1. A method of tracking vocal tract resonant frequencies in a speech signal, the method comprising: determining an observation probability of an observation acoustic feature vector given a set of vocal tract resonant frequencies; determining a transition probability of a transition from a first set of vocal tract resonant frequencies to a second set of vocal tract resonant frequencies based in part on a target-guided constraint for the vocal tract resonant frequencies; and using the observation probability and the transition probability to select a set of vocal tract resonant frequencies corresponding to an observation acoustic feature vector.