[PD] obscuring voices while maintaining intelligibility

katja katjavetter at gmail.com
Wed Sep 21 23:29:46 CEST 2011

On Wed, Sep 21, 2011 at 10:44 PM, Hans-Christoph Steiner <hans at at.or.at>wrote:

> I'm trying to come up with a simple voice scrambling technique that
> leaves voices understandable, but makes them unrecognizable.  A key part
> of this is to make it very hard to reverse the scrambling to make the
> voice more recognizable.
> I'm currently thinking that a ring modulator would work well for this,
> and it uses minimal CPU.  Can anyone think of a way to reverse the ring
> modulation?  I attached my quick sketch.

That's a nice challenge. If I were CIA, I'd try to descramble like
this: demodulate (division instead of multiplication) with a sine sweep
while analyzing the spectrum. At the sweep frequency where the spectrum is a
harmonic recipe: bingo, a human voice. Then you could demodulate that 0.1
second of sound with the found frequency. Not something to quickly do in a
Pd patch though.

If your scrambler would modulate the modulation frequency continuously, with
a noise signal, speech is still intelligible, but descrambling in the above
described way would no longer be possible, as you can't find a harmonic
recipe from a one sample fourier transform.

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