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 ' ((Token.punc in ("?" "." ":")) ((BB)) ((Token.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.
The following variables are used for parameters. They are all set in the file `lib/phrase.scm'.
break_pos_ngram_name
break_ngram_name
pos_p_start_tag
pos_pp_start_tag
pos_n_start_tag
phrase_type_tree
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