1. A method of a continuous speech recognition system for discriminatively training hidden Markov models, the method comprising: for a mixture component of a hidden Markov model state, calculating a gradient adjustment of the standard deviation of the mixture component, and i. if the calculated gradient adjustment is greater than a first threshold amount, performing an adjustment of the standard deviation of the mixture component using the first threshold, or ii. if the calculated gradient adjustment is less than a second threshold amount, performing an adjustment of the standard deviation of the mixture component using the second threshold, or else iii. performing an adjustment of the standard deviation of the mixture component using the calculated gradient adjustment.