A new method is presented for text-to-speech synthesis using diphones. The diphone database consists of the diphone waveforms labeled with pitch-marks indicating the pitch-periods. At synthesis time, the diphone waveforms are processed through a new analysis-synthesis system, providing an independent control of all prosodic parameters, while retaining a good degree of naturalness. This system is based on a representation of the speech signal by its short-time Fourier transform (STFT) at a pitch-synchronous sampling rate. The synthesis part of the system works by overlap-adding the modified short-term signals and it ensures a smooth concatenation of the diphone waveforms. The synthetic speech obtained by this method sounds more natural than with the conventional LPC method.