In phonetics, a diphone is an adjacent pair of phones in an utterance.
For example, in [daɪfəʊn], the diphones are [da], [aɪ], [ɪf], [fə], [əʊ], [ʊn].
The term is usually used to refer to a recording of the transition between two phones.
When pre-recorded diphones are combined to create synthesized speech, the resulting sounds are much more natural than just combining simple phones.
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