  This paper proposes a controllable end-to-end text-to-speech (TTS) system to
control the speaking speed (speed-controllable TTS; SCTTS) of synthesized
speech with sentence-level speaking-rate value as an additional input. The
speaking-rate value, the ratio of the number of input phonemes to the length of
input speech, is adopted in the proposed system to control the speaking speed.
Furthermore, the proposed SCTTS system can control the speaking speed while
retaining other speech attributes, such as the pitch, by adopting the global
style token-based style encoder. The proposed SCTTS does not require any
additional well-trained model or an external speech database to extract
phoneme-level duration information and can be trained in an end-to-end manner.
In addition, our listening tests on fast-, normal-, and slow-speed speech
showed that the SCTTS can generate more natural speech than other phoneme
duration control approaches which increase or decrease duration at the same
rate for the entire sentence, especially in the case of slow-speed speech.
