The quality of the speech produced depends greatly on the techniques employed at both the lexical analysis phase and the synthesis phase. The particular problem with phoneme concatenation is how to actually perform the join since fluent speech requires fluent transitions. Other commonly used elements of speech are demisyllables, syllables, words, and word systems. As the units become longer, so the quality increases, but then so does the storage requirement and the processing overhead.
"speech synthesis." A Dictionary of Computing. . Encyclopedia.com. (August 16, 2018). http://www.encyclopedia.com/computing/dictionaries-thesauruses-pictures-and-press-releases/speech-synthesis
"speech synthesis." A Dictionary of Computing. . Retrieved August 16, 2018 from Encyclopedia.com: http://www.encyclopedia.com/computing/dictionaries-thesauruses-pictures-and-press-releases/speech-synthesis