13.5.2 Use

HEREST is invoked via the command line

   HERest [options] hmmList trainFile ...

This causes the set of HMMs given in 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, 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 HEREST is controlled by the following command line options

-c f
Set the threshold for tied-mixture observation pruning to f. For tied-mixture TIEDHS systems, only those mixture component probabilities which fall within f of the maximum mixture component probability are used in calculating the state output probabilities (default 10.0).

-d dir
Normally HEREST looks for HMM definitions (not already loaded via MMF files) in the current directory. This option tells HEREST to look in the directory dir to find them.

-m N
Set the minimum number of training examples required for any model to 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).

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

-p N
This switch is used to set parallel mode operation. If p is set to a positive integer N, then HEREST will process the training files and then dump all the accumulators into a file called HERN.acc. If p is set to 0, then it treats all file names input on the command line as the names of .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.

-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 HPARM1 and the variables describing the new parameterisation are qualified by the name HPARM2. The stream widths for the old and the new must be identical.

-s file
This causes statistics on occupation of each state to be output to the named file. This file is needed for the RO command of HHEd but it is also generally useful for assessing the amount of training material available for each HMM state.

-t f [i l]
Set the pruning threshold to f. During the backward probability calculation, at each time t all (log) values falling more than f below the maximum value at that time are ignored. During the subsequent forward pass, (log) values are only calculated if there are corresponding valid values. Furthermore, if the ratio of the product divided by the total probability (as computed on the backward pass) falls below a fixed threshold then those values of and are ignored. Setting f to zero disables pruning (default value 0.0). Tight pruning thresholds can result in HEREST failing to process an utterance. if the i and l options are given, then a pruning error results in the threshold being increased by i and utterance processing restarts. If errors continue, this procedure will be repeated until the limit l is reached.

-u flags
By default, 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 flags argument to be updated, this argument is a string containing one or more of the letters m (mean), v (variance) , t (transition) and w (mixture weight). The presence of a letter enables the updating of the corresponding parameter set.

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

-w f
Any mixture weight which falls below the global constant MINMIX is treated as being zero. When this parameter is set, all mixture weights are floored to f * MINMIX.

-x ext
By default, HEREST expects a HMM definition for the label X to be stored in a file called X. This option causes HEREST to look for the HMM definition in the file X.ext.

-B
Output HMM definition files in binary format.
-F fmt
Set the source data format to fmt.
-G fmt
Set the label file format to fmt.
-H mmf
Load HMM macro model file mmf. This option may be repeated to load multiple MMFs.
-I mlf
This loads the master label file mlf. This option may be repeated to load several MLFs.
-L dir
Search directory dir for label files (default is to search current directory).
-M dir
Store output HMM macro model files in the directory dir. If this option is not given, the new HMM definition will overwrite the existing one.
-X ext
Set label file extension to ext (default is lab).

HEREST also supports the standard options -A, -C, -D, -S, -T, and -V as described in section 4.4.