  This paper investigates the differences occuring in the excitation for
different voice qualities. Its goal is two-fold. First a large corpus
containing three voice qualities (modal, soft and loud) uttered by the same
speaker is analyzed and significant differences in characteristics extracted
from the excitation are observed. Secondly rules of modification derived from
the analysis are used to build a voice quality transformation system applied as
a post-process to HMM-based speech synthesis. The system is shown to
effectively achieve the transformations while maintaining the delivered
quality.
