Excitation synchronous time encoding vocoder and method

An apparatus for excitation synchronous time encoding of speech signals, said apparatus comprising:

• a frame synchronous linear predictive coding (LPC) device having an input and an output, said input for accepting input speech signals, said output for providing a first group of LPC coefficients describing a first portion of said input speech signal and an excitation waveform describing a second portion of said input speech signal;
• an autocorrelator coupled to said frame synchronous LPC device, said autocorrelator for estimating an epoch length of said excitation waveform;
• a pitch filter having an input coupled to said autocorrelator and having an output signal comprising a multiplicity of coefficients describing characteristics of said excitation waveform;
• frame voicing decision means coupled to an output of said pitch filter, an output of said autocorrelator and said output of said frame synchronous LPC device, said frame voicing decision means for determining whether a frame is voiced or unvoiced;
• means for computing representative excitation levels in a series of contiguous time slots coupled to said frame voicing decision means and operating when said frame voicing decision means determines that said series of contiguous time slots is unvoiced;
• and
• encoding means coupled to said means for computing representative excitation levels, said encoding means for providing an encoded digital signal corresponding to said excitation waveform.

A method for excitation synchronous time encoding of speech signals, said method comprising steps of:

• providing an input signal;
• processing the input speech signal to characterize qualities including linear predictive coding coefficients, epoch length and voicing;
• determining from said voicing when said input speech signal comprises voiced speech;
• characterizing the input speech signals on a single epoch time domain basis when the input speech signals comprise voiced speech to provide a parameterized excitation function;
• determining epoch excitation positions within a frame of excitation when the input speech signals comprise voiced speech;
• determining epoch lengths for each epoch within the frame of parameterized excitation function;
• averaging the epoch lengths to provide fractional pitch;
• and
• encoding the parameterized excitation function and the fractional pitch to provide a digital output signal representing the input speech signal.

A method for excitation synchronous time encoding of speech signals, said method comprising steps of:

• providing an input speech signal;
• processing the input speech signal to characterize qualities including linear predictive coding (LPC) coefficients, epoch length and voicing;
• determining from said voicing when said input speech signal comprises voiced speech;
• characterizing the input speech signals on a single epoch time domain basis when the input speech signals comprise voiced speech to provide a parameterized voiced excitation function by substeps of;
• determining epoch excitation positions within, and a frame of excitation data from, a frame of speech data;
• performing excitation synchronous linear predictive coding (LPC) to provide synchronous LPC coefficients, the synchronous LPC coefficients corresponding to the epoch excitation positions from said determining step;
• selecting an interpolation excitation target from within the frame of excitation data based on minimum envelope error to provide a target excitation function, wherein the target excitation function comprises single-epoch speech parameters including the synchronous LPC coefficients;
• correlating the interpolation excitation target selected in said selecting step with an interpolation excitation target selected in an adjacent frame of excitation data to provide an optimum interpolation offset;
• and
• rotating the interpolation excitation target selected in said selecting step by said interpolation offset to provide new values for said interpolation excitation target;
• and
• determining when the input speech comprises unvoiced speech and characterizing the input speech signals for at least a portion of a frame when the input speech signals comprise unvoiced speech to provide a parameterized unvoiced excitation function;
• and
• encoding a composite excitation function including the parameterized unvoiced excitation function and the parameterized voiced excitation function to provide a digital output signal representing the input speech signal.

A communications apparatus including:

• an encoder for excitation synchronous time encoding of input speech signals, said encoder comprising:
• an input for receiving said input speech signals;
• a speech digitizer coupled to said input for digitally encoding said input speech signals;
• said speech digitizer comprising:
• a frame synchronous linear predictive coding (LPC) device having an input and an output, said input for accepting input speech signals, said output for providing a first group of LPC coefficients describing a first portion of said input speech signal and an excitation waveform describing a second portion of said input speech signal;
• an autocorrelator coupled to said frame synchronous LPC device, said autocorrelator for estimating an epoch length of said excitation waveform;
• a pitch filter having an input coupled to said autocorrelator and having an output signal comprising a multiplicity of coefficients describing characteristics of said excitation waveform;
• frame voicing decision means coupled to an output of said pitch filter, an output of said autocorrelator and said output of said frame synchronous LPC device, said frame voicing decision means for determining whether a frame is voiced or unvoiced;
• means for computing representative excitation levels in a series of contiguous time slots coupled to said frame voicing decision means and operating when said frame voicing decision means determines that said series of contiguous time slots is unvoiced;
• and
• encoding means coupled to said means for computing representative excitation levels, said encoding means for providing an encoded digital signal corresponding to said excitation waveform;
• an output for transmitting said digitally encoded input speech signals, said output coupled to said speech digitizer;
• and a decoder comprising:
• a digital input for receiving digitally encoded speech signals;
• speech synthesizer means coupled to said digital input for synthesizing speech signals from said digitally encoded speech signals, wherein said speech synthesizer means further comprises:
• frame voicing decision means coupled to vector quantizer codebooks, said frame voicing decision means for determining when quantized signals from said vector quantizer codebooks represent voiced speech and when said quantized signals represent unvoiced speech;
• means for interpolating between contiguous signal levels representative of unvoiced excitation coupled to said frame voicing decision means;
• and
• a random noise generator coupled to said interpolating means, said random noise generator for providing noise signals modulated to a level determined by said interpolating means;
• and
• output means coupled to said random noise generator for synthesizing unvoiced speech from said modulated noise signals.