Most state-of-the-art TTS synthesizers are based on a technique known as synthesis by concatenation, in which speech is produced by concatenating elementary speech units. The design of a high-quality TTS system implies the storage of a large number of segments. To facilitate the storage of these segments, this paper proposes a very low complexity coder to compress unit databases with a toll quality. A particular interest has been taken in the databases used by the MBROLA synthesizer, composed of fixed-length pitch periods with constrained harmonic phases. The coder developed here uses this special characteristic to reach compression rates from 7 to 9 without degrading the speech quality produced by the synthesizer, and with very limited computational cost.