Salient phonetic features of Indian languages in speech technology

Speech signal is the basic study and analysis material in speech technology as well phonetics. To form meaningful chunks of language, the speech signal should have dynamically varying spectral characteristics, sometimes varying within a stretch of a few milliseconds. Phonetics groups these temporally varying spectral chunks into abstract classes roughly called as allophones. Distribution of these allophones into higher level classes called phonemes takes us closer to their function in a language. Phonemes and letters in the scripts of literate languages - languages which use writing have varying degrees of correspondence. As such a relationship exists, a major part of speech technology deals with the correlation of script letters with chunks of time-varying spectral stretches in that language. Indian languages are said to have a more direct correlation between their sounds and letters. Such similarity gives a false impression of similarity of text-to-sound rule sets across these languages. A given letter which has parallels across various languages may have different degrees of divergence in its phonetic realization in these languages. We illustrate such differences and point out the problem areas where speech scientists need to pay greater attention in building their systems, especially multilingual systems for Indian languages.