The basic synthesis process in Festival is viewed as applying
a set of modules to an utterance. Each module will access
various relations and items and potentially generate new features,
items and relations. Thus as the modules are applied the utterance
structure is filled in with more and more relations until
ultimately the waveform is generated.
Modules may be written in C++ or Scheme. Which modules are executed are
defined in terms of the utterance type, a simple feature on the
utterance itself. For most text-to-speech cases this is defined to be
of type Tokens. The function utt.synth simply looks up an
utterance's type and then looks up the definition of the defined
synthesis process for that type and applies the named modules.
Synthesis types maybe defined using the function defUttType.
For example definition for utterances of type Tokens
is
(defUttType Tokens
(Token_POS utt)
(Token utt)
(POS utt)
(Phrasify utt)
(Word utt)
(Pauses utt)
(Intonation utt)
(PostLex utt)
(Duration utt)
(Int_Targets utt)
(Wave_Synth utt)
)
While a simpler case is when the input is phone names
and we don't wish to do all that text analysis and prosody
prediction. Then we use the type
Phones which simply
loads the phones, applies fixed prosody and the synthesizes
the waveform
(defUttType Phones
(Initialize utt)
(Fixed_Prosody utt)
(Wave_Synth utt)
)
In general the modules named in the type definitions are general and
actually allow further selection of more specific modules within
them. For example the
Duration module respects the global
parameter
Duration_Method and will call then desired duration
module depending on this value.
When building a new voice you will probably not need to change any of
these definitions, though you may wish to add a new module and we will
show how to do that without requiring any change to the synthesis
definitions in a later chapter.
There are many modules in the system, some simply wraparounds
to choose between other modules. However the basic modules
used for text-to-speech have the basic following function
- Token_POS
basic token identification, used for homograph disambiguation
- Token
Apply the token to word rules building the Word relation.
- POS
A standard part of speech tagger (if desired)
- Phrasify
Build the Phrase relation using the specified method. Various
are offered, from statistically trained models to simple CART trees.
- Word
Lexical look up building the Syllable and Segment
relations and the SylStructure related these together.
- Pauses
Prediction of pauses, inserting silence into the Segment
relation, again through a choice of different prediction mechanisms.
- Intonation
Prediction of accents and boundaries, building the IntEvent
relation and the Intonation relation that links IntEvents
to syllables. This can easily be parameterized for most practical
intonation theories.
- PostLex
Post lexicon rules that can modify segments based on their
context. This is used for things like vowel reduction,
contractions, etc.
- Duration
Prediction of durations of segments.
- Int_Targets
The second part of intonation. This creates the Target
relation representing the desired F0 contour.
- Wave_Synth
A rather general function that in turn calls the appropriate
method to actually generate the waveform.
