Whereas our previous speech recognition systems incorporated specific rules about phoneme structures and sequences explicitly coded by human linguists, the new HHMM-based system was not explicitly told that there are forty-four phonemes in English, the sequences of vectors that were likely for each phoneme, or what phoneme sequences were more likely than others. We let the system discover these “rules” for itself from thousands of hours of transcribed human speech data. The advantage of this approach over hand-coded rules is that the models develop probabilistic rules of which human experts
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