Because of the problems including those mentioned above, continuous recognition is more complicated than the isolated recognition. However HMMs provide a good frame work for continuous mode of speech recognition. In this case we connect the HMMs for each speech unit in a particular sentence to form a large HMM for the sentence, as depicted in fig 1.2. This represents the clamped grammar case. The HMM, which represents the free grammar case, is obtained by connecting all the speech units in the vocabulary as shown in fig 1.3.The transitions between speech units are determined using the so called language model.
Figure 1.2: A sentence model using HMMs, the clamped grammar
Figure 1.3: A language model (with vocabulary size 4) using HMMs, the free grammar. Transition probabilities are not shown.