On Sun, Oct 23, 2011 at 10:09:44PM -0400, Jeremy Salwen wrote: > Hmm... it seems to me that if the watermark does not cause audible > distortion, then shouldn't there be a simple algorithm to remove the > watermark: namely, to watermark the same file again with different > information? Not always. For example, a spread-spectrum signal can co-exist with others if it uses a different spreading code. And even if the algorithm is know, the actual code used can be kept secret. > I suppose this requires you to know the algorithm used to watermark the file > in the first place. All the algorithms discussed here: > http://www.ece.uvic.ca/~aupward/w/watermarking.htm seem like they would be > pretty trivially removed by the applying them again with different watermark > data. See above. And there are *much* more sophisticated ways than those discussed in that page. There are three steps involved in adding a watermark to an audio file, and they are similar to those found in any digital telecom system: 1. Encryption, 2. Coding - inserting redundancy to allow error recovery, 3. Modulation - representing the digital data in audio form, and of course the inverse sequence to recover a watermark: 3. Demodulation - extracting features from the audio signal to produce 'soft bits' (bits which are not hard 0 or 1 but are represented by a probability function), 2. Decoding - combining soft bits to produce 'hard' ones, 1. Decryption. As long as these steps are separate, having the source code reveals all about 2 and 3, and that is usually enough to allow removing a watermark without introducing significant damage to the audio. In digital telecoms all 'advanced' systems combine 2 and 3 to some extent, mainly to improve both bandwidth and power efficiency. There's been a lot of research into such this, and the times when e.g. 16-QPSK + Viterbi + Reed-Solomon was 'state-of-the-art' are well past. What is required for a robust watermarking scheme is to mix in (1) as well, and one way is using cryptographically strong spreading codes. Ciao, -- FA _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
