Voice quality is currently a key issue in speech synthesis research. The lack of realistic intra-speaker voice quality variation is an important source of concern for concatenation-based synthesis methods. A challenging problem is to reproduce the voice quality changes that are occuring in natural speech when the vocal eort is varying. A new method for voice quality modification is presented. It takes advantage of a spectral theory for voice source signal representation. An algorithm based on periodic-aperiodic decomposition and spectral processing (using the short-term Fourier transform) is described. The use of adaptive inverse filtering in this framework is also discussed. Applications of this algorithm may include: pre-processing of speech corpora, modification of voice quality parameters together with intonation in synthesis, voice transformation. Some experiments are reported, showing convincing voice quality modifications for various speakers.