Speech mode based multi-stage vector quantizer

A line spectral frequency (LSF) vector quantizer for use in encoding an LSF vector in a digital communication system, comprising:

• a mode classifier that classifies the LSF vector as being associated with one of a plurality of modes;
• a first multi-stage LSF vector quantizer section having multiple stages that quantize the LSF vector when the LSF vector is associated with a first one of the plurality of modes, the multiple stages of the first multi-stage section being arranged in a backward predictive network to reduce correlation between adjacent frames of a signal associated with the LSF vector when the LSF vector is associated with the first one of the plurality of modes;
• and
• a second LSF vector quantizer section having multiple stages that quantize the LSF vector when the LSF vector is associated with a second one of the plurality of modes, the multiple stages of the second multi-stage section being arranged in a backward predictive network to reduce correlation between adjacent frames of the signal associated with the LSF vector when the LSF vector is associated with the second one of the plurality of modes.

A method of encoding a speech signal, comprising the steps of:

• dividing the speech signal into a series of speech frames;
• converting each of the speech frames into a vector;
• identifying a mode associated with each of the speech frames as a first mode or a second mode;
• encoding the vectors for the speech frames associated with the first mode using a first multi-stage LSF vector encoder including a first backward predictive network to reduce correlation between adjacent speech frames of the speech signal;
• and,
• encoding the vectors for the speech frames associated with the second mode using a second multi-stage LSF vector encoder including a second backward predictive network to reduce correlation between adjacent speech frames of the speech signal.