CREATING THE HMM TOPOLOGY FILE

TRAINING CONTINUOUS MODELS

The HMM topology file consists of a matrix with boolean entries, each entry indiactes whether a specific transition from state=row_number to state=column_number is permitted in the HMMs or not. For example a 3-state HMM with no skips permitted beteen states would have a topology file with the following entries:

4
1.0     1.0     0.0     0.0
0.0     1.0     1.0     0.0
0.0     0.0     1.0     1.0 

The number 4 is total the number of sates in an HMMs. The SPHINX automatically appends a fourth non-emitting terminating state to the 3 state HMM. The first entry of 1.0 means that a transition from state 1 to state 1 (itself) is permitted. Accordingly, the transition matrix estimated for any phone would have a "transition-probability" in place of this boolean entry. Where the entry is 0.0, the corresponding transition probability will not be estimated (will be 0). You can either write out the topology file manually, or use the script script make_topology.pl provided with the SPHINX package to do this. The script needs the following arguments:

        states_per_hmm : this is merely an integer specifying the
                         number of states per hmm
        skipstate      : "yes" or "no" depending on whether you
                         want the HMMs to have skipped state transitions
                         or not.

Note that the topology file is common for all HMMs and is a single file containing the topology definition matrix. This file also defines your model architecture and is usually placed in the model_architecture directory. This is however optional, but recommended. If you are running scripts from the SPHINX training package, you will find the file created in the model_architecture directory.