| 摘要 |
To watermark a covertext comprising audio or video signals 10, a two-dimensional pattern of predetermined size is used to generate at least one key 1 which is used by an encoder 2 to watermark the covertext and which can be used subsequently by a decoder 3 to decode the resulting stegotext 15. In the encoder, multiples of the key(s) are added or subtracted to blocks of the covertext signal transformed into a power spectrogram 11 by fast Fourier transformation and rectangular polar conversion of the covertext signal to represent a desired code. The stegotext is then obtained by polar rectangular conversion and inverse fast Fourier transformation. In the decoder 3, the stegotext is transformed into the log power spectrogram domain by fast Fourier transformation and rectangular polar conversion. Two-dimensional correlation 17 is then performed between the original key(s) and blocks of the transformed stegotext so as to generate a correlation function from which data bits representing the code are extracted at 18. The origin of the covertext can thus be identified in an imperceptible way, and unauthorised reproduction of the covertext may be prevented. |
