The present invention provides a technique for shifting pitch to target
pitch without detecting the original pitch directly, and for extracting
the pitch of the audio waveform exactly. A phase compensator extracts 2
or more frequency channels each having frequency components of a harmonic
overtone whose frequency is 1 or more times as higher than frequency of a
fundamental tone of the original sound, from the frequency channels from
which the frequency components are extracted by fast Fourier transform.
The phase compensator calculates a scaling value to be used for
converting the fundamental tone to another target fundamental tone, and
performs phase compensation in accordance with the scaling value. A pitch
shifter performs pitch scaling in accordance with the scaling value onto
the audio data resultant from inverse fast Fourier transform onto the
phase compensated frequency components. Thus, audio data representing the
target fundamental tone are generated.