1. A method of speech recognition, the method comprising: dividing an input signal into frames; calculating a vocal tract resonance trajectory value for a frame of an input signal and a decoding state based on vocal tract resonance trajectories determined for two immediately preceding frames and a target for the vocal tract resonance trajectory using a recursive definition; using the vocal tract resonance trajectory for the state to determine a probability distribution for the state by determining a mean for the probability distribution by multiplying a difference between an expected value for the vocal tract resonance trajectory and the vocal tract resonance trajectory by a value associated with the state to form a product and adding the product to a second mean associated with the state and separate from the target; determining a probability for an observation value associated with the frame based on the probability distribution for the state; and using the probability for the observation value to produce a word represented by the frame.