  Time- and pitch-scale modifications of speech signals find important
applications in speech synthesis, playback systems, voice conversion,
learning/hearing aids, etc.. There is a requirement for computationally
efficient and real-time implementable algorithms. In this paper, we propose a
high quality and computationally efficient time- and pitch-scaling methodology
based on the glottal closure instants (GCIs) or epochs in speech signals. The
proposed algorithm, termed as epoch-synchronous overlap-add time/pitch-scaling
(ESOLA-TS/PS), segments speech signals into overlapping short-time frames and
then the adjacent frames are aligned with respect to the epochs and the frames
are overlap-added to synthesize time-scale modified speech. Pitch scaling is
achieved by resampling the time-scaled speech by a desired sampling factor. We
also propose a concept of epoch embedding into speech signals, which
facilitates the identification and time-stamping of samples corresponding to
epochs and using them for time/pitch-scaling to multiple scaling factors
whenever desired, thereby contributing to faster and efficient implementation.
The results of perceptual evaluation tests reported in this paper indicate the
superiority of ESOLA over state-of-the-art techniques. ESOLA significantly
outperforms the conventional pitch synchronous overlap-add (PSOLA) techniques
in terms of perceptual quality and intelligibility of the modified speech.
Unlike the waveform similarity overlap-add (WSOLA) or synchronous overlap-add
(SOLA) techniques, the ESOLA technique has the capability to do exact
time-scaling of speech with high quality to any desired modification factor
within a range of 0.5 to 2. Compared to synchronous overlap-add with fixed
synthesis (SOLAFS), the ESOLA is computationally advantageous and at least
three times faster.
