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add watermark twice #28
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Yes, as long as you use two different keys, it should be possible to add two different watermarks. However of course if you can avoid it, it is better to use just one watermark which is better for the quality. |
what's the theory and process of two different watermark by only two keys? |
First of all, if you haven't read the README section on recommendations for the payload, you can do so here: https://github.com/swesterfeld/audiowmark#recommendations-for-the-watermarking-payload We kind-of assume that 128-bit messages ought to be enough for everyone.
As a user create two keys using To see why using two different keys works you could look at the decoder code for single raw bits from the watermark. The simplest possible decoder is here: Line 101 in eced66f
This computes umag and dmag as sum of pseudo randomly selected amplitude bins from the FFT spectrum of each frame. Simplified if (umag > dmag) we decode an 1 bit, otherwise 0. Which amplitude bins are used is depends on the key. So if you use different keys for watermarking you can embed / decode different message bits in the same spectrum. Btw I didn't do excessive testing for using two keys, it appears to work but it may be a little less robust than using just one key.
Yes you could do that. However there are cases in Lets assume that we have a "short" N seconds clip. At any possible location, See also this comment: Line 437 in eced66f
So it wouldn't be totally easy to change the code to no longer make the assumption that A and B blocks can be decoded together. |
is it possible to support adding watermark twice, that means I could get two different watermarking from the same wav?
for example, I add watermarking for user copyright information first, then after transfered the audio file to client, add watermarking of user who download or play it.
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