voice morphing

Voice morphing (also known as voice transformation and voice conversion) is the software-generated alteration of a person's natural voice. The purpose may be to add audio effects to the voice, to obscure the identity of the person or to impersonate another individual.

Voice morphing software that alters a voice to sound like a target individual is the most sophisticated technology and the most difficult to create. The CUED project at the University of Cambridge explains: 

There are basically three inter-dependent issues that must be solved before building a voice morphing system. Firstly, it is important to develop a mathematical model to represent the speech signal so that the synthetic speech can be regenerated and prosody can be manipulated without artifacts. Secondly, the various acoustic cues which enable humans to identify speakers must be identified and extracted. Thirdly, the type of conversion function and the method of training and applying the conversion function must be decided.

At Los Alamos National Laboratory in New Mexico, researchers developed voice morphing technology that allowed them to create convincing representations of the voices of a number of United States generals. In 1999, Los Alamos researchers replicated speech patterns from a 10-minute recording of General Carl W. Steiner and played the general a message: "Gentlemen! We have called you together to inform you that we are going to overthrow the United States government." Other voice clones included Colin Powell saying "I am being treated well by my captors." 

There are various consumer voice morphing applications available. An iPhone app called Voice Morphing offers modes including helium, gangster, robot and alien. Second Life voice morphing effects include techie, masculine, feminine, scary and tiny.