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Objective intelligibility measurements

Hi all,

I am in need of a bit of advice regarding objective measures of
intelligibility such as the AI and the STI. I am interested in assessing
comparative intelligibility of 2 communication systems in noise with regard
to their frequency spectra, however the systems in question use vocoder type
codecs to transmit/recreate the speech, introducing non linear distortion
that the above mentioned methods do not account for. However, my proposal is
this; my signal path in both systems involves CELP algorithms and therefore
a direct comparison using AI or STI to assess the intelligibility is not
valid, however, it should be possible to gain a score for each system with
no additive noise, then repeat the tests with additive noise in the acoustic
interface (i.e. in the room with the loudspeaker, so not passing through the
electronic signal path, algorithms etc) and see which system 'survives' the
noise better with respect to its AI/STI score. This seems to be a valid
approach in gaining an idea as to which system will perform better in
acoustic noise, although obviously will not produce 'absolute' results for
each system. 

Ideally I'd like to do this with AI as it involves measuring signal to noise
ratios in 1/3 octave bands, something which I can readily do, whereas to
implement an STI measurement will require purchasing some (probably
expensive) software.

Thanks in advance for your help
Matt Reynolds