Speech recognition systems generally assume that the speech signal is a realisation of some message encoded as a sequence of one or more symbols (see Fig. 1.1). To effect the reverse operation of recognising the underlying symbol sequence given a spoken utterance, the continuous speech waveform is first converted to a sequence of equally spaced discrete parameter vectors. This sequence of parameter vectors is assumed to form an exact representation of the speech waveform on the basis that for the duration covered by a single vector (typically 10ms or so), the speech waveform can be regarded as being stationary. Although this is not strictly true, it is a reasonable approximation. Typical parametric representations in common use are smoothed spectra or linear prediction coefficients plus various other representations derived from these.
The rôle of the recogniser is 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 similar speech sounds. Furthermore, there are large variations in the realised speech waveform due to speaker 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 speech waveform as a sequence of concatenated static patterns.
The second problem of not knowing the word boundary locations can be avoided by restricting the task to isolated word recognition. As shown in Fig. 1.2, this implies that the speech waveform corresponds to a single underlying symbol (e.g. word) chosen from a fixed vocabulary. Despite the fact that this simpler problem is somewhat artificial, it nevertheless has a wide range of practical applications. Furthermore, it serves as a good basis for introducing the basic ideas of HMM-based recognition before dealing with the more complex continuous speech case. Hence, isolated word recognition using HMMs will be dealt with first.
