  Speech is the fundamental mode of human communication, and its synthesis has
long been a core priority in human-computer interaction research. In recent
years, machines have managed to master the art of generating speech that is
understandable by humans. But the linguistic content of an utterance
encompasses only a part of its meaning. Affect, or expressivity, has the
capacity to turn speech into a medium capable of conveying intimate thoughts,
feelings, and emotions -- aspects that are essential for engaging and
naturalistic interpersonal communication. While the goal of imparting
expressivity to synthesised utterances has so far remained elusive, following
recent advances in text-to-speech synthesis, a paradigm shift is well under way
in the fields of affective speech synthesis and conversion as well. Deep
learning, as the technology which underlies most of the recent advances in
artificial intelligence, is spearheading these efforts. In the present
overview, we outline ongoing trends and summarise state-of-the-art approaches
in an attempt to provide a comprehensive overview of this exciting field.
