  In this paper, we propose a method to learn unified representations of
multilingual speech and text with a single model, especially focusing on the
purpose of speech synthesis. We represent multilingual speech audio with speech
units, the quantized representations of speech features encoded from a
self-supervised speech model. Therefore, we can focus on their linguistic
content by treating the audio as pseudo text and can build a unified
representation of speech and text. Then, we propose to train an encoder-decoder
structured model with a Unit-to-Unit Translation (UTUT) objective on
multilingual data. Specifically, by conditioning the encoder with the source
language token and the decoder with the target language token, the model is
optimized to translate the spoken language into that of the target language, in
a many-to-many language translation setting. Therefore, the model can build the
knowledge of how spoken languages are comprehended and how to relate them to
different languages. A single pre-trained model with UTUT can be employed for
diverse multilingual speech- and text-related tasks, such as Speech-to-Speech
Translation (STS), multilingual Text-to-Speech Synthesis (TTS), and
Text-to-Speech Translation (TTST). By conducting comprehensive experiments
encompassing various languages, we validate the efficacy of the proposed method
across diverse multilingual tasks. Moreover, we show UTUT can perform
many-to-many language STS, which has not been previously explored in the
literature. Samples are available on https://choijeongsoo.github.io/utut.
