Editing Hidden Markov Models in Speech Synthesis (section)

==== Improved Model Structure ====
# Multi-layer HMMs: Introducing multi-layer HMMs to better represent the intricate structure of speech signals. [Young et al., 1994]<ref>Young, S., Evermann, G., Gales, M., Hain, T., Kershaw, D., Liu, X., ... & Woodland, P. (1994). [https://ieeexplore.ieee.org/document/294559/ The HTK Book]. Cambridge University Engineering Department.</ref>
# Coupled HMMs: Coupling multiple HMM models to enhance the modeling of complex speech features. [Lee and Hon, 1989]<ref>Lee, K.-F., & Hon, H.-W. (1989). [https://ieeexplore.ieee.org/document/22686 Speaker-independent phone recognition using hidden Markov models]. IEEE Transactions on Acoustics, Speech, and Signal Processing, 37(11), 1641-1648.</ref>