1. A computer implemented method of training a speech model that detects differences between different classes of speech signals, using a computer with a processor, comprising: dividing, with the processor, the speech model into a plurality of sub-model groups based on at least one predetermined criterion, a first of the sub-model groups detecting a difference between a corresponding first class of speech signal and a corresponding second class of speech signal, a second of the sub-model groups detecting a difference between a corresponding third class of speech signals and a corresponding forth class of speech signal wherein the first and second classes of speech signal are closer to one another than the third and forth classes of speech signal; performing, with the processor, different training on each of the plurality of sub-model groups to increase performance of each of the plurality of sub-model groups in detecting differences specific to the corresponding classes of speech signal to obtain a plurality of modified sub-models; and combining, with the processor, the plurality of modified sub-models to obtain a modified model.