1. A method of generating grapheme-to-phoneme rules for text-to-speech conversion based on a lexicon having words and phonetic transcriptions associated with the words, executed by a computer programmed to perform the method, the method comprising: an alignment phase, using the computer, for aligning phonemes, belonging to a phoneme set, to graphemes, belonging to a grapheme set; and a rule-set extraction phase, using the computer, for generating a set of rules for automatic grapheme to phoneme conversion, said alignment phase comprising the following steps: aligning said lexicon in a preliminary alignment step, using the computer, by generating a first plurality of grapheme and phoneme clusters, each cluster comprising a sequence of at least two components; enlarging at least one of said phoneme and grapheme sets, using the computer, by adding at least one of the grapheme or phoneme clusters generated in said preliminary alignment step into at least one of the phoneme and grapheme sets; rewriting said lexicon, using the computer, according to said at least one enlarged phoneme and grapheme sets; aligning said lexicon in a further alignment step, using the computer, by generating a second plurality of phoneme and grapheme clusters; and the steps of: a) selecting, using the computer, potential grapheme clusters whose occurrence is higher than a first predetermined threshold; b) enlarging, using the computer, said grapheme set by adding said selected potential grapheme clusters; c) selecting, using the computer, potential phoneme clusters whose occurrence is higher than a second predetermined threshold; d) enlarging, using the computer, said phoneme set by adding said selected potential phoneme clusters; and e) rewriting, using the computer, said lexicon by replacing each sequence of components of corresponding grapheme and phoneme clusters in said lexicon with the selected potential grapheme and phoneme clusters, f) generating, using the computer, a lexicon alignment for said rule-set extraction phase in the further alignment step, and g) calculating, using the computer, a statistical distribution of the second plurality of grapheme and phoneme clusters generated in said further alignment step, and repeating, using the computer, said steps a) to f) in case a number of said grapheme and phoneme clusters generated in said further alignment step is greater than a third predetermined threshold.