The HTK Book Steve Young Gunnar Evermann Dan Kershaw ...

3.1 Data Preparation 24onetwodialthree... etcsent-startzerosent-endJulianOdellphonecallDave... etcSteveOllasonYoungFig. 3.1Grammar for Voice DiallingGrammar(gram)HPARSEWord Net(w d n e t )Fig. 3.2Step 1The above high level representation of a task grammar is provided for user convenience. TheHTK recogniser actually requires a word network to be defined using a low level notation calledHTK Standard Lattice Format (SLF) in which each word instance and each word-to-word transitionis listed explicitly. This word network can be created automatically from the grammar above usingthe HParse tool, thus assuming that the file gram contains the above grammar, executingHParse gram wdnetwill create an equivalent word network in the file wdnet (see Fig 3.2).3.1.2 Step 2 - the DictionaryThe first step in building a dictionary is to create a sorted list of the required words. In the telephonedialling task pursued here, it is quite easy to create a list of required words by hand. However, ifthe task were more complex, it would be necessary to build a word list from the sample sentencespresent in the training data. Furthermore, to build robust acoustic models, it is necessary to trainthem on a large set of sentences containing many words and preferably phonetically balanced. Forthese reasons, the training data will consist of English sentences unrelated to the phone recognitiontask. Below, a short example of creating a word list from sentence prompts will be given. As notedabove the training sentences given here are extracted from some prompts used with the TIMITdatabase and for convenience reasons they have been renumbered. For example, the first few itemsmight be as follows