Lattice tool

[Andreas Stolcke]

In message <00d701c45df4$f9d70c00$34284484 at cs.technion.ac.il>you wrote:
> Hi,
> 
> I have a few questions about lattices:
> 
> 1. Is it possible to get the n-best word sequences from a lattice,
> according to Viterbi decoding, and not only the 1-best? If not, what is
> the function of  n-best lists in SRILM? How they are created?

There is currently no facility in SRILM to generate N-best lists from
lattices, although that is a perfectly legitimate thing to want to do.
It just hasn't been a high-priority thing for us because we generate
N-best with our recognizer, not from lattices directly.

I think HTK has some tool that does it.  So at least for HTK lattices you
could try that.

> 2. When applying lattice-expansion in lattice-tool: are the original
> probabilities in the lattice just ignored?

Yes, except if you preprocess the lattices in some way that relies on
the probabilities, e.g., by pruning.
Then the "old" probabilities are used in the pruning step, prior to
expansion.

> 3. How does lattice-expansion work, for instance, for a trigram backoff
> model. How do the states and transitions change? I would be grateful to
> see a toy example that clarifies that, or to get a reference for such an
> explanation.

Try the paper referenced in the lattice-tool man page.
A postscript file can be found at
http://www.speech.sri.com/papers/icslp98-lattices.ps.gz

> 4. Just to make sure: if the transition prbablity is p, should I encode
> it as 10000.5*log(p) (log is the natural log) in the pfsg?

Correct.

> 5. In pfsg format, are n1 n2 in the transition lines state numbers, or
> the actual words? If they are numbers, does the numbering start with 0
> or with 1?

The numbers are the state indices, starting at 0.

--Andreas