search_bestfirst_v1.c File Reference

Viterbi path update and hypothesis score calculation on the 2nd pass, using backscan algorithm. More...

#include <julius.h>

Include dependency graph for search_bestfirst_v1.c:

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Functions
void	free_node (NODE *node)
	< Define if want triphone debug messages
NODE *	cpy_node (NODE *dst, NODE *src)
NODE *	newnode ()
void	malloc_wordtrellis ()
void	free_wordtrellis ()
void	scan_word (NODE *now, HTK_Param *param)
void	next_word (NODE *now, NODE *new, NEXTWORD *nword, HTK_Param *param, BACKTRELLIS *backtrellis)
void	start_word (NODE *new, NEXTWORD *nword, HTK_Param *param, BACKTRELLIS *backtrellis)
void	last_next_word (NODE *now, NODE *new, HTK_Param *param)
Variables
static LOGPROB *	wordtrellis [2]
	Buffer to compute viterbi path of a word.
static int	tn
	Temporal pointer to current buffer.
static int	tl
	Temporal pointer to previous buffer.
static LOGPROB *	g
	Buffer to hold source viterbi scores.
static HMM_Logical **	phmmseq
	Phoneme sequence to be computed.
static int	phmmlen_max
	Maximum length of phmmseq.

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Detailed Description

Viterbi path update and hypothesis score calculation on the 2nd pass, using backscan algorithm.

Author:

Akinobu Lee

Date:

Sun Sep 11 23:54:53 2005

This file has functions for score calculations on the 2nd pass. It includes Viterbi path update calculation of a hypothesis, calculations of scores and word trellis connection at word expansion.

The cross-word triphone will be computed not at word expansion time, but at later pop up for rapid decoding. This is called "backscan" altgorithm. These functions are enabled when PASS2_STRICT_IWCD is UNDEFINED in config.h. If defined, "nextscan" functions in search_bestfirst_v2.c are used instead.

Here we use "delayed cross-word context handling" method for connection of next word and last word of the hypothesis for speeding up decoding:

1. Only right context of the tail phone in the next word is considered when generating a new hypothesis (next_word()).

1. The whole context dependency will be fully computed when the hypothesis is once pushed to stack and later popped in scan_word().

This method avoid computing full context-dependency handling for all generated hypothesis in next_word(), and only re-compute it after primising ones are popped from stack later. This speeds up decoding. But since the context dependency is not considered in the total hypothesis score (computed in next_word()).

The actual implimentation:

1. In nextword(), the tail phone in the new word is modified considering the right context (= head phone in the last word of source hypothesis), and the outprob on the connection point between backtrellis and forward trellis is computed using the triphone.

1. In scan_word(), not only the new word but also the head phone in the previous word should be modified and re-scanned. To realize this '1-phoneme backscan' procedure, hypothesis nodes have to keep forward scores not only at the last HMM state (g[] in NODE), but also at the backscan restart point (= before the head phone in the previous word, g_prev[] in NODE).

Note that the actual implementation becomes a little more complicated to handle 1-phoneme words...

Revision

1.1.1.1

Definition in file search_bestfirst_v1.c.

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Function Documentation

void free_node (  NODE *  node  )

< Define if want triphone debug messages Free a hypothesis node. Parameters: node  [in] hypothesis node Definition at line 129 of file search_bestfirst_v1.c. Referenced by free_all_nodes(), put_all_in_stack(), put_to_stack(), result_reorder_and_output(), and wchmm_fbs().

NODE* cpy_node (  NODE *  dst, NODE *  src )

Copy the content of node to another. Parameters: dst  [out] target hypothesis src  [in] source hypothesis Returns: the value of dst. Definition at line 166 of file search_bestfirst_v1.c. Referenced by last_next_word(), and wchmm_fbs().

NODE* newnode (   )

Allocate a new hypothesis node. Returns: pointer to the newly allocated node. Definition at line 240 of file search_bestfirst_v1.c. Referenced by wchmm_fbs().

void malloc_wordtrellis (   )

Allocate work area for trellis computation of a word. Definition at line 321 of file search_bestfirst_v1.c. Referenced by wchmm_fbs().

void free_wordtrellis (   )

Free the work area for trellis computation of a word. Definition at line 359 of file search_bestfirst_v1.c. Referenced by wchmm_fbs().

void scan_word (  NODE *  now, HTK_Param *  param )

Compute the forward viterbi for the last word to update forward scores and ready for word connection. Parameters: now  [i/o] hypothesis param  [in] input parameter vectors Definition at line 458 of file search_bestfirst_v1.c. Referenced by wchmm_fbs().

void next_word (  NODE *  now, NODE *  new, NEXTWORD *  nword, HTK_Param *  param, BACKTRELLIS *  backtrellis )

Connect a new word to generate a next hypothesis. The optimal connection point and new sentence score of the new hypothesis will be estimated by looking up the corresponding words on word trellis. Parameters: now  [in] source hypothesis new  [out] pointer to save the newly generated hypothesis nword  [in] next word to be connected param  [in] input parameter vector backtrellis  [in] word trellis Definition at line 1256 of file search_bestfirst_v1.c. Referenced by wchmm_fbs().

void start_word (  NODE *  new, NEXTWORD *  nword, HTK_Param *  param, BACKTRELLIS *  backtrellis )

Generate an initial hypothesis from given word. Parameters: new  [out] pointer to save the newly generated hypothesis nword  [in] words of the first candidates param  [in] input parameter vector backtrellis  [in] word trellis Definition at line 1544 of file search_bestfirst_v1.c. Referenced by wchmm_fbs().

void last_next_word (  NODE *  now, NODE *  new, HTK_Param *  param )

Hypothesis termination: set the final sentence scores of hypothesis that has already reached to the end. Parameters: now  [in] hypothesis that has already reached to the end new  [out] pointer to save the final sentence information param  [in] input parameter vectors Definition at line 1650 of file search_bestfirst_v1.c. Referenced by wchmm_fbs().

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