The basic cluster unit selection code available in festival uses
segments as the size of unit. However the acoustic distance measure
used in cluster uses significant portions of the previous segment. Thus
the cluster unit selection effectively selects diphones from the
database.
The type of units the cluster selection code uses is based on the
segment name, by default. In the case of limited domain synthesis we
have found that constraining this further gives both better, and faster
synthesis. Thus we allow for the unit type to be defined by an
arbitrary feature. In the default limited domain set up
we use
SEGMENT_WORD
That is, the segment plus the word the segment comes from. Note
this doesn't mean we are doing
word concatenation in our
synthesizer. We are still selecting phone units but that the
these phone are differentiated depending on the word they come from
thus a /t/ from the word "unit" cannot be used to synthesis a /t/ in
"table". The primary reason for us doing this was to cut down
the search, though it notable improves synthesis quality to. As we
have constructed the database to have good coverage this is a practical
thing to do.
The feature function clunit_name constructs the unit type for a
particular segment item. We have provided the above default (segment
name plus (downcased) word name), but it is easy to extend this.
In one domain we have worked in we wish to differentiate between words
in different prosody contexts. Particularly we wished to mark words us
"questionable" so we can ask users for confirmation. To do this we
marked the "questionable" words in the prompts with a question mark
prefix. We then recorded them with appropriate intonation and then
defined our clunit_name function in include "C_" is the word was
prefixed by a question mark. For example the following two prompts
will be read in a different manner
theater is Squirrel Hill Theater
theater is ?Squirrel ?Hill ?Theater
Likewise in unit selection the units in the word "Squirrel" will not be
used to synthesize the word "?Squirrel" and vice versa. Although crude,
this does give simple control over prosody variation though this technique
can require the vocabulary of the units to increase to where this
technique ceases to be practical.
It would be good if this technique had a back-off strategy where if no
unit can be found for a particular word it would allow other words to
contribute candidates. This is ultimately what general unit selection
is. We do consider this our goal in unit type but in the interest of
building quick and reliable limited domain synthesizers we do not yet do
this but consider it an area we will experiment with. One specific area
that only partially cross this line is in the synthesis of numbers. It
seem very reasonable to allow selection of units from simple numbers
(e.g. "seven" and "seventy") but we have not experimented on that yet.
One further important point should be highlights about this method
for defining unit types. Although including the word name in the unit
name does greatly encourage whole words to be selected it does not
mean that joins in the synthesize utterances only occur at word
boundaries. It is common that contiguous units are selection from
different occurrences of the same word. Mid-word (e.g. within
vowels, or at stops) joins at stable places are common. The optimal
coupling technique selects the best place within a word for the cross
over between two different parts of the database.
