Abstract: A speech coding system and associated method relies on a speech encoder (15) and a speech decoder (20). The speech decoder (20) includes an LPC synthesis filter (90), a Gaussian noise generator (80) for generating unvoiced excitation, an epoch-adaptive harmonic generator (70) for generating voiced excitation for pitch harmonics below voicing cutoff frequency, and an excitation summer (72) for summing the voiced and unvoiced excitation generated by the Gaussian noise generator (80) and the harmonic generator (70). The output of the excitation summer (72) is provided to the LPC synthesis filter (90) to generate synthesized speech. The system and method provides natural sounding synthesized speech at a low bit rate.

Abstract: A speech coding system (10) and associated method rely on a speech encoder (15) and a speech decoder (20). The speech decoder (20) includes a Linear Predictive Coding (LPC) filter (90) having an input and an output. The LPC filter (90) provides synthesized speech at its output in response to voiced and unvoiced excitation provided at its input. A harmonic generator (70) for providing voiced excitation to the input of the LPC filter includes a pitch-related randomized adaptive phase off-setter. The adaptive phase off-setter off-sets the harmonic phases of the voiced excitation in accordance with the fundamental pitch frequency of respective frames of speech. The system and method thereby reduce perceived buzziness of synthesized speech provided at the output of the LPC filter.

Abstract: A speech coding system and associated method rely on a speech encoder and a speech decoder. The speech decoder includes a Linear Predictive Coding (LPC) filter having an input and an output. The LPC filter provides synthesized speech at the output in response to voiced and unvoiced excitation provided at the input. A harmonic generator for providing voiced excitation to the input of the LPC filter includes a spectral formant enhancer for attenuating the amplitude of harmonics generate by the harmonic generator in spectral valleys between format peaks of respective frames of voiced speech. The system and method reduce perceived buzziness while increasing perceived spectral depth of synthesized speech at the output of the LPC filter.

Abstract: A speech coding system and associated method relies on a speech encoder and a speech decoder. The encoder includes a spectral quantizer for computing line spectral frequencies (LSFs) for respective frames of speech and for quantizing the LSFs to obtain a minimum bit representation of a spectral envelope of each respective frame of speech. For even numbered frames of speech the LSFs are quantized using a vector quantization technique. For odd numbered frames of speech samples the LSFs are quantized using a dynamic bit allocation (DBA) method. The dynamic bit allocation method determines an interpolation factor for interpolating between the LSFs of the previous and next frames. According to the dynamic bit allocation method the most perceptually important LSFs are represented by relatively more bits, while the least perceptually important LSFs are represented by relatively fewer bits.

Abstract: A speech coding system and associated method relies on a speech coder and a speech decoder. The speech coder provides speech spectrum information, including Line Spectral Frequency (LSF) information, for respective frames of speech. The LSF information is provided to the speech decoder. The speech decoder includes an LSF smoothing filter comprising an adaptive finite impulse response filter for averaging consecutive LSFs which are similar. Consecutive LSFs which are dissimilar are not averaged. The system and method causes a reduction in the shaky quality of synthesized speech without causing a corresponding decrease in the clarity of the synthesized speech.

Abstract: A speech coding system and associated method relies on a speech encoder (15) and a speech decoder (20). The speech encoder (15) includes a voicing cut off frequency analyzer (60). Voicing cut off frequency analyzer (60) includes voicing cut off frequency estimator (61) and voicing cut off frequency quantizer (62). Voicing cut off frequency estimator (61) estimates a voicing cut off frequency value for respective samples of an input speech waveform (1). To accomplish this, voicing cut off frequency estimator (61) utilizes a bandpass filter to estimate a frequency above which a sample of speech is voiced and below which the sample of speech is unvoiced. Voicing cut off frequency quantizer (62) quantizes the estimated voicing cut off frequency value and provides, for respective samples, a voicing cut off frequency index signal (6) which may be stored or transmitted. Voicing cut off frequency index signal (6) may comprise as few as 1 bit, and in a preferred embodiment, as few as 3 bits.

Abstract: A speech coding system (10) and associated method relies on a speech encoder (15) and a speech decoder (20). The speech decoder (20) includes a harmonic generator (70) which modulates the phase of each generated harmonic with a low frequency, low bandwidth signal to remove the buzzy quality of the speech and to produce natural sounding speech. The amplitude of the phase modulating signal is adjusted in accordance with the harmonic magnitude. For harmonics residing in a spectral valley the amplitude of the modulating signal is relatively large and for harmonics residing near spectral peaks, the amplitude of the modulation signal is relatively small.