly, repeating the above process generates additional pseudo-random bits which are very likely to differ significantly from the previously generated pseudo-random bits. A basis signal is generated by spread-spectrum chipping in accordance with the stream of pseudo-random bits and watermark data is encoded in the basis signal to form a watermark signal which is added to a digitized signal. Dans le procédé de l'invention on code des données de filigrane (116), dans un signal numérisé (110), en formant un spectre sonore seuil (210) représentant une quantité maximale de bruit imperceptible, en transformant par étalement de spectre (206) ce spectre sonore, à l'aide d'un flux pratiquement sans fin de bits pseudo-aléatoires (204), afin de former un signal de base (112), en divisant ce signal en segments et en filtrant les segments afin d'en lisser les contours. Les données codées dans le signal de filigrane sont préalablement codées pour résister à leur inversion et elles sont codées par convolution afin d'accroître encore plus la difficulté d'extraction ultérieure de ces données, bien que ce traitement entraîne la perte du signal de filigrane. Afin de produire le flux sans fin de données pseudo-aléatoires, des bits ultérieurs de la séquence sont produits de manière pseudo-aléatoire