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

Katja