The patent for Autotune can be found here: https://www.google.com/patents/US5973252?dq=5,973,252

My (very non-expert) reading of the patent is that it uses sample reduction and some specific features of the periodic form of the human voice to simplify the math involved in the auto-correlation routine. Although this routine does seem to be unique as far as I know, I wouldn't be surprised if it shares similarities to other techniques to reduce correlation complexity.