15. The apparatus of claim 14 , wherein: the speech encoder means determines a first set of line spectral pairs (LSPs); the speech recognizer means determines a second set of LSPs; and the difference encoder means calculates, for each LSP in the first set of LSPs, an LSP difference between the LSP in the first set of LSPs and a corresponding LSP in the second set of LSPs.
16. The apparatus of claim 15 , wherein the difference encoder means further quantizes the set of formant filter parameter differences using a formant filter parameter difference codebook that includes multiple entries, each entry having a set of LSP differences.