natural - sounding synthesized speech is obtained from pieced elemental speech units that have their super - class identities known , and their line spectral frequencies set in accordance with a correlation between the desired fundamental frequency and the lsf vectors that are known for different classes in the super - class . the correlation between a fundamental frequency in a class and the corresponding lsf is obtained by , for example , analyzing the database of recorded speech of a person and , more particularly , by analyzing frames of the speech signal .