19. An apparatus comprising: a speech encoder, which processes an input speech signal, resulting in a compressed encoder representation of the input speech signal; a speech recognizer, which processes the input speech signal; and a difference encoder, which determines a compressed recognizer representation of a corresponding dictionary speech element that approximates the input speech signal when the speech recognizer identifies the corresponding dictionary speech element, calculates one or more differences between the compressed encoder representation and the compressed recognizer representation, and compiles compressed speech information that includes representations of the one or more differences; and a transmitter, which transmits the compressed speech information that includes representations of the one or more differences when the speech recognizer identifies the corresponding dictionary speech element and transmits the compressed encoder representation of the input speech signal when the speech recognizer does not identify a dictionary speech element that approximates the input speech signal.
19. An apparatus comprising: a speech encoder, which processes an input speech signal, resulting in a compressed encoder representation of the input speech signal; a speech recognizer, which processes the input speech signal; and a difference encoder, which determines a compressed recognizer representation of a corresponding dictionary speech element that approximates the input speech signal when the speech recognizer identifies the corresponding dictionary speech element, calculates one or more differences between the compressed encoder representation and the compressed recognizer representation, and compiles compressed speech information that includes representations of the one or more differences; and a transmitter, which transmits the compressed speech information that includes representations of the one or more differences when the speech recognizer identifies the corresponding dictionary speech element and transmits the compressed encoder representation of the input speech signal when the speech recognizer does not identify a dictionary speech element that approximates the input speech signal. 20. The apparatus of claim 19 , wherein: the speech encoder includes an analysis-by-synthesis encoder, which determines an encoder representation of a formant filter for the input speech signal; the speech recognizer determines a recognizer representation of a formant filter for the corresponding dictionary speech element; and the difference encoder calculates a set of formant filter parameter differences between the encoder representation of the formant filter and the recognizer representation of the formant filter.