Each combination of signal class and feature set must be
accompanied by a PDF estimator (See section 14.3.1).
We need to define which PDF type is used to estimate these.
We have defined two types gauss and gmm,
Gaussian and Gaussian mixtire, respectively.
An arbitrary PDF type can be user-defined,
but the function pdf_xxx.m must exist
(use software/pdf_gauss.m or software/pdf_gmm.m as templates).
If the suffix _yyy or _yyy_zzz, etc, is added to the pdf_type variable,
the character string , 'yyy' or yyy_zzz will be passed to the function
pdf_xxx.m as input variable pstr. To see how this works,
look at software/pdf_gmm.m. The parameter 'yyy' defines the number of
mixture components, for example
specifies a Gaussian mixture with two mixture components.