Bangla Speech Analysis, Synthesis, and Vowel Nasality

1.1. Brief Description of Bangla

Bangla, one of the more prominent Indo-Iranian languages, is the sixth-most popular in the world and spoken by a population that now exceeds 250 million. Geographical distribution of Bangla-speaking population percentages are as follows: Bangladesh (over 95%), and the Indian States of Andaman & Nicobar Islands (26%), Assam (28%), Tripura (67%), and West Bengal (85%). The global total includes those who are now in diaspora in Canada, Malawi, Nepal, Pakistan, Saudi Arabia, Singapore, United Arab Emirates, United Kingdom, and United States. Bangla has two literary styles: one is called Sadhubhasa (elegant form) and the other Chaltibhasa (commonly used form). The differences between the two styles are not huge and involve mainly forms of pronouns and verb conjugations.

The origin of modern Bangla derives from Vedic Sanskrit (1500 BC – 1000 BC). Bangla writing system evolved from the Brahmi script, which is closely related to the Devanagari alphabet, from which it started to diverge in the 11th century AD. The current printed form of Bangla alphabet first appeared in 1778 when Charles Wilkins developed printing in Bangla. Its script includes two types of symbols, the letters (Varnas) and signs (Cinhas). The letters are vowels, consonants and conjunct consonants. The signs used are vowel signs and prosodic signs. When the vowel is used with the consonant except in the first position, it is written in sign symbols. Among prosodic signs Chandrabindu [(] and Hasant [$] are noticeable. Chandrabindu or moon dot is a graphic sign for nasalization and is written on top of a letter when the vowel following immediately is to be expressly nasalized. The writing system is more syllabic than phonemic as there is an inherent vowel [ɔ] with all the consonant phonemes except three consonants. It has the ambiguity of being phonetic, syllabic or alphabetic in nature (Hai, 1985; Chaudhury, 1984). When Hasant is appended to a consonant, the pronunciation is consonantal alone, so the inherent vowel [ɔ] is not functioning anymore. Bangla is an intonation language having no tone, accent or stress (Shaw, 1996). Normal Bangla speech is heard more or less on a monotone with slight rise and fall of pitch and loudness within the sentence. Just as the written form of a language is a sequence of elementary alphabet, speech is also a sequence of elementary acoustic sounds or symbols known as phonemes that convey the spoken form of a language (Hai, 1985). Bangla is heard more or less in monotone. But the interesting point in Bangla is nasality. All the 7 vowels in Bangla have their corresponding nasal counterparts. Nasalization of vowel changes meaning of some words in Bangla. The contrast lies in the spectrum of nasal and oral vowels as shown in Figure 1. Besides, there are 29 simple consonants, 19 diphthongs and two semivowel phonemes as shown in Table 1.There are monosyllabic to seven syllabic words in Bangla. Vowel is the nucleus of a syllable. Some vowel itself has meaning and is treated as a word. The general syllable structure of Bangla is:(C)(C)(C)V(V)(C)

Figure 1.

Spectrum of oral Bangla vowel /i/ and its nasal counterpart /i)/