Threshold Generation Using Multiple Verification Sessions
Currently, the threshold is set based on the average distortion calculated
by one verification session. If for some reason there is a large variance
in that one session, then in future verification sessions, there may be
very high false acceptances or very high false rejections.
To account for this, several verification sessions should be held and
the average distortion factors from each session should be summed and
averaged before scaling as the threshold.
Weighting the Code Vectors
If the codebooks for several users were compared, it can be seen that
certain code words are generated relatively close to each other among
the different users. Accuracy could be improved by creating some sort
of weighting for each code word in each codebook. This way, code words
that occur frequently would be weight less in the average distortion calculations.
Code Book Adaptation Over Time
An idea that was discussed was allowing the codebook of each user to adapt
over successful verification sessions. This would modify the thresholds
and codebooks each time the user successfully passed the verification
test. However, it was debated on whether it would then be plausible to
convert a given user's codebook to match that of a different user. Thus
defeating the purpose of the system.
In the Mel-Frequency transform, the output of the filterbanks depends
on the power of the signal. This implies that speaking loudly will be
seen differently than quietly. By normalizing the recording signal, this
effect can be reduced.