I am trying to generate vowels on MATLAB by using source-filter model. In case of monophthongs I'm using Rosenberg pulse as my source signal and then this source signal is convoluted with sounds generated from formants. Basically I'm using the approach used in the following project http://in.mathworks.com/matlabcentral/fileexchange/45449-vowel-synthesis
Now I'm trying to do synthesis of diphthongs which are basically generated by gliding from one monophthong to another. I've tried weighted-average but it's not working. So how can I model this transitioning from formants of one monophthong to other to generate the IPA diphthongs ?