#herest.tex#

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transforms generated. Transforms may either be stored in 
a single TMF. These TMFs may be loaded using the {\tt -H} option.
When macros are specified for the regression class trees and 
the base classes the following search order is used
\begin{enumerate}
\item Any loaded macro that matches the macro name.
\item The path specified by the configuration variable.
\item The list of directories specified with the {\tt -J} option.
The directories are searched in the order that they are specified
in the command line.
\end{enumerate}
Baseclasses and regression classes may also be loaded using the 
{\tt -H} option.
\mysubsect{Use}{HERest-Use}

\htool{HERest} is invoked via the command line
\begin{verbatim}
   HERest [options] hmmList trainFile ...
\end{verbatim}
This causes the set of HMMs given in {\tt hmmList} to be loaded.
The given list of
training files is then used to perform one re-estimation cycle. As always,
the list of training files can be stored in a script file if required.  On
completion, \htool{HERest} outputs new updated versions of each HMM definition. If
the number of training examples falls below a specified threshold 
for some particular HMM, then
the new parameters for that HMM are ignored and the original parameters are used 
instead.

The detailed operation of \htool{HERest} is controlled by the following
command line options
\begin{optlist}

  \ttitem{-a} Use an input transform to obtain alignments for updating
      models or transforms (default off).
 
  \ttitem{-c f} Set the threshold for tied-mixture observation
      pruning to {\tt f}.
      For tied-mixture \texttt{TIEDHS} systems, only those 
      mixture component probabilities which fall within {\tt f} of
      the maximum mixture component probability are used in calculating
      the state output probabilities (default 10.0).
 
  \ttitem{-d dir} 
      Normally \htool{HERest} looks for HMM definitions
       (not already loaded via MMF files) 
      in the current directory.  This option tells \htool{HERest} to look in
      the directory {\tt dir} to find them.

  \ttitem{-h mask} Set the mask for determining which transform names are 
	to be used for the output transforms. If \texttt{PAXFORMMASK}
       	or \texttt{INXFORMMASK} are not specified then the input
  	transform mask is assumed for both output and parent transforms.

  \ttitem{-l N} Set the maximum number of files to use for each 
	speaker, determined by the output transform speaker mask,
	to estimate the transform with.(default $\infty$).

  \ttitem{-m N}  Set the minimum number of training examples 
    required for any model to {\tt N}.  If the actual number
    falls below this value, the HMM is not updated and the original
    parameters are used for the new version (default value 3).

  \ttitem{-o ext}  This causes the file name extensions of the
      original models (if any) to be replaced by {\tt ext}.

  \ttitem{-p N}  This switch is used to set parallel mode operation.
      If {\tt p} is set to a positive integer {\tt N}, then \htool{HERest} will
      process the training files and then dump all the accumulators
      into a file called {\tt HERN.acc}.  If {\tt p} is set to 0, then
      it treats all file names input on the command line as the names
      of {\tt .acc} dump files.  It reads them all in, adds together
      all the partial accumulations and then re-estimates all the
      HMM parameters in the normal way. 

  \ttitem{-r}  This enables single-pass retraining.  The list of training
      files is processed pair-by-pair.  For each pair, the first file
      should match the parameterisation of the original model set.  The
      second file should match the parameterisation of the required new
      set.  All speech input processing is controlled by configuration
      variables in the normal way except that the variables describing
      the old parameterisation are qualified by the name \texttt{HPARM1}
      and the variables describing the new parameterisation are
      qualified by the name \texttt{HPARM2}.  The stream widths for the
      old and the new must be identical.

  \ttitem{-s file} This causes statistics on occupation of each
      state to be output to the named file.  This file
      is needed for the {\tt RO} command of HHEd but it is also
      generally useful for assessing the amount of training material
      available for each HMM state.
      
  \ttitem{-t f [i l]} Set the pruning threshold to {\tt f}.  During the 
      backward probability calculation, at
      each time $t$ 
      all (log) $\beta$ values falling more than {\tt f} below the
      maximum $\beta$ value at that time are ignored.  During the
      subsequent forward pass, (log) $\alpha$ values are only
      calculated if there are corresponding valid $\beta$ values.
      Furthermore, if the ratio of the $ \alpha \beta $ product divided
      by the total probability (as computed on the backward pass)
      falls below a fixed threshold then those values of $\alpha$
      and $\beta$ are ignored. Setting {\tt f} to zero disables
      pruning  (default value 0.0).  Tight pruning thresholds can
       result in \htool{HERest} failing to process an utterance.
      if the {\tt i} and {\tt l} options are given, then a pruning
      error results in the threshold being increased by {\tt i} and
      utterance processing restarts.  If errors continue, this procedure will 
      be repeated until the limit {\tt l} is reached.
      
  \ttitem{-u flags} By default, \htool{HERest} updates all of the HMM parameters,
      that is, means, variances, mixture weights and 
      transition probabilies. This 
      option causes just the parameters indicated by the {\tt flags}
      argument to be updated, this argument is a string containing one
      or more of the letters {\tt m} (mean), {\tt v} (variance) ,
      {\tt t} (transition), {\tt a} (linear transform), {\tt p} (use 
	MAP adaptation), and {\tt w} (mixture weight).  The 
      presence of a letter enables
      the updating of the corresponding parameter set.

  \ttitem{-v f}  This sets the minimum variance (i.e. diagonal element of
      the covariance matrix) to the real value {\tt f} (default value
      0.0).

  \ttitem{-w f}  Any mixture weight which falls below the global
            constant {\tt MINMIX} is treated as being zero.
      When this parameter is  set,  all mixture weights  are floored
      to {\tt f * MINMIX}.
      
  \ttitem{-x ext}  By default, \htool{HERest} expects a HMM definition for 
      the label X to be stored in a file called {\tt X}.  This
      option causes \htool{HERest} to look for the HMM definition in the
      file {\tt X.ext}.

  \ttitem{-z file} Save all output transforms to file. Default
	is TMF.

\stdoptB
\stdoptE
\stdoptF
\stdoptG
\stdoptH
\stdoptI
\stdoptJ
\stdoptK
\stdoptL
\stdoptM
\stdoptX

\end{optlist}
\stdopts{HERest}

\mysubsect{Tracing}{HERest-Tracing}

\htool{HERest} supports the following trace options where each
trace flag is given using an octal base
\begin{optlist}
   \ttitem{00001} basic progress reporting.
   \ttitem{00002} show the logical/physical HMM map.
   \ttitem{00004} list the updated model parameters.
           of tied mixture components.
\end{optlist}


Trace flags are set using the \texttt{-T} option or the  \texttt{TRACE} 
configuration variable.
\index{herest@\htool{HERest}|)}


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