  Emotional voice conversion (EVC) focuses on converting a speech utterance
from a source to a target emotion; it can thus be a key enabling technology for
human-computer interaction applications and beyond. However, EVC remains an
unsolved research problem with several challenges. In particular, as speech
rate and rhythm are two key factors of emotional conversion, models have to
generate output sequences of differing length. Sequence-to-sequence modelling
is recently emerging as a competitive paradigm for models that can overcome
those challenges. In an attempt to stimulate further research in this promising
new direction, recent sequence-to-sequence EVC papers were systematically
investigated and reviewed from six perspectives: their motivation, training
strategies, model architectures, datasets, model inputs, and evaluation
methods. This information is organised to provide the research community with
an easily digestible overview of the current state-of-the-art. Finally, we
discuss existing challenges of sequence-to-sequence EVC.
