22. The apparatus of claim 21 , wherein the difference encoder further quantizes the set of form ant filter parameter differences using a formant filter parameter difference codebook that includes multiple entries, each entry having a set of LSP differences.
21. The apparatus of claim 20 , wherein: the speech encoder determines a first set of line spectral pairs (LSPs); the speech recognizer determines a second set of LSPs; and the difference encoder 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.