In this book, hierarchical structures based on neural networks are
investigated for automatic speech recognition. These structures are
mainly evaluated within the phoneme recognition task under the Hybrid
Hidden Markov Model/Artificial Neural Network (HMM/ANN) paradigm. The
baseline hierarchical scheme consists of two levels each which is based
on a Multilayered Perceptron (MLP). Additionally, the output of the
first level is used as an input for the second level. This system can be
substantially speeded up by removing the redundant information contained
at the output of the first level.