decode.tex

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\htool{HVite} can be made to compute forced alignments by not 
specifying a network with the \texttt{-w} option but by specifying
the \texttt{-a} option instead.  In this mode, \htool{HVite} 
computes a new network for each input utterance using the word
level transcriptions and a dictionary.  By default, the output
transcription will just contain the words and their boundaries.
One of the main uses of forced alignment\index{forced alignment}, 
however, is to 
determine the actual pronunciations used in the utterances
used to train the HMM system in this case, the \texttt{-m}
option can be used to generate model level output transcriptions.
}  
This type of forced alignment is usually part of a \textit{bootstrap}
process, initially models are trained on the basis of one fixed
pronunciation per \index{hled@\htool{HLEd}}
\index{ex@\texttt{EX} command}word\footnote{
The \htool{HLEd} \texttt{EX} command can be used to compute phone
level transcriptions when there is only one possible 
phone transcription
per word}.  
Then \htool{HVite} is used in forced alignment mode
to select the best matching pronunciations.  The new phone level
transcriptions can then be used to retrain the HMMs.  Since training
data may have leading and trailing silence, it is usually
necessary to insert a silence model at the start and end of the
recognition network.  The  \texttt{-b} option can be used to do this.

As an illustration, executing
\begin{verbatim}
    HVite -a -b sil -m -o SWT -I words.mlf \
       -H hmmset dict hmmlist file.mfc
\end{verbatim}
would result in the following sequence of events (see Fig.~\href{f:hvalign}).
The input file name \texttt{file.mfc} would have its extension replaced by
\texttt{lab} and then a label file of this name would be searched for.
In this case, the MLF file \texttt{words.mlf} has been loaded. 
Assuming that this file contains a word level transcription called
\texttt{file.lab}, this transcription along with the dictionary \texttt{dict}
will be used to construct a network equivalent to \texttt{file.lab}
but with alternative pronunciations included in parallel.  Since \texttt{-b}
option has been set, the specified \texttt{sil} model will be inserted
at the start and end of the network.  The decoder then finds the best
matching path through the network and constructs a lattice which
includes model alignment information.  Finally, the lattice is converted
to a transcription and output to the label file \texttt{file.rec}.
As for testing on a database, alignments will normally be computed on
a large number of input files so in practice the input files would be listed
in a \texttt{.scp} file and the output transcriptions would be written
to an MLF using the \texttt{-i} option.

When the \texttt{-m} option is used, the transcriptions output by \htool{HVite} 
would by default contain both the model level and 
word level transcriptions
\index{transcriptions!word level}.
\index{transcriptions!model level}
\index{transcriptions!phone level}
For example, a typical fragment of the output might be
\begin{verbatim}
    7500000  8700000 f  -1081.604736 FOUR 30.000000
    8700000  9800000 ao  -903.821350
    9800000 10400000 r   -665.931641
   10400000 10400000 sp    -0.103585
   10400000 11700000 s  -1266.470093 SEVEN 22.860001
   11700000 12500000 eh  -765.568237
   12500000 13000000 v   -476.323334
   13000000 14400000 n  -1285.369629
   14400000 14400000 sp    -0.103585
\end{verbatim}
Here the score alongside each model name is the acoustic score for that segment.
The score alongside the word is just the language model score.

Although the above information can be useful for some purposes, for example
in bootstrap training, only the model names are required.
The formatting option \texttt{-o SWT} in the above suppresses all output
except the model names.\index{decoder!output formatting}

\mysect{Recognition using Direct Audio Input}{recaudio}

\index{decoder!live input}
In all of the preceding discussion, it has been assumed that input was
from speech files stored on disk.  These files would normally have
been stored in parameterised form so that little or no conversion
of the source speech data was required.   When \htool{HVite}
is invoked with no files listed on the command line, it assumes that
input is to be taken directly from the audio input.  In this case,
configuration variables must be used to specify firstly how the
speech waveform is to be captured and secondly, how the captured
waveform is to be converted to parameterised form. 

Dealing with waveform capture\index{waveform capture} first, as described in
section~\ref{s:audioio}, \HTK\ provides two main forms of control over speech
capture: signals/keypress and an automatic speech/silence
detector\index{speech/silence detector}. To use the speech/silence detector
alone, the configuration file would contain the following
\begin{verbatim}
    # Waveform capture
    SOURCERATE=625.0
    SOURCEKIND=HAUDIO
    SOURCEFORMAT=HTK
    USESILDET=T
    MEASURESIL=F
    OUTSILWARN=T
    ENORMALISE=F
\end{verbatim}

where the source sampling rate is being set to 16kHz.  Notice that the
\texttt{SOURCEKIND}\index{sourcekind@\texttt{SOURCEKIND}} must be set to
\texttt{HAUDIO} and the \texttt{SOURCEFORMAT} must be set to 
\texttt{HTK}. Setting the Boolean variable 
\texttt{USESILDET}\index{usesildet@\texttt{USESILDET}} causes the
speech/silence detector to be used, and the
\texttt{MEASURESIL}\index{measuresil@\texttt{MEASURESIL}}
\texttt{OUTSILWARN}\index{outsilwarn@\texttt{OUTSILWARN}} 
variables result in a measurement being taken of the background silence level
prior to capturing the first utterance.  To make sure that each input utterance
is being captured properly, the \htool{HVite} option \texttt{-g} can be set to
cause the captured wave to be output after each recognition attempt. Note that
for a live audio input system, the configuration variable
\texttt{ENORMALISE} should be explicitly set to \texttt{FALSE} both when training models and when performing recognition. Energy normalisation cannot
be used with live audio input, and the default setting for this variable
is \texttt{TRUE}.

As an alternative to using the speech/silence detector, a
signal\index{signals!for recording control} can be used to start and stop
recording.  For example,
\begin{verbatim}
    # Waveform capture
    SOURCERATE=625.0
    SOURCEKIND=HAUDIO
    SOURCEFORMAT=HTK
    AUDIOSIG=2
\end{verbatim}
would result in the Unix interrupt signal (usually the Control-C key) being
used as a start and stop control\footnote{ The underlying signal number must be
given, \HTK\ cannot interpret the standard Unix signal names such as
\texttt{SIGINT} }. Key-press control of the audio input can be obtained by
setting \texttt{AUDIOSIG} to a negative number.

Both of the above can be used together, in this case, audio capture is disabled
until the specified signal is received.  From then on control is in the hands
of the speech/silence detector.

The captured waveform must be converted to the required 
target parameter kind.  Thus, the configuration file must define
all of the parameters needed to control the
conversion of the waveform to the required target kind.
This process is described in detail in Chapter~\ref{c:speechio}.
As an example, the following parameters would allow conversion
to Mel-frequency cepstral coefficients with delta and acceleration
parameters.
\begin{verbatim}
    # Waveform to MFCC parameters
    TARGETKIND=MFCC_0_D_A
    TARGETRATE=100000.0
    WINDOWSIZE=250000.0
    ZMEANSOURCE=T
    USEHAMMING = T
    PREEMCOEF = 0.97
    USEPOWER = T
    NUMCHANS = 26
    CEPLIFTER = 22
    NUMCEPS = 12
\end{verbatim}
Many of these variable settings are the default settings
and could be omitted, they are included explicitly here as a reminder
of the main configuration options available.

When \htool{HVite} is executed in direct audio input mode,
it issues a prompt prior to each input and it is normal to enable
basic tracing so that the recognition results can be seen.
A typical terminal output might be
\begin{verbatim}
    READY[1]>
    Please speak sentence - measuring levels
    Level measurement completed
    DIAL ONE FOUR SEVEN  
         ==  [258 frames] -97.8668 [Ac=-25031.3 LM=-218.4] (Act=22.3)

    READY[2]>
    CALL NINE TWO EIGHT  
         ==  [233 frames] -97.0850 [Ac=-22402.5 LM=-218.4] (Act=21.8)

    etc
\end{verbatim}
If required, a transcription of each spoken input can be output 
to a label file or an MLF in the usual way by setting the \texttt{-e} option.  
However, to do this
a file name must be synthesised.  This is done by using a counter
prefixed by the value of the
\htool{HVite} configuration variable
\texttt{RECOUTPREFIX}\index{recoutprefix@\texttt{RECOUTPREFIX}} and 
suffixed by the value of \texttt{RECOUTSUFFIX}
\index{recoutsuffix@\texttt{RECOUTSUFFIX}}.
For example, with the settings
\begin{verbatim}
    RECOUTPREFIX = sjy
    RECOUTSUFFIX = .rec
\end{verbatim}
then the output transcriptions would be stored as 
\texttt{sjy0001.rec},  \texttt{sjy0002.rec} etc.


\mysect{N-Best Lists and Lattices}{nbest}

\index{decoder!N-best}
As noted in section~\ref{s:decop}, \htool{HVite} can generate 
lattices\index{lattice generation}
and N-best\index{N-best} outputs.  To generate an N-best list, the \texttt{-n} option
is used to specify the number of N-best tokens to store per state and
the number of N-best hypotheses to generate.  The result is that
for each input utterance, a multiple alternative 
transcription\index{multiple alternative transcriptions} is generated.
For example, setting \texttt{-n 4 20} with a digit 
recogniser would generate an output of the form
\begin{verbatim}
    "testf1.rec"
    FOUR
    SEVEN
    NINE
    OH
    /// 
    FOUR
    SEVEN
    NINE
    OH
    OH
    /// 

    etc
\end{verbatim}


The lattices from which the N-best lists are generated can be output by setting
the option \texttt{-z ext}.  In this case, a lattice called \texttt{testf.ext} will
be generated for each input test file \texttt{testf.xxx}.  By default, these lattices
will  be stored in the same directory as the test files, but they can be redirected
to another directory using the \texttt{-l} option.

\index{output lattice format}
The lattices generated by \htool{HVite} have the following general form
\begin{verbatim}
    VERSION=1.0
    UTTERANCE=testf1.mfc
    lmname=wdnet
    lmscale=20.00  wdpenalty=-30.00
    vocab=dict
    N=31   L=56   
    I=0    t=0.00  
    I=1    t=0.36  
    I=2    t=0.75  
    I=3    t=0.81
    ... etc
    I=30   t=2.48  
    J=0     S=0    E=1    W=SILENCE   v=0  a=-3239.01  l=0.00    
    J=1     S=1    E=2    W=FOUR      v=0  a=-3820.77  l=0.00    
    ... etc
    J=55    S=29   E=30   W=SILENCE   v=0  a=-246.99   l=-1.20   
\end{verbatim}

The first 5 lines comprise a header which records names of the files used to
generate the lattice along with the settings of the language model scale and
penalty factors. Each node in the lattice represents a point in time measured in
seconds and each arc represents a word spanning the segment of the input
starting at the time of its start node and ending at the time of its end node.  
For each such span, \texttt{v} gives the number of the pronunciation used, 
\texttt{a} gives the acoustic score and \texttt{l} gives the language model
score.

The language model scores in output lattices do not include the scale factors
and penalties.  These are removed so that the lattice can be used as a
constraint network for subsequent recogniser testing.  When using \htool{HVite}
normally, the word level network file is specified using the \texttt{-w}
option.  When the \texttt{-w} option is included but no file name is included,
\htool{HVite} constructs the name of a lattice file from the name of the test
file and inputs that.  Hence, a new recognition network is created for each
input file and recognition is very fast.  For example, this is an efficient way
of experimentally determining optimum values for the language 
model scale\index{lattice!language model scale factor} and
penalty factors.


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