nbest-format.5

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.\" $Id: nbest-format.5,v 1.4 2001/08/11 20:03:03 stolcke Exp $
.TH nbest-format 5 "$Date: 2001/08/11 20:03:03 $" "SRILM File Formats"
.SH NAME
nbest-format \- File formats for N-best hypotheses lists
.SH DESCRIPTION
SRILM currently understands three different formats for 
lists of N-best hypotheses for rescoring or 1-best hypothesis extraction.
The first two formats originated in the SRI Decipher(TM) recognition
system, the third format is particular to SRILM.
.PP
The first format consists of the header
.br
	NBestList1.0
.br
followed by one or more lines of the form
.br
	(\fIscore\fP) \fIw1 w2 w3\fP ...
.br
where
.I score
is a composite acoustic/language model score
from the recognizer, on the bytelog scale.
(A bytelog is a logarithm to base 1.0001, divided by 1024 and 
rounded to an integer.)
This format is output by the SRI Decipher(TM) recognizer,
by the
.BR "ngram \-nbest" ,
and by
.BR "nbest-lattice \-write-nbest \-decipher-nbest" .
.PP
The second Decipher(TM) format is an extension of the first format
that encodes word-level scores and time alignments.
It is marked by a header of the form
.br
	NBestList2.0
.br
The hypotheses are in the format
.br
	(\fIscore\fP) \fIw1\fP ( st: \fIst1\fP et: \fIet1\fP g: \fIg1\fP a: \fIa1\fP ) \fIw2\fP ...
.br
where words are followed by start and end times, language model and 
acoustic scores (bytelog-scaled), respectively.  
This format may also contain scores and time marks for sub-word units
(phones and HMM states), in the same format as above, but with the
.IR w 's
denoting phone and state names.  Sub-word units will have 
time marks that are contained in the duration of the preceding word units,
and may thus be easily identified.
.PP
The third format understood by SRILM lists
hypotheses in the format
.br
	\fIascore\fP \fIlscore\fP \fInwords\fP \fIw1 w2 w3\fP ...
.br
where the first three columns contain the
acoustic model log probability, the language model log probability,
and the number of words in the hypothesis string, respectively.
All scores are logarithms base 10.
(This format must not be preceded by an ``NBestList'' header.)
This format is output by the
.B "ngram \-rescore"
and by
.B nbest-lattice \-write-nbest 
without the
.B \-decipher-nbest
option.
.SH "SEE ALSO"
ngram(1), nbest-lattice(1), segment-nbest(1), nbest-scripts(1), pfsg-scripts(1).
.SH BUGS
All these formats are somewhat ad hoc and could use a more rational
design.
The ``NBestList1.0'' format is particularly cumbersome because it 
conflates acoustic and language model scores.
.br
A generalization to an arbitrary number of separate scores would be nice.
.SH AUTHOR
Manual page written by Andreas Stolcke <[e-mail]>.
.br
Copyright 1999-2001 SRI International