1. A method comprising: receiving a speech input; generating an interpretation of the speech input using hidden trajectory modeling with one or more observation vectors that are based at least in part on cepstra and on differential cepstra derived from the cepstra wherein using the hidden trajectory modeling comprises using prediction functions for the cepstra to estimate a mean for a distribution of target vocal tract resonances, and using prediction functions for the differential cepstra to estimate a vector-valued parameter related to the mean for the distribution of target vocal tract resonances, wherein the prediction functions are applied to time differences of sequential vocal tract resonance trajectories; and providing an output based at least in part on the interpretation of the speech input.