I have a man talking for a documentary movie whose voice needs to be distorted so that he could not be recognized.

Now, in most cases, the pitch shift is being used for that purpose. But it has some drawbacks. One, this process is reversible, so you can "unpitch" the result and aquire the original voice. The second, the result sounds very very unnatural, robot-like. I'm looking for a solution that would allow to have both unreversible distortion process yet still remaining the naturality of normal voice (just sounding like a different man).

Some time ago there was the first Sound Blaster Live! card that had this realtime female to male conversion that worked pretty well on male voices, maybe that is the solution?

I often do some work for a Broadcast TV Network, you are right flangers, pitch changing often ends up 'robotic'.
The easiest way for 'News' / 'Current Affairs' to disguise the voice to use an actor....And repeat the recorded dialogue from the original person, doing it this way there is NO way to identify the person and no need to do electronic trickery. We have 2 males and 2 females who are actually office staff that are happy to do it on the odd occasion.

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Cheers.