Diphone
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In phonetics, a diphone is an adjacent pair of phones. It is usually used to refer a recording of the transition between two phones.
In the following diagram, a stream of phones are represented by P1, P2, etc., and the corresponding diphones are represented by D1-2, D2-3, etc:
|P1===|P2===|P3===|P4===|P5===|P6===| ===|D1-2=|D2-3=|D3-4=|D4-5=|D5-6=|===
If the number of phones in a language is P, then the theoretical number of possible diphones is P2, although since all languages have restrictions about what sounds can occur next to each other, the number of diphones in each language is usually much smaller than P2.
Spanish has about 800 diphones and German has about 2,500.
Diphones are useful in speech synthesis because combining pre-recorded diphones to create synthesized speech sounds much more natural than combining just simple phones, because the pronunciations of each phones varies based on the surrounding phones.