17 Phrase breaks

There are two methods for predicting phrase breaks in Festival, one simple and one sophisticated. These two methods are selected through the parameter Phrase_Method and phrasing is achieved by the module Phrasify.

The first method is by CART tree. If parameter Phrase_Method is cart_tree, the CART tree in the variable phrase_cart_tree is applied to each word to see if a break should be inserted or not. The tree should predict categories BB (for big break), B (for break) or NB (for no break). A simple example of a tree to predict phrase breaks is given in the file lib/phrase.scm.

(set! simple_phrase_cart_tree
'
((R:Token.parent.punc in ("?" "." ":"))
  ((BB))
  ((R:Token.parent.punc in ("'" "\"" "," ";"))
   ((B))
   ((n.name is 0)
    ((BB))
    ((NB))))))

The second and more elaborate method of phrase break prediction is used when the parameter Phrase_Method is prob_models. In this case a probabilistic model using probabilities of a break after a word based on the part of speech of the neighbouring words and the previous word. This is combined with a ngram model of the distribution of breaks and non-breaks using a Viterbi decoder to find the optimal phrasing of the utterance. The results using this technique are good and even show good results on unseen data from other researchers’ phrase break tests (see black97b). However sometimes it does sound wrong, suggesting there is still further work required.

Parameters for this module are set through the feature list held in the variable phr_break_params, and example of which for English is set in english_phr_break_params in the file lib/phrase.scm. The features names and meaning are

pos_ngram_name

The name of a loaded ngram that gives probability distributions of B/NB given previous, current and next part of speech.

pos_ngram_filename

The filename containing pos_ngram_name.

break_ngram_name

The name of a loaded ngram of B/NB distributions. This is typically a 6 or 7-gram.

break_ngram_filename

The filename containing break_ngram_name.

gram_scale_s

A weighting factor for breaks in the break/non-break ngram. Increasing the value insertes more breaks, reducing it causes less breaks to be inserted.

phrase_type_tree

A CART tree that is used to predict type of break given the predict break position. This (rather crude) technique is current used to distinguish major and minor breaks.

break_tags

A list of the break tags (typically (B NB)).

pos_map

A part of speech map used to map the pos feature of words into a smaller tagset used by the phrase predictor.