A number of different duration prediction modules are available with varying levels of sophistication.
Segmental duration prediction is done by the module Duration
which calls different actual methods depending on the parameter
Duration_Method.
All of the following duration methods may be further affected by both a global duration stretch and a per word one.
If the parameter Duration_Stretch is set, all absolute durations
predicted by any of the duration methods described here are multiplied by
the parameter's value. For example
(Parameter.set 'Duration_Stretch 1.2)
will make everything speak more slowly.
In addition to the global stretch method, if the feature
dur_stretch on the related Token is set it will also be
used as a multiplicative factor on the duration produced by the selected
method. That is R:Syllable.parent.parent.R:Token.parent.dur_stretch.
There is a lisp function duration_find_stretch wchi will return
the combined gloabel and local duration stretch factor for a given
segment item.
Note these global and local methods of affecting the duration produced by models are crude and should be considered hacks. Uniform modification of durations is not what happens in real speech. These parameters are typically used when the underlying duration method is lacking in some way. However these can be useful.
Note it is quite easy to implement new duration methods in Scheme directly.