High quality low bit rate celp-based speech codec

A method of encoding and decoding a speech signal comprising the steps of:

• receiving a speech signal and dividing the speech signal into speech frames;
• performing linear predictive code analysis on a speech frame in each of a first and a second linear prediction window, the first linear prediction window being centered at the middle of the speech frame and the second linear prediction window being centered at the edge of the speech frame;
• generating a first set of filter coefficients for the first linear prediction window and a second set of filter coefficients for the second linear prediction window;
• generating a first pitch estimate for a first pitch estimation window and a second pitch estimate for a second pitch estimation window, the first pitch estimation window being centered at the middle of the speech frame and the second pitch estimation window being centered at the edge of the speech frame;
• classifying the speech frame into one of a plurality of modes based on the first and the second sets of filter coefficients and the first and the second pitch estimates, wherein a first mode is predominantly voiced and a second mode is not predominantly voiced;
• encoding the speech frame based on the classified mode of the speech frame, wherein, for a speech frame classified in the first mode, the encoded speech frame encodes information derived from the second set of linear coefficients and the second pitch estimate, and for a speech frame classified in the second mode, the encoded speech frame encodes information derived from the first and the second sets of linear coefficients;
• transmitting the encoded speech frame;
• receiving a transmission for an encoded speech frame and identifying the transmitted speech frame as one of a first mode and a second mode speech frame;
• and
• decoding the transmitted speech frame in a mode-specific manner, based on the identified mode of the transmitted speech frame.

A coder for encoding a speech signal comprising:

• a receiver for receiving the speech signal and dividing the speech signal into speech frames;
• a linear predictor for performing linear predictive code analysis on a first and a second linear prediction window, the first linear prediction window being centered at the middle of the speech frame and the second linear prediction window being centered at the edge of the speech frame, wherein the linear predictor generates a first set of filter coefficients for the first linear prediction window and a second set of filter coefficients for the second linear prediction window;
• a pitch estimator for generating a pitch estimate for each of a first and a second pitch estimation window, the first pitch estimation window being centered at the middle of the speech frame and the second pitch estimation window being centered at the edge of the speech frame;
• a mode classifier responsive to the first and the second sets of filter coefficients and the first and the second pitch estimates, for classifying the speech frame into one of a plurality of modes, wherein a first mode is predominantly voiced and a second mode is not predominantly voiced;
• and
• an encoder for encoding the speech frame based on the classified mode of the speech frame.

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

• receiving a speech signal and dividing the speech signal into speech frames;
• performing linear predictive code analysis on a speech frame in a first and a second linear prediction window, the first linear prediction window being centered at the middle of the speech frame and the second linear prediction window being centered at the edge of the speech frame;
• generating a first set of filter coefficients for the first linear prediction window and a second set of filter coefficients for the second linear prediction window;
• generating a first pitch estimate for a first pitch estimation window and a second pitch estimate for a second pitch estimation window, the first pitch estimation window being centered at the middle of the speech frame and the second pitch estimation window being centered at the edge of the speech frame;
• classifying the speech frame into one of a plurality of modes based on the first and the second sets of filter coefficients and the first and the second pitch estimates, wherein a first mode is predominantly voiced and a second mode is predominantly not voiced;
• encoding the speech frame based on the classified mode of the speech frame;
• and
• transmitting the encoded speech frame.