  In this paper, we propose a dictionary update method for Nonnegative Matrix
Factorization (NMF) with high dimensional data in a spectral conversion (SC)
task. Voice conversion has been widely studied due to its potential
applications such as personalized speech synthesis and speech enhancement.
Exemplar-based NMF (ENMF) emerges as an effective and probably the simplest
choice among all techniques for SC, as long as a source-target parallel speech
corpus is given. ENMF-based SC systems usually need a large amount of bases
(exemplars) to ensure the quality of the converted speech. However, a small and
effective dictionary is desirable but hard to obtain via dictionary update, in
particular when high-dimensional features such as STRAIGHT spectra are used.
Therefore, we propose a dictionary update framework for NMF by means of an
encoder-decoder reformulation. Regarding NMF as an encoder-decoder network
makes it possible to exploit the whole parallel corpus more effectively and
efficiently when applied to SC. Our experiments demonstrate significant gains
of the proposed system with small dictionaries over conventional ENMF-based
systems with dictionaries of same or much larger size.
