Sphinx-3 is CMU's state-of-the-art large vocabulary speech recognition system.
It uses Hidden Markov Models (HMM) with continuous output probability density
functions (PDF). It supports several modes of operation. The more accurate
mode, known as the "flat decoder", is descended from the original Sphinx-3
release. The faster mode, known as the "tree decoder", was developed
separately. The two decoders were merged in Sphinx 3.5, though the flat
decoder was not fully functional until Sphinx 3.7.
