Vocal-tract models, synthesis with

That said, formant synthesis does share much in common with the all-pole vocal-tract model. As with the tube model, the formant synthesiser is modular with respect to the source and vocal-tract filter. The oral-cavity component is formed from the connection of between three and six individual formant resonators in series, as predicted by the vocal-tract model, and each formant resonator is a second-order filter of the type discussed in Section 10.5.3. 

Cook, 1990] Cook, P. R. (1990). Identification of Control Parameters in an Articulatory Vocal Tract Model, with Applications to the Synthesis of Singing. PhD thesis, Elec. Eng. Dept., Stanford University. 

Despite its ability to faithfiilly mimic the target and transition patterns of natural speech, standard LP synthesis has a significant unnatural quality to it, often impressionisticly described as buzzy or metallic sounding. Recall that while we measured the vocal tract model parameters directly from real speech, we still used an explicit impulse/noise model for the source. As we will now see, it is this, and specifically the interaction of this with the filter, which creates the unnaturalness. 

---