In a large vocabulary speech recognition system the broad phonetic classification
technique is used instead of detailed phonetic analysis to overcome the variability in the
acoustic realisation of utterances. The broad phonetic description of a word is used as a
means of lexical access, where the lexicon is structured into sets of words sharing the
same broad phonetic labelling.
This approach has been applied to a large vocabulary isolated word Arabic speech
recognition system. Statistical studies have been carried out on 10,000 Arabic words
(converted to phonemic form) involving different combinations of broad phonetic
classes. Some particular features of the Arabic language have been exploited. The results
show that vowels represent about 43% of the total number of phonemes. They also show
that about 38% of the words can uniquely be represented at this level by using eight
broad phonetic classes. When introducing detailed vowel identification the percentage of
uniquely specified words rises to 83%. These results suggest that a fully detailed
phonetic analysis of the speech signal is perhaps unnecessary.
In the adopted word recognition model, the consonants are classified into four broad
phonetic classes, while the vowels are described by their phonemic form. A set of 100
words uttered by several speakers has been used to test the performance of the
In the implemented recognition model, three procedures have been developed, namely
voiced-unvoiced-silence segmentation, vowel detection and identification, and automatic
spectral transition detection between phonemes within a word. The accuracy of both the
V-UV-S and vowel recognition procedures is almost perfect. A broad phonetic
segmentation procedure has been implemented, which exploits information from the
above mentioned three procedures. Simple phonological constraints have been used to
improve the accuracy of the segmentation process. The resultant sequence of labels are
used for lexical access to retrieve the word or a small set of words sharing the same broad
phonetic labelling. For the case of having more than one word-candidates, a verification
procedure is used to choose the most likely one.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.