Harold Hallikainen wrote:
> I'm looking at sending some extremely low speed data under audio to
> identify the audio. The receiver would be an Android device with internal
> microphone. I'm thinking of using direct sequence spread spectrum with th=
e
> level being substantially below the audio so it would appear to be very
> low level noise. But, my experience with spread spectrum is extremely
> limited. I'd appreciate any pointers to information on it, especially at
> audio frequencies, and any open source projects on it.

I used to work for Aris Technologies, a predecessor to Verance Corporation,
one of the leaders in this field. (https://www.verance.com/)

Making an audio watermark both *unobtrusive* and *robust* is a serious
challenge. Noise-based systems score poorly on both aspects.

Wideband noise is not really all that unobtrusive, unless the audio "cover"
signal itself is also fairly wideband, like a rock band. Solo instruments o=
r
singers will not hide the noise well at all.

Wideband noise is also not robust. First of all, the microphone in a typica=
l
Android device is not going to give you a high SNR to begin with, which mea=
ns
you'll need to have a significant amount of power in your watermark, which
gets back to the obtrusiveness point above. Second, any kind of encoding
algorithm used to compress the audio (e.g., MP3) will be expressly designed
to ignore any wideband noise in the original signal, pretty much wiping out
your watermark.

Some of Verance's algorithms are patended; you might try searching for pate=
nts
assigned to them in order to get a feel for what the state of the art in th=
is
area is. I know what they were doing back in 2000, but I'm not free to disc=
uss
the details. Besides, I'm sure things have evolved significantly since then=
..

-- Dave Tweed
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