الفهرس 
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Abstract
Ahmed Ismail Ahmed Ismail. Devising a Very Low Bit-Rate VoIP
System Using an Arabic Segmental Vocoder. Master of Science
dissertation, Ain Shams University, Faculty of Engineering, 2008.
Voice over Internet Protocol (VoJP) has been an interesting topic in the
recent years. It is cheaper and well positioned for providing a multitude of
different applications other than the conventional two peers’ handset
conversation; low bandwidth conversations, multi-conferencing, audio
commentary archival and serving over the web, encryption, and internet
messaging. This thesis describes research investigating the use of
segmental vocoders to provide a very low bit-rate codec.for use in VoIP
applications. The research was done with emphasis on the Arabic
language, where there’s a lack of reliable publicly available speech
corpora. Hence, efforts were directed towards creating a segmental
vocoder with unsupervised training and the capability of performing well
using training-sets of small size. Initial obtained results had unsatisfactory
performance, from the quality and bit-rate perspectives, so several
enhancements were investigated and were eventually used successfully.
The introduced enhancements included: using shorter length segments,
using segment piecewise linearization, using variable bit rate coding, and
introducing two novel distortion measures: the low-band Time Domain
Phase Aware CTDPA) distortion measure [1], and the high-band Energy
Mass distortion measure [2, 3]. The TDPA distortion measure introduces
a novel technique to jointly encode magnitude and phase information in a
single codebook, avoiding the complexity of the iterative methods
typically needed to create separate magnitude and phase codebooks. Also,
since the TDPA is time-domain based, it showed a superior ability to
maintain phase coherence and provide smooth time evolution of the
synthesized speech signal, an attribute often linked to improved speech
quality. On the other hand, the introduced Energy Mass distortion
measure provided a means to exploit a physical property of the speech
spectrum where the adjacent frequency bins were related to each other
while assessing distortion. The introduced TDPA and Energy Mass