Method and apparatus for speech compression using multi-mode code excited linear predictive coding

A multi-mode linear predictive coder for processing digital speech signals, said digital speech signals being partitioned into frames of a first predetermined length, where each frame is partitioned into subframes of a second predetermined length, said coder comprising:

• a short-term prediction analyzer responsive to said digital speech signals, said short-term prediction analyzer for generating linear prediction parameters and line spectrum parameters;
• a variable rate encoder, coupled to said short-term prediction analyzer, for coding differences of said line spectrum parameters by a predetermined variable rate code;
• a ringing removal and perceptual weighting circuit for ringing removal and perceptual weighting said digital speech signals to produce predistorted speech vectors for successive subframes;
• a multi-mode excitation analyzer, coupled to said ringing removal and perceptual weighting circuit, for generating a set of excitations, a set of match values, and a set of parameters, each excitation in said set of excitations corresponding to a maximal value of a match function in said set of match values;
• a pause analyzer, responsive to said digital speech signals, for pause detecting and producing a pause mode signal;
• a comparator and controller, coupled to said multi-mode excitation analyzer and said pause analyzer, for weighting and comparing said match function values for each of a plurality of excitation search modes, and for generating a current excitation search mode corresponding to one of said plurality of excitation search modes with a maximal weighted match function value;
• a selector of parameters, coupled to said multi-mode excitation analyzer, for generating selected parameters from said set of parameters corresponding to said current excitation search mode;
• and
• a selector of excitations, coupled to said multi-mode excitation analyzer, for selecting a current excitation from said set of excitations corresponding to said current excitation search mode.

A method of selecting encoding parameters, said method for use in a speech synthesizer to improve the subjective speech quality, said method comprising the steps of:

• constructing a pulse based upon the time inversion of a pulse response of a response filter;
• generating an excitation vector in the form of multiple pitch spaced pulses using a set of pitch values, a set of phase values, and said pulse, said set of pitch values and said set of phase values derived from a perceptually weighted speech signal;
• computing energy values and correlation values, said energy values determined using a filtered vector, said correlation values representing the correlation between said filtered vector and said perceptually weighted speech signal, said filtered vector corresponding to said excitation vector;
• and
• selecting the pulse excitation from said excitation vector corresponding to correlation values and energy values that maximize a pulse mode match function.

A pulse train analyzer for use in a speech synthesizer comprising:

• a pulse generator coupled to receive a set of pitch values, a set of phase values, and a set of linear prediction coefficients (LPCs), said set of pitch values and said set of phase values derived from a perceptually weighted speech signal, said set of LPCs derived from an input speech signal, said pulse generator producing an excitation vector based upon said set of pitch values, said set of phase values, and said set of LPCs;
• a correlation circuit coupled to said pulse generator and further coupled to receive said perceptually weighted speech signal, said correlation circuit using a pulse mode match function to determine a set of match values, said set of match values based upon said excitation vector and said perceptually weighted speech signal;
• and
• a pulse train selector coupled to receive said set of match values, said pulse train selector selecting the excitation from said excitation vector that corresponds to the maximal value in said set of match values as a selected pulse excitation.