In this thesis, the study and design of Voice Change and Voice Conversion systems are
presented. Particularly, a voice change system manipulates a speaker’s voice to be perceived
as it is not spoken by this speaker; and voice conversion system modifies a speaker’s voice,
such that it is perceived as being spoken by a target speaker.
This thesis mainly includes two sub-parts. The first part is to develop a low latency and low
complexity voice change system (i.e. includes frequency/pitch scale modification and formant
scale modification algorithms), which can be executed on the smartphones in 2012 with very
limited computational capability. Although some low-complexity voice change algorithms
have been proposed and studied, the real-time implementations are very rare. According to the
experimental results, the proposed voice change system achieves the same quality as the
baseline approach but requires much less computational complexity and satisfies the
requirement of real-time. Moreover, the proposed system has been implemented in C
language and was released as a commercial software application. The second part of this
thesis is to investigate a novel low-complexity voice conversion system (i.e. from a source
speaker A to a target speaker B) that improves the perceptual quality and identity without
introducing large processing latencies. The proposed scheme directly manipulates the
spectrum using an effective and physically motivated method – Continuous Frequency
Warping and Magnitude Scaling (CFWMS) to guarantee high perceptual naturalness and
quality. In addition, a trajectory limitation strategy is proposed to prevent the frame-by-frame
discontinuity to further enhance the speech quality. The experimental results show that the
proposed method outperforms the conventional baseline solutions in terms of either objective
tests or subjective tests
