Recognizer = to effect a mapping between sequences of speech vectors and the wanted underlying symbol sequences.
Two problems make this very difficult.
Firstly, the mapping from symbols to speech is not one-to-one since different underlying symbols can give rise to similarspeech sounds. Furthermore, there are large variations in the realised speech waveform due tospeaker variability, mood, environment, etc.
Secondly, the boundaries between symbols cannot be identified explicitly from the speech waveform. Hence, it is not possible to treat the speechwaveform as a sequence of concatenated static patterns.
Isolated word recognition
Objective: To overcome problem of not knowing the word boundary locations
Means: the speech waveform corresponds to a single underlying symbol (e.g. word) chosen from a fixed vocabulary.
Limitation: this simpler problem is somewhat artificial, real life: continuous speech case.
The isolated word recognition problem can thenbe regarded as that of computing
How to compute probability: Bayes Rule:
P(wi|O) = (P(O|wi)P(wi)) /P(O)
prior probabilities P(wi),