This invention relates to the field of tonal language speech signal processing. We describe a computer system for characterizing samples of a tonal language. These are analyzed to identify one or more vocal tract characterizing parameters of the user and synthesized speech data is generated by modifying a variation of fundamental frequency with time using a set of standard tones. The synthesized speech data represents the user speaking the tonal language with the modified fundamental frequency. Graphical feedback to guide the user can also be provided.