13. A method of speech recognition comprising: representing a length of speech as a temporal sequence of frames, with each frame representing speech sounds at one of a succession of brief time periods, and with each frame containing a plurality of acoustic parameters; calculating one or more difference parameters in association with each of a plurality of said frames, with each difference parameter being calculated as a function of the difference between a first acoustic parameter in one such frame and a second acoustic parameter in a nearby frame, in which for one or more of said difference parameters, said first parameter and said second parameter are associated with different frequencies; and comparing the difference parameters associated with individual frames against each of a plurality of acoustic models representing speech units, where each such speech-unit model has a model for the difference parameters associated with frames that correspond to the speech unit it represents.
16. A method as described in claim 13, wherein: each such speech-unit model has a model of the spectral parameters, as well as the difference parameters, associated with frames that correspond to the speech unit it represents; and said comparing includes comparing the spectral parameters, as well as the difference parameters, from individual frames against each of said plurality of speech-unit models.