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The module is the basic unit that does the work of synthesis. Within Festival there are duration modules, intonation modules, wave synthesis modules etc. As stated above the utterance type defines the set of modules which are to be applied to the utterance. These modules in turn will create relations and items so that ultimately a waveform is generated, if required.
Many of the chapters in this manual are solely concerned with particular
modules in the system. Note that many modules have internal choices,
such as which duration method to use or which intonation method to
use. Such general choices are often done through the Parameter
system. Parameters may be set for different features like
Duration_Method, Synth_Method etc. Formerly the values
for these parameters were atomic values but now they may be the
functions themselves. For example, to select the Klatt duration rules
(Parameter.set 'Duration_Method Duration_Klatt)
This allows new modules to be added without requiring changes to
the central Lisp functions such as Duration, Intonation,
and Wave_Synth.