A method for performing waveform invariant time stretching and pitch shifting on a quasi harmonic and sinusoidally modeled sound is presented. The method is based on the relative phase delay representation of the phase, defined as the difference between the phase delay of the partials and the phase delay of the fundamental. This representation makes the waveform characterization independent from the phase of the first partial. It is therefore possible to compute a smooth trajectory for the phase of the modified fundamental and rebuild the waveform on the synthesis frame boundaries by adding the relative phase delays to the new fundamental phase delay.