Digital Audio Watermarking

The outstanding progress of digital technology has increased the ease with which digital data is reproduced and retransmitted. However, since the advantages of such a progress are broadly available, they offer equally increasing potential to both legal and unauthorized data manipulation. Consequently, the necessity arises for copyright protection of digital products against unauthorized recording attempts, knows as data piracy. Current research in image, audio and video copyright protection exploits the fact that the human visual and audio perception cannot detect slight changes in certain temporal or frequency domains of the image and the audio signal, respectively. This property is called masking, according to which a faint but perceptible signal becomes non-perceptible in the presence of another one under certain conditions. Most research methods consider a watermark signal produced in a unique way by a function of one or more input keys. These keys can be both owner and signal dependent and generate a signal which is embedded on the original one. The embedding signal is known as a watermark or copyright label. Temporal and frequency characteristics of the original signal should be taken into account in the watermark casting process to reduce perceptible distortions in the watermarked signal. Each individual that produces or possesses digital data owns a unique key that identifies its legal possession and is required for the watermark detection. Besides copyright purposes, a watermark serves authentication purposes, as well. A watermark has to be statistically undetectable by others to prevent the efforts of its unauthorized removal. This condition is fulfilled if the potential number of keys that produce distinct watermarks is large enough to ensure statistical safety. The detection scheme should be as statistically reliable as possible. False rejection or acceptance of the existence of the watermark should be minimal. Finally, a watermark has to be robust to signal manipulation and impossible to be removed without significant alteration of the signal. In other words, a pirate should have to destroy the audio signal before he accomplishes to destroy the watermark. The robustness should extend to common signal processing operations, such as filtering, compression, resampling, requantization, cropping, noise, D/A conversion.