18-07-2011, 11:14 AM
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1. INTRODUCTION
Voice morphing means the transition of one speech signal
into another. Like image morphing, speech morphing aims to preserve
the shared characteristics of the starting and final signals, while
generating a smooth transition between them. Speech morphing is
analogous to image morphing. In image morphing the in-between
images all show one face smoothly changing its shape and texture until
it turns into the target face. It is this feature that a speech morph should
possess. One speech signal should smoothly change into another,
keeping the shared characteristics of the starting and ending signals but
smoothly changing the other properties. The major properties of
concern as far as a speech signal is concerned are its pitch and
envelope information. These two reside in a convolved form in a
speech signal. Hence some efficient method for extracting each of
these is necessary. We have adopted an uncomplicated approach
namely cepstral analysis to do the same. Pitch and formant information
in each signal is extracted using the cepstral approach. Necessary
processing to obtain the morphed speech signal include methods like
Cross fading of envelope information, Dynamic Time Warping to
match the major signal features (pitch) and Signal Re-estimation to
convert the morphed speech signal back into the acoustic waveform.
Voice Morphing www.bestneo.com
2. AN INTROSPECTION OF THE MORPHING
PROCESS
Speech morphing can be achieved by transforming the
signal’s representation from the acoustic waveform obtained by
sampling of the analog signal, with which many people are familiar
with, to another representation. To prepare the signal for the
transformation, it is split into a number of 'frames' - sections of the
waveform. The transformation is then applied to each frame of the
signal. This provides another way of viewing the signal information.
The new representation (said to be in the frequency domain) describes
the average energy present at each frequency band.
Further analysis enables two pieces of information to be
obtained: pitch information and the overall envelope of the sound. A
key element in the morphing is the manipulation of the pitch
information. If two signals with different pitches were simply crossfaded
it is highly likely that two separate sounds will be heard. This
occurs because the signal will have two distinct pitches causing the
auditory system to perceive two different objects. A successful morph
must exhibit a smoothly changing pitch throughout. The pitch
information of each sound is compared to provide the best match
between the two signals' pitches. To do this match, the signals are
stretched and compressed so that important sections of each signal
match in time. The interpolation of the two sounds can then be
performed which creates the intermediate sounds in the morph. The
final stage is then to convert the frames back into a normal waveform.