1. A speech recognition system comprising: a line spectral pair generator to generate line spectral pair frequencies from a speech input signal; and a first speech classifier comprising: a quantizer for determining a distance measure between an ith speech input signal line spectral pair frequency and an ith reference speech signal line spectral pair frequency, wherein the distance measure, for i=1 to N.sub.1, is proportional to (i) a difference between the ith speech input signal line spectral pair frequency and the ith reference speech signal line spectral pair frequency and (ii) a shift of the difference by an ith frequency shifting factor to at least partially compensate for frequency shifting of the ith speech input signal line spectral pair frequency by acoustic noise, wherein N.sub.1 is greater than or equal to one and less than or equal to P; wherein the first speech classifier is capable of receiving output data based on the distance measures and is capable of generating speech classification output data for classifying the speech input signal.