1. A method for synthesizing a portion or all of a speech signal from harmonics, the method comprising the steps of: processing a signal containing information on a plurality of harmonics that consist of at least two groups of said harmonics; generating said first group of harmonics from said signal using a time domain synthesis method; generating a second signal from said information on said second group of harmonics using a frequency domain synthesis method; and combining said generated first group of harmonics and said generated second signal to generate said portion or all of a speech signal.