a) Unit Selection Synthesis
This type uses a large speech database that is recorded
"utterances". Utterances are expressions or statements. It gives the greatest naturalness and the output from
the best systems is usually indistinguishable from real human voices.
b) Diphone Synthesis
This type uses a minimal speech database containing diphones
(sound-to-sound transitions) in a language. The quality of speech is generally not as good, but it is more natural than
other technologies. The use of this type of synthesizer in commercial applications is declining.
c) Domain-specific Synthesis
This type concatenates pre-recorded words and phrases
to create complete utterances. It is limited to such things as transit schedule announcements and, therefore, is not
used to help those people who are disabled. The reason for this is that it can only synthesize the combinations of words
and phrases that it has been programmed with.