In previous work we have presented a new method for improving the quality of LPC synthetic speech, where the excitation signal was modelled with a polynomial function followed by an adaptive filter. This scheme provides the properties of mathematical models which permits avoiding the problems related to prosody control [1], [2]. In order to reduce the storage needs, a segmentation technique was developed which grouped together several pitch periods based on spectral similarity. For every segment the same coefficient set (both the polynomial function and the post-processing filter) was used. These techniques were applied to a codification/decodification task were the resulting speech quality was promising [1], [2]. In this paper we present some results concerning prosodic modification, i.e. duration and fundamental frequency arbitrary changes which show the suitability of these methods for text-to-speech applications. We also present some results of the extension of the model to unvoiced segments of speech.