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.
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. 14. A method as described in claim 13, wherein: each frame contains a plurality of spectral parameters, each of which represents the energy at one of a series of different frequency bands; said calculating of difference parameters includes calculating one or more slope parameters in association with each of a plurality of said frames, with each slope parameter being calculated as a function of the difference between the energy of a spectral parameter in one such frame and the energy of a spectral parameter representing a different frequency band in a nearby frame; and said comparing of difference parameters against speech-unit models includes comparing slope parameters against such speech-unit models, where each such speech-unit model has a model for the slope parameters associated with frames that correspond to the speech unit it represents.