Method and system for speech reconstruction from speech recognition features, pitch and voicing with resampled basis functions providing reconstruction of the spectral envelope

A method for accepting a series of indices of speech frames in a speech database, a series of respective pitch values and voicing decisions and a series of respective energy values, and generating speech therefrom, the method comprising:

• (a) creating a database containing coded or uncoded feature vectors, the feature vectors being obtained as follows:
• i) deriving at successive instances of time an estimate of the spectral envelope of the digitized original speech signal,
• ii) multiplying each estimate of the spectral envelope by a predetermined set of frequency domain window functions, wherein each window is non-zero over a narrow range of frequencies, and computing the integrals thereof, and
• iii) assigning said integrals or a set of pre-determined functions thereof to respective components of a corresponding feature vector in a series of feature vectors;
• (b) producing a series of features vectors from frames selected from the database according to the series of indices and the series of respective energy values, and
• (c) reconstructing speech from the series of feature vectors and the series of respective pitch values and voicing decisions by:
• i) converting each feature vector into a binned spectrum,
• ii) generating harmonic frequencies and weights according to the corresponding pitch and voicing decision,
• iii) generating for each harmonic frequency a respective phase, depending on the corresponding pitch value and voicing decision and possibly on the binned spectrum,
• iv) sampling a predetermined set of basis functions each being a function in a set of frequency domain functions with bounded supports at all harmonic frequencies which are within its support, and multiplying by the respective harmonic weight, so as to produce for each sampled basis function a respective line spectrum having multiple components,
• v) combining each component of each respective line spectrum with the respective phase thereof so as to produce a complex line spectrum for each basis function,
• vi) generating gain coefficients of the basis functions,
• vii) multiplying each complex line spectrum of each basis function by the respective basis function gain coefficient, and summing up all resulting complex line spectra to generate a single complex line spectrum having a respective component for each of the harmonic frequencies, and
• viii) generating a time signal from complex line spectra computed at successive instances of time.

A speech reconstruction device for converting a series of feature vectors and a series of respective pitch values and voicing decisions of an original input speech signal into a reconstructed speech signal, the feature vectors being obtained as follows:

• (i) deriving at successive instances of time an estimate of a spectral envelope SE(i), i being a frequency index, of the digitized original speech signal,
• (ii) multiplying each estimate of the spectral envelope by a predetermined set of frequency domain window functions, BW(i,k), i being a frequency index and k being the window function index, wherein each window is non-zero over a narrow range of frequencies, and computing the integrals thereof, according to the expression:
• BI ⁢ ( k ) = ∑ i ⁢ SE ⁢ ( i ) · BW ⁢ ( i , k )
• where BI(k) is the kth component or “bin” of a “binned spectrum”, and
• (iii) assigning said integrals or a set of pre-determined functions thereof to respective components of a corresponding feature vector in a series of feature vectors;
• said device comprising:
• an input stage for inputting said series of feature vectors and a respective series of pitch values and voicing decisions, and converting the feature vectors into binned spectra,
• a frequency and weight generator coupled to the input stage for generating harmonic frequencies and weights,
• a phase generator coupled to the input stage for generating phases for each harmonic frequency,
• a basis function sampler for sampling a predetermined set of basis functions each being a function in a set of frequency domain functions with bounded supports at all harmonic frequencies which are within its support, and multiplying by the respective harmonic weights, so as to produce for each sampled basis function a respective line spectrum having multiple components,
• a phase combiner coupled to the basis function sampler and the phase generator for combining each component of the respective line spectrum with the respective phase thereof so as to produce a complex line spectrum for each basis function,
• a coefficient generator for generating gain coefficients of the basis functions,
• a linear combination unit for multiplying each complex line spectrum of each basis function by the respective basis function gain coefficient and summing up all the resulting complex line spectra to generate a complex line spectrum with respective components for all harmonic frequencies, and
• a line spectrum to signal converter coupled to the linear combination unit for generating a time signal from a series of complex line spectra.

A decoder for decoding speech, said decoder being responsive to a received bit stream representing an encoded series of feature vectors, pitch values and voicing decisions, the decoder including:

• a decompression module for decompressing the series of respective feature vectors, pitch values and voicing decisions,
• a conversion unit for converting the feature vectors into binned spectra,
• a frequency and weight generator responsive to the pitch values and voicing decisions for generating harmonic frequencies and weights,
• a phase generator responsive to the pitch values, voicing decisions and possibly to the binned spectra for generating phases for each harmonic frequency,
• a basis function sampler for sampling a predetermined set of basis functions each being a function in a set of frequency domain functions with bounded supports at all harmonic frequencies which are within its support, and multiplying by the respective harmonic weights, so as to produce for each sampled basis function a respective line spectrum having multiple components,
• a phase combining device coupled to the basis function sampler and the phase generator for combining each component of the respective line spectrum with the respective phase thereof so as to produce a complex line spectrum for each basis function,
• a coefficient generator for generating gain coefficients of the basis functions,
• a linear combination unit for multiplying each complex line spectrum of each basis function by the respective basis function gain coefficient and summing up all the resulting complex line spectra to generate a complex line spectrum with respective components for all harmonic frequencies, and
• a line spectrum to signal converter coupled to the linear combination unit for generating a time signal from a series of complex line spectra.

A speech coding/decoding system comprising:

• an encoder for coding speech, said encoder being responsive to an input speech signal and including:
• a feature extraction module for computing feature vectors from the input speech signal at successive instances of time, the feature extraction module including:
• a spectrum estimator for deriving at each said instances of time an estimate of the spectral envelope of the input speech signal.
• an integrator coupled to the spectrum estimator for multiplying the spectral envelope by a predetermined set of frequency domain window functions, wherein each window occupies a narrow range of frequencies, and computing the integral thereof, and
• an assignment unit coupled to the integrator for deriving a set of predetermined functions of said integrals and assigning to respective components of a corresponding feature vector in said series of feature vectors;
• a pitch detector for computing respective pitch values and voicing decisions at said successive instances of time, and
• a compression module for compressing the series of respective feature vectors, pitch values and voicing decisions into a bit-stream;
• a decoder for decoding speech, said decoder being responsive to a received bit stream representing an encoded series of respective feature vectors, pitch values and voicing decisions, the decoder including:
• a decompression module for decompressing the series of respective feature vectors, pitch values and voicing decisions,
• a conversion unit for converting the feature vectors into binned spectra,
• a frequency and weight generator responsive to the pitch values and voicing decisions for generating harmonic frequencies and weights,
• a phase generator responsive to the pitch values, voicing decisions and possibly to the binned spectra for generating phases for each harmonic frequency,
• a basis function sampler for sampling a predetermined set of basis functions each being a function in a set of frequency domain functions with bounded supports at all harmonic frequencies which are within its support, and multiplying by the respective harmonic weights, so as to produce for each sampled basis function a respective line spectrum having multiple components,
• a phase combining device coupled to the basis function sampler and the phase generator for combining each component of the respective line spectrum with the respective phase thereof so as to produce a complex line spectrum for each basis function,
• a coefficient generator for generating gain coefficients of the basis functions,
• a linear combination unit for multiplying each complex line spectrum of each basis function by the respective basis function gain coefficient and summing up all the resulting complex line spectra to generate a complex line spectrum with respective components for all harmonic frequencies, and
• a line spectrum to signal converter coupled to the linear combination unit for generating a time signal from a series of complex line spectra.

A dual purpose speech recognition/playback system, for continuous speech recognition and reproduction of an encoded speech signal, said system comprising a decoder and a recognition unit:

• the decoder for decoding and playback of encoded speech being responsive to a received bit stream representing an encoded series of respective feature vectors, pitch values and voicing decisions, the decoder including:
• a decompression module for decompressing the series of respective feature vectors, pitch values and voicing decisions,
• a conversion unit for converting the feature vectors into binned spectra,
• a frequency and weight generator responsive to the pitch values and voicing decisions for generating harmonic frequencies and weights,
• a phase generator responsive to the pitch values, voicing decisions and possibly to the binned spectra for generating phases for each harmonic frequency,
• a basis function sampler for sampling a predetermined set of basis functions each being a function in a set of frequency domain functions with bounded supports at all harmonic frequencies which are within its support, and multiplying by the respective harmonic weights, so as to produce for each sampled basis function a respective line spectrum having multiple components,
• a phase combining device coupled to the basis function sampler and the phase generator for combining each component of the respective line spectrum with the respective phase thereof so as to produce a complex line spectrum for each basis function,
• a coefficient generator for generating gain coefficients of the basis functions,
• a linear combination unit for multiplying each complex line spectrum of each basis function by the respective basis function gain coefficient and summing up all the resulting complex line spectra to generate a complex line spectrum with respective components for all harmonic frequencies, and
• a line spectrum to signal converter coupled to the linear combination unit for generating a time signal from a series of complex line spectra;
• and
• the recognition unit being responsive to the decompressed feature vectors for continuous speech recognition.

A speech recognition system comprising:

• an encoder for coding speech so as to derive low bit rate bit stream, said encoder being responsive to an input speech signal and including:
• a feature extraction module for computing feature vectors from the input speech signal at successive instances of time, the feature extraction module including:
• a spectrum estimator for deriving at each said instances of time an estimate of the spectral envelope of the input speech signal,
• an integrator coupled to the spectrum estimator for multiplying the spectral envelope by a predetermined set of frequency domain window function, wherein each window occupies a narrow range of frequencies, and computing the integral thereof, and
• an assignment unit coupled to the integrator for deriving a set of predetermined functions of said integrals and assigning to respective components of a corresponding feature vector in said series of feature vectors;
• a pitch detector for computing respective pitch values and voicing decisions at said successive instances of time,
• a compression module for compressing the series of respective feature vectors, pitch values and voicing decisions into a bit-stream,
• a transmitter coupled to the encoder for transmitting the low bit rate bit stream,
• a recognition unit responsive to the low bit rate bit stream for decompressing the feature vectors and performing continuous speech recognition on the feature vectors, and
• a transmitter within the speech recognition unit for retransmitting the results of the recognition and the low bit rate bit stream to a remote device for displaying the results of the recognition;
• said remote device including a speech decoder, comprising:
• a decompression module for decompressing the series of respective feature vectors, pitch values and voicing decisions,
• a conversion unit for converting the feature vectors into binned spectra,
• a frequency and weight generator responsive to the pitch values and voicing decisions for generating harmonic frequencies and weights,
• a phase generator responsive to the pitch values, voicing decisions and possibly to the binned spectra for generating phases for each harmonic frequency,
• a basis function sampler for sampling a predetermined set of basis functions each being a function in a set of frequency domain functions with bounded supports at all harmonic frequencies which are within its support, and multiplying by the respective harmonic weights, so as to produce for each sampled basis function a respective line spectrum having multiple components,
• a phase combiner coupled to the basis function sampler and the phase generator for combining each component of the respective line spectrum with the respective phase thereof so as to produce a complex line spectrum for each basis function,
• a coefficient generator for generating gain coefficients of the basis functions,
• a linear combination unit for multiplying each complex line spectrum of each basis function by the respective basis function gain coefficient and summing up all the resulting complex line spectra to generate a complex line spectrum with respective components for all harmonic frequencies, and
• a line spectrum to signal converter coupled to the linear combination unit for generating a time signal from a series of complex line spectra.

A speech generator for accepting a series of indices of speech frames in a speech database, a series of respective pitch values and voicing decisions and a series of respective energy values and generating speech, the device comprising:

• a database containing coded or uncoded feature vectors, the feature vectors being obtained as follows:
• i) deriving at successive instances of time an estimate of the spectral envelope of the digitized original speech signal,
• ii) multiplying each estimate of the spectral envelope by a predetermined set of frequency domain window functions, wherein each window is non-zero over a narrow range of frequencies, and computing the integrals thereof, and
• iii) assigning said integrals or a set of predetermined functions thereof to respective components of a corresponding feature vector in a series of feature vectors;
• a features generator responsive to the series of indices and the series of respective energy values for producing a series of feature vectors using frames selected from the database, and
• a speech reconstruction unit for reconstructing speech from a series of features vectors and the series of respective pitch values and voicing decisions, said reconstruction unit comprising:
• a conversion unit for converting the feature vectors into binned spectra,
• a frequency and weight generator responsive to the pitch values and voicing decisions for generating harmonic frequencies and weights,
• a phase generator responsive to the pitch values, voicing decisions and possibly to the binned spectra for generating phases for each harmonic frequency,
• a basis function sampler for sampling a predetermined set of basis functions each being a function in a set of frequency domain functions with bounded supports at all harmonic frequencies which are within its support, and multiplying by the respective harmonic weights, so as to produce for each sampled basis function a respective line spectrum having multiple components,
• a phase combiner coupled to the basis function sampler and the phase generator for combining each component of the respective line spectrum with the respective phase thereof so as to produce a complex line spectrum for each basis function,
• a coefficient generator for generating gain coefficients of the basis functions,
• a linear combination unit for multiplying each complex line spectrum of each basis function by the respective basis function gain coefficient and summing up all the resulting complex line spectra to generate a complex line spectrum with respective components for all harmonic frequencies, and
• a line spectrum to signal converter coupled to the linear combination unit for generating a time signal from a series of complex line spectra.

A computer program product comprising a computer useable medium having computer readable program code embodied therein for converting a series of feature vectors and a series of respective pitch values and voicing decisions of an original input speech signal into a reconstructed speech signal, the feature vectors being obtained as follows:

• i) deriving at successive instances of time an estimate of the spectral envelope of the digitized original speech signal,
• ii) multiplying each estimate of the spectral envelope by a predetermined set of frequency domain window functions, wherein each window is non-zero over a narrow range of frequencies, and computing the integrals thereof, and
• iii) assigning said integrals or a set of predetermined functions thereof to respective components of a corresponding feature vector in a series of feature vectors;
• said computer program product comprising:
• computer readable program code for inputting said series of feature vectors and a respective series of pitch values and voicing decisions, and converting the feature vectors into binned spectra,
• computer readable program code for causing the computer to generate harmonic frequencies and weights according to the pitch value and voicing decision,
• computer readable program code for causing the computer to generate phases for each harmonic frequency depending on the pitch value, voicing decision and possibly on the binned spectrum,
• computer readable program code for causing the computer to sample a predetermined set of basis functions each being a function in a set of frequency domain functions with bounded supports at all harmonic frequencies which are within its support, and multiply by the respective harmonic weights, so as to produce for each sampled basis function a respective line spectrum having multiple components,
• computer readable program code for causing the computer to combine each component of the respective line spectrum with the respective phase thereof so as to produce a complex line spectrum for each basis function,
• computer readable program code for causing the computer to generate coefficients of the basis functions,
• computer readable program code for causing the computer to multiply each complex line spectrum of each basis function by the respective basis function coefficient and sum up all the resulting complex line spectra to generate a complex line spectrum with respective components for all harmonic frequencies, and
• computer readable program code for causing the computer to generate a time signal from a series of complex line spectra.

A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform method steps for converting a series of feature vectors and a series of respective pitch values and voicing decisions of an original input speech signal into a reconstructed speech signal, the feature vectors being obtained as follows:

• i) deriving at successive instances of time an estimate of the spectral envelope of the digitized original speech signal,
• ii) multiplying each estimate of the spectral envelope by a predetermined set of frequency domain window functions, wherein each window is non-zero over a narrow range of frequencies, and computing the integrals thereof, and
• iii) assigning said integrals or a set of pre-determined functions thereof to respective components of a corresponding feature vector in a series of feature vectors,
• said method steps comprising:
• (a) converting each feature vector into a binned spectrum,
• (b) generating harmonic frequencies and weights according to the corresponding pitch and voicing decision,
• (c) generating for each harmonic frequency a respective phase, depending on the corresponding pitch value and voicing decision and possibly on the binned spectrum,
• (d) sampling a predetermined set of basis functions each being a function in a set of frequency domain functions with bounded supports at all harmonic frequencies which are within its support, and multiplying by the respective harmonic weight, so as to produce for each sampled basis function a respective line spectrum having multiple components,
• (e) combining each component of each respective line spectrum with the respective phase thereof so as to produce a complex line spectrum for each basis function,
• (f) generating gain coefficients of the basis functions,
• (g) multiplying each complex line spectrum of each basis function by the respective basis function gain coefficient, and summing up all resulting complex line spectra to generate a single complex line spectrum having a respective component for each of the harmonic frequencies, and
• (h) generating a time signal from complex line spectra computed at successive instances of time.