Voice Conversion: a Critical Survey

Voice conversion is an emergent problem in voice and speech processing with increasing commercial interest, due to applications such as Speech-to-Speech Translation (SST) and personalized Text-To-Speech (TTS) systems. A Voice Conversion system should allow the mapping of acoustical features of sentences pronounced by a source speaker to values corresponding to the voice of a target speaker, in such a way that the processed output is perceived as a sentence uttered by the target speaker. In the last two decades the number of scientific contributions to the voice conversion problem has grown considerably, and a solid overview of the historical process as well as of the proposed techniques is indispensable for those willing to contribute to the field. The goal of this text is to provide a critical survey that combines historical presentation to technical discussion while pointing out advantages and drawbacks of each technique, and to bring a discussion of future directions, specially referring to the development of a perceptual benchmark process for voice conversion systems.