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Subject: Re: hacksdmi?
From: Erhard Schwenk (eschwenkFTO.DE)
Date: Fri Oct 20 2000 - 02:56:38 CDT

On 16-Oct-00 Brooke, O'neil (EXP) wrote:
> This maybe a silly question but, couldn't they put a watermark
> into the sound by using sounds that we cannot hear? Very high or very
> low
> frequencies could be used, so could sounds that will be overpowered
> by the
> watermarked content.

This is NOT so easy as it looks like.

First of all, a normal ear can detect frequencies between about 20 and
20000 Hz, the same Bandwidth you have with an Audio CD or a 44.1 Khz
sampling rate in any other file (no, you won't hear, let's say, 2 Hz,
but most audio player have a High-Pass filter with about 16 Hz, so this
will get filtered out anyway). So in the principle, you cannot record a
frequency which you cannot hear since the recorder won't recognize it
(and if, it would be cut out at Mastering the final CD). So the only way
is to hide your information as a silent addition to a loud part of the
signal, so that noone realizes it.

Then, modern lossy Compression algorithms base on things "we can't
hear" because they are overloaded by other things in the sound. So, the
risk to cut off the watermark by simply encoding it with a mp3 Encoder
or some future (and optimized) Version of lossy Compression algorithms
is evident. The better the Compressor is, the more "you can't hear" it
cuts out and the lesser space remains for watermarking. Since you need
many different watermarks (if you want to sue someone, you have to
identify him) I do not think this is very simple. And the watermark
needs to be very robust since you can simply record the signal once
over a standard analog out and in of a Soundcard. You will get marginal
quality losses then, but a normal consumer won't recognize them. 

--
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     Erhard Schwenk - alias Bitrunner  =)B==o)
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