1. One or more computer-readable storage media comprising computer-executable instructions that, when executed by one or more processors, configure the one or more processors to perform acts comprising: imposing a constraint for discriminative training, the constraint limiting a difference between an initial continuous density hidden Markov model (CDHMM) parameter value and an updated CDHMM parameter value; approximating an objective function as a smooth function of CDHMM parameters; and performing a constrained line search on the smoothed function to optimize values of the CDHMM parameters.