Audio Time-Scale Modification

By David Dorran

Abstract:

Audio time-scale modification is an audio effect that alters the duration of an audio
signal without affecting its perceived local pitch and timbral characteristics. There
are two broad categories of time-scale modification algorithms, time-domain and
frequency-domain. The computationally efficient time-domain techniques produce
high quality results for single pitched signals such as speech, but do not cope well
with more complex signals such as polyphonic music. The less efficient frequencydomain
techniques have proven to be more robust and produce high quality results for a variety of signals; however they introduce a reverberant artefact into the output.
This dissertation focuses on incorporating aspects of time-domain techniques into
frequency-domain techniques in an attempt to reduce the presence of the reverberant
artefact and improve upon computational demands.
From a review of prior work it was found that there are a number of time-domain
algorithms available and that the choice of algorithm parameters varies considerably
in the literature. This finding prompted an investigation into the effects of the choice
of parameters and a comparison of the various techniques employed in terms of
computational requirements and output quality. The investigation resulted in the
derivation of an efficient and flexible parameter set for use within time-domain
implementations.
Of the available frequency-domain approaches the phase vocoder and timedomain/
subband techniques offer an efficiency and robustness advantage over
sinusoidal modelling and iterative phase update techniques, and as such were
identified as suitable candidates for the provision of a framework for further
investigation. Following from this observation, improvements in the quality produced
by time-domain/subband techniques are realised through the use of a bark based
subband partitioning approach and effective subband synchronisation techniques.
In addition, computational and output quality improvements within a phase vocoder
implementation are achieved by taking advantage of a certain level of flexibility in the
choice of phase within such an implementation. The phase flexibility established is used to push or pull phase values into a phase coherent state. Further improvements
are realised by incorporating features of time-domain algorithms into the system in
order to provide a ‘good’ initial set of phase estimates; the transition to ‘perfect’ phase coherence is significantly reduced through this scheme, thereby improving the overall output quality produced. The result is a robust and efficient time-scale modification algorithm which draws upon various aspects of a number of general approaches to time-scale modification.

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Hello David,

Your thesis is very good, and covers a lot of ground in Audio Research.

Keep up the good work!!

thanks