nbest-lattice

nbest-lattice

NAME

nbest-lattice - rescore N-best lists and lattices

SYNOPSIS

nbest-lattice [ -help ] option ...

DESCRIPTION

nbest-lattice rescores N-best lists or optimizes word-level recognition scores (as opposed to sentence-level scores). There are two rescoring modes. In N-best word error minimization mode, the program computes the posterior expected word error for each hypothesis relative to all hypotheses in the N-best list, choosing the one with the lowest value.

In lattice word error minimization mode, the program constructs a word lattice from all the N-best hypotheses and extracts the path with the lowest expected word error. This is similar to N-best word error minimization but allows hypotheses not contained in the N-best list. A variant of this mode uses a word ``mesh'' instead of a word lattice, in which all hypotheses are aligned into a grid of word positions, and one is allowed to chose a word from each grid position, thus allowing an even greater number of potential hypotheses.

OPTIONS

Each filename argument can be an ASCII file, or a compressed file (name ending in .Z or .gz), or ``-'' to indicate stdin/stdout.

-help
Print option summary.
-version
Print version information.
-debug level
Controls the amount of output (the higher the level, the more). At level 1, the expected word error counts for the chosen hypotheses are printed. At level 2, the word posterior probabilities are printed in addition (only for lattice mode, similar to -dump-posteriors).
-wer
Chooses N-best word error minimization mode.
-lattice-wer
Chooses lattice word error minimization mode (the default).
-use-mesh
Choose the variant of lattice mode that uses word meshes instead of simple lattices.
-deletion-bias D
Causes the probabilities of deletions to be biased by a factor D in doing mesh-based word error minimization. This controls the trade-off between insertion and deletion errors. The default is 1 (no bias).
-random-tie-break
Break ties between words with equal probability pseudo-randomly when doing mesh-based word error minimization. The default is to decide for the word with the lowest internal index (which reflects the order in the vocab file, or in which they are encountered in the input data).
-no-tie-break
Disable all explicit tie breaking, for backward compatibility.
-rescore file
Reads the N-best list from file. The N-best list can be in any of the formats described in nbest-format(5).
-nbest file
A synonym for -rescore.
-write-nbest file
Outputs the N-best list to a file, after sorting and processing (for validation or format conversion purposes).
-nbest-files file-list
Rescores multiple N-best lists whose filenames are read from file-list.
-write-nbest-dir directory
Outputs N-best lists to directory, to files named after the input N-best lists, for when multiple N-best lists are processed (see -nbest-files).
-write-vocab file
Outputs vocabulary used in N-best list.
-decipher-nbest
Output N-best list in Decipher nbest-format(5), rather than the default native SRILM format. (All N-best formats are accepted for input regardless of this option.)
-no-rescore
Suppress rescoring of lattices; useful if only the operations of lattice/N-best list reading/writing are desired.
-max-nbest n
Limits the number of hypotheses read from each N-best list to the first n.
-max-rescore m
In N-best mode, only choose among the top m hypotheses when optimizing word error. This is convenient to limit computation for long N-best lists. The cutoff is made after reading all hypotheses (subject to -max-nbest) and reordering them according to the posterior probabilities.
The worst-case time taken in N-best error minimization is proportional to m times n, where n is the length of the N-best list (or the value given to -max-nbest). However, in practice the average time per sentence is independent of m, so this option is usually not necessary.
In lattice mode, only align the top m scoring hypotheses (after reweighting and sorting) into the lattice.
-posterior-prune threshold
Don't process N-best hypotheses whose cumulative posterior probability is below threshold. This is another strategy to speed up the algorithm.
-no-reorder
Process N-best hypotheses in the order in which they appear. By default, hypotheses are first sorted by their aggregate scores.
-nbest-backtrace
Preserve backtrace information (word-level timemarks and scores) when reading N-best lists containing such information (see nbest-format(5)). The default is to ignore backtrace information and record only sentence-level scores and the word identities.
-output-ctm
Output word hypotheses in NIST CTM (conversation time mark) format. Note that word start times will be relative to the segment start times, the first column will contain the N-best filename, and the channel field is always 1. The word confidence field contains posterior probabilities. This option also implies -nbest-backtrace.
-rescore-lmw lmw
Sets the language model weight used in combining the language model log probabilities with acoustic log probabilities (only relevant if separate scores are given in the N-best input).
-rescore-wtw wtw
Sets the word transition weight used to weight the number of words relative to the acoustic log probabilities (only relevant if separate scores are given in the N-best input).
If -no-reorder is not specified, and either lmw or wtw are specified to be non-zero, the aggregate scores are recomputed using those weights; otherwise aggregate scores supplied in the input N-best lists are used to sort hypotheses.
-posterior-scale scale
Divide the total weighted log score by scale when computing normalized posterior probabilities. This controls the peakedness of the posterior distribution. The default value is whatever was chosen for -rescore-lmw, so that language model scores are scaled to have weight 1, and acoustic scores have weight 1/lmw.
-posterior-amw amw
Sets the acoustic model weight for computing posteriors; the default is 1. This and the next two options allow posteriors to be computed using a different weighting than that used in ranking and reordering the hypotheses.
-posterior-lmw lmw
Sets the language model weight for computing posteriors. The default is to use whatever was specified for -rescore-lmw.
-posterior-wtw wtw
Sets the word transition weight for computing posteriors. The default is to use whatever was specified for -rescore-wtw.
If all three of amw, lmw, and wtw are set to zero the posteriors are computed directly from the aggregate scores stored in the N-best input.
-vocab file
Read the N-best list vocabulary from file. This option is mostly redundant since words found in the N-best input are implicitly added to the vocabulary.
-vocab-aliases file
Reads vocabulary alias definitions from file, consisting of lines of the form
	alias word
This causes all tokens alias to be mapped to word.
-tolower
Map vocabulary to lowercase, eliminating case distinctions.
-multiwords
Split multiwords (words joined by '_') into their components when reading N-best lists.
-multi-char C
Character used to delimit component words in multiwords (an underscore character by default).
-noise noise-tag
Designate noise-tag as a vocabulary item that is to be ignored in aligning hypotheses with each other (the same as the -pau- word). This is typically used to identify a noise marker.
-noise-vocab file
Read several noise tags from file, instead of, or in addition to, the single noise tag specified by -noise.
-keep-noise
Do not remove pause or noise tokens from hypotheses. The default is to preserve noise tags but still eliminate pauses.
-nbest-error
Compute the N-best error (minimum word error) of the N-best list read with -nbest. Pause and noise tokens (as specified with -noise) in the N-best list are ignored.
-dump-posteriors
Output posterior probabilities of all N-best hypotheses instead of choosing the best hypothesis. In N-best mode, only the posterior probability for each hypothesis is output. In lattice mode, the hyp posterior is followed by word posterior probabilities for each (non-pause, non-noise) token in the hypothesis. The -max-rescore option limits the number of hypotheses per N-best list processed.
-dump-errors
Output word correctness indicators for all N-best hypotheses instead of choosing the best hypothesis. For each hypothesis, a line is output containing first the total number of errors and the list of indicators of whether the corresponding word is correct, substituted or inserted relative to the reference string. The location of deleted words is also indicated by a corresponding marker. The -max-rescore option limits the number of hypotheses per N-best list processed.
-reference w1 w2 ...
Specifies a reference word string for -dump-errors, -nbest-error, and -lattice-error options. Additionally, in -use-mesh mode, the reference words are recorded in the word mesh and can be output with -write, indicating which word in each alignment position is the correct one.
-refs references
Read a table of reference transcripts from file reference, for when multiple N-best lists are processed (see -nbest-files). Each line in references must contain the sentence ID (the last component in the N-best filename path, minus any suffixes) followed by zero or more reference words.

The following options only affect lattice mode.

-read file
Reads an initial lattice from file, to be merged with additional paths constructed from the N-best hypotheses.
-lattice-files file
Reads the names of one or more lattices from file and aligns those lattices with the main lattice being built. Each line of file must contain a lattice filename, optionally followed by a weight.
-dump-lattice-alignments
Causes -lattice-files to write out the position alignments between the -read input lattice and each of the lattices in file, as well as their alignment costs.
-write file
Writes the resulting word posterior lattice or mesh to file, in wlat-format(5).
-write-dir directory
Write the resulting N-best lattices to directory, in files named after the input N-best lists, for when multiple N-best lists are processed (see -nbest-files).
-prime-lattice
Start building the lattice with the best hypothesis obtained from N-best error minimization. This produces slightly better alignments and sometimes lower error rates. The default is to start with the top-scoring hypothesis.
-prime-with-1best
Similar to -prime-lattice, but uses the top-ranked sentence hypothesis for priming. (Experience shows that -no-reorder -prime-lattice gives best results.)
-prime-with-refs
Similar to -prime-lattice, but uses the reference words for priming.
-no-merge
Build a lattice from the N-best hypotheses without merging edges (string/lattice alignment). This creates a lattice with one disjoint path per hypothesis, and is useful mainly for debugging purposes. This option has no effect with -use-mesh since word meshes can represent only one word type per alignment position.
-lattice-error
Compute the lattice error (minimum word error) of the lattice read with -read or built with -nbest.
-dictionary file
Use word pronunciations listed in file to construct word alignments when building word meshes. This will use an alignment cost function that reflects the number of inserted/deleted/substituted phones, rather than words. The dictionary file should contain one pronunciation per line, each naming a word in the first field, followed by a string of phone symbols.
-hidden-vocab file
Read a subvocabulary from file and constrain word meshes to only align those words that are either all in or outside the subvocabulary. This may be used to keep ``hidden event'' tags from aligning with regular words.
-suppress-vocab file
Read a subvocabulary from file and disallow its words when decoding the best word string from a lattice or mesh. If such a word has the highest posterior probability at a given position, the word with next highest posterior is chosen instead, or a null word if no other word choice is available. This is useful when special tokens are included in the nbest inputs to mediate alignments, but are not meant to be included in the output.
-time-penalty p
Apply soft time constraints during word alignment (in word mesh mode only). In addition to the expected word error, a penalty term is added to the cost function minimized during alignment. The penality term is only applied if word meshes or input N-best lists contain backtrace information (see -nbest-backtrace) and is scaled by the factor p (which is zero by default). The penality term for aligning two word hypotheses or word mesh columns is the absolute difference in their times (the time of a word mesh column is the posterior-averaged time of all its component words). If two successive words or word columns have time stamps in the wrong temporal order, their time difference is added to the penalty term.
-average-times
When aligning two instances of the same word, average their times (if available) instead of adopting the one with the time information associated with the highest poosterior probability.
-record-hyps
Record the ranks of the hyps contributing to each word hypothesis in the resulting word lattice; the information is included in -write output.

SEE ALSO

ngram(1), nbest-optimize(1), nbest-scripts(1), nbest-format(5), wlat-format(5).
A. Stolcke, Y. Konig, and M. Weintraub, ``Explicit Word Error Minimization in N-best List Rescoring,'' Proc. Eurospeech, 163-166, 1997.
The ``word meshes'' used here are equivalent to the ``confusion networks'' described in: L. Mangu, E. Brill, and A. Stolcke, ``Finding Consensus Among Words: Lattice-based Word Error Minimization.'' Proc. Eurospeech, vol. 1, 495-498, 1999.

BUGS

Several functions are not uniformly implemented for all rescoring modes (e.g., -lattice-files, -dictionary, -record-hyps, and -nbest-backtrace are currently effective only in mesh-lattice mode).
It is a common mistake (not a bug) to use the default LM weight with N-best lists directly from Decipher. Decipher N-best lists have the recognizer's LM weight already built in, so they should be processed with
	nbest-lattice -rescore-lmw 1 -posterior-scale LMW
where LMW is the LM weight during recognition. This is not an issue if the N-best lists have been rescored with rescore-decipher.

AUTHOR

Andreas Stolcke <stolcke@icsi.berkeley.edu>
Copyright (c) 1996-2010 SRI International
Copyright (c) 2011-2019 Andreas Stolcke
Copyright (c) 2011-2019 Microsoft Corp.