Pitch extraction methods and systems for speech coding using interpolation techniques

1. A method of searching for an interpolated peak of a Normalized Correlation Square (NCS) signal derived from an audio signal, the NCS signal being represented as a first ratio of a correlation square signal c 2 (k) to an energy signal E(k), where k represents time lags spanning a range of integer k-values, the interpolated peak being near a known local peak c 2 (k p )/E(k p ) of the NCS signal, comprising:

(a) producing quadratically interpolated correlation (QIC) signal values (ci) at interpolated time lags between time lag k p and an adjacent time lag;

(b) squaring each of the QIC signal values to produce square QIC signal values (ci 2 );

(c) producing an individual interpolated energy signal value (ei) corresponding to each of the square QIC signal values, wherein second ratios of the square QIC signal values (ci 2 ) to their corresponding interpolated energy values (ei) represent interpolated NCS signal values; and

(d) selecting, as the interpolated peak, a largest interpolated NCS signal value among the interpolated NCS signal values without evaluating the second ratios.

2. The method of claim 1 , wherein step (d) comprises:

comparing the interpolated NCS signal values to each other using cross-multiply compare operations, so as to avoid evaluating the second ratios representing the NCS values; and

selecting the largest interpolated NCS signal value among the interpolated NCS signal values based on said comparing step.

3. The method of claim 1 , wherein the NCS signal includes multiple known local peaks c 2 (k p (j))/E(k p (j)), including the known local peak searched in steps (a), (b), (c) and (d), where j=1, 2, . . . N p , the method further comprising:

(e) repeating steps (a), (b), (c) and (d) for each of the remaining known local peaks among the N p local peaks, thereby selecting an interpolated peak near each of the N p local peaks.

4. The method of claim 3 , further comprising:

determining a largest interpolated peak among the N p interpolated peaks; and

an interpolated time lag corresponding to the largest interpolated peak.

5. The method of claim 1 , further comprising:

prior to step (a), comparing NCS signal values c 2 (k p +1)/E(k p +1) and c 2 (k p −1)/E(k p −1), that are adjacent neighbors of the local peak c 2 (k p )/E(k p ); and

wherein step (a) comprises

interpolating between time lags k p and k p +1 when said comparing step indicates the interpolated peak resides between time lags k p and k p +1, and otherwise

interpolating between time lags k p and k p −1.

6. The method of claim 1 , wherein the NCS signal is a decimated signal such that k represents decimated time lags, the time lags k p is a decimated time lag, and the adjacent time lag is a decimated time lag.

7. The method of claim 1 , wherein the interpolated time lag selected in step (c) is representative of the audio signal pitch period.

8. A method of searching for an interpolated time lag representative of an audio signal pitch period, the method using a correlation-based signal derived from an audio signal swd(n), the correlation-based signal having N p local peaks at corresponding known time lags k p (j), where j=1, 2, . . . N p , each of the N p local peaks being near a corresponding one of interpolated correlation-based peaks, each of the interpolated correlation-based peaks corresponding to an interpolated time lag, the method comprising:

(a) determining if any of the time lags k p (j) are within a predetermined time lag range, the predetermined time lag range including a time lag representative of a past pitch period of a past portion of the audio signal;

(b) comparing the interpolated peaks corresponding to the time lags determined to be within the predetermined time lag range; and

(c) selecting the interpolated time lag corresponding to a largest interpolated peak among the interpolated peaks compared in step (b).

9. The method of claim 8 , wherein the interpolated correlation-based peaks are Normalized Correlation Square (NCS) peaks represented as respective ratios of interpolated correlation square values to corresponding interpolated energy values, and step (b) includes performing a cross-multiply comparison operation between at least two of the interpolated peaks so as to avoid evaluating the ratios representing the at least two of the interpolated peaks.

10. A computer readable medium carrying one or more sequences of one or more instructions for execution by one or more processors to perform a method of searching for an interpolated peak of a Normalized Correlation Square (NCS) signal derived from an audio signal, the NCS signal being represented as a first ratio of a correlation square signal c 2 (k) to an energy signal E(k), where k represents time lags spanning a range of integer k-values, the interpolated peak being near a known local peak c 2 (k p )/E(k p ) of the NCS signal, the instructions when executed by the one or more processors, causing the one or more processors to perform the steps of:

(a) producing quadratically interpolated correlation (QIC) signal values (ci) at interpolated time lags between time lag k p and an adjacent time lag;

(b) squaring each of the QIC signal values to produce square QIC signal values (ci 2 );

(c) producing an individual interpolated energy signal value (ei) corresponding to each of the square QIC signal values, wherein second ratios of the square QIC signal values (ci 2 ) to their corresponding interpolated energy values (ei) represent interpolated NCS signal values; and

(d) selecting, as the interpolated peak, a largest interpolated NCS signal value among the interpolated NCS signal values without evaluating the second ratios.

11. The computer readable medium of claim 10 , wherein step (d) comprises:

comparing the interpolated NCS signal values to each other using cross-multiply compare operations, so as to avoid evaluating the second ratios representing the NCS values; and

selecting the largest interpolated NCS signal value among the interpolated NCS signal values based on said comparing step.

12. The computer readable medium of claim 10 , wherein the NCS signal includes multiple known local peaks c 2 (k p (j))/E(k p (j)), including the known local peak searched in steps (a), (b), (c) and (d), where j=1, 2, . . . N p , and wherein the one or more instructions carried by the computer readable medium cause the one or more processors to perform the further step of:

(e) repeating steps (a), (b), (c) and (d) for each of the remaining known local peaks among the N p local peaks, thereby selecting an interpolated peak near each of the N p local peaks.

13. The computer readable medium of claim 12 , wherein the one or more instructions carried by the computer readable medium cause the one or more processors to perform the further steps of:

determining a largest interpolated peak among the N p interpolated peaks; and

an interpolated time lag corresponding to the largest interpolated peak.

14. The computer readable medium of claim 10 , wherein the one or more instructions carried by the computer readable medium cause the one or more processors to perform, prior to step (a), the step of:

comparing NCS signal values c 2 (k p +1)/E(k p +1) and c 2 (k p −1)/E(k p −1), that are adjacent neighbors of the local peak c 2 (k p )/E(k p ),

wherein step (a) comprises

interpolating between time lags k p and k p +1 when said comparing step indicates the interpolated peak resides between time lags k p and k p +1, and otherwise

interpolating between time lags k p and k p −1.

15. The computer readable medium of claim 10 , wherein the NCS signal is a decimated signal such that k represents decimated time lags, the time lags k p is a decimated time lag, and the adjacent time lag is a decimated time lag.

16. A computer readable medium carrying one or more sequences of one or more instructions for execution by one or more processors to perform a method of searching for an interpolated time lag representative of an audio signal pitch period, the method using a correlation-based signal derived from an audio signal swd(n), the correlation-based signal having N p local peaks at corresponding known time lags k p (j), where j=1, 2, . . . N p , each of the N p local peaks being near a corresponding one of interpolated correlation-based peaks, each of the interpolated correlation-based peaks corresponding to an interpolated time lag, the instructions when executed by the one or more processors, causing the one or more processors to perform the steps of:

(a) determining if any of the time lags k p (j) are within a predetermined time lag range, the predetermined time lag range including a time lag representative of a past pitch period of a past portion of the audio signal;

(b) comparing the interpolated peaks corresponding to the time lags determined to be within the predetermined time lag range; and

(c) selecting the interpolated time lag corresponding to a largest interpolated peak among the interpolated peaks compared in step (b).

17. The computer readable medium of claim 16 , wherein the interpolated correlation-based peaks are Normalized Correlation Square (NCS) peaks represented as respective ratios of interpolated correlation square values to corresponding interpolated energy values, and step (b) includes performing a cross-multiply comparison operation between at least two of the interpolated peaks so as to avoid evaluating the ratios representing the at least two of the interpolated peaks.

18. An apparatus for searching for an interpolated peak of a Normalized Correlation Square (NCS) signal derived from an audio signal, the NCS signal being represented as a first ratio of a correlation square signal c 2 (k) to an energy signal E(k), where k represents time lags spanning a range of integer k-values, the interpolated peak being near a known local peak c 2 (k p )/E(k p ) of the NCS signal, comprising:

a first module for

producing quadratically interpolated correlation (QIC) signal values (ci) at interpolated time lags between time lag k p and an adjacent time lag, and

squaring each of the QIC signal values to produce square QIC signal values (ci 2 );

a second module for producing an individual interpolated energy signal value (ei) corresponding to each of the square QIC signal values, wherein second ratios of the square QIC signal values (ci 2 ) to their corresponding interpolated energy values (ei) represent interpolated NCS signal values; and

a third module for selecting, as the interpolated peak, a largest interpolated NCS signal value among the interpolated NCS signal values without evaluating the second ratios.

19. The apparatus of claim 18 , wherein the third module is configured to:

compare the interpolated NCS signal values to each other using cross-multiply compare operations, so as to avoid evaluating the second ratios representing the NCS values; and

select the largest interpolated NCS signal value among the interpolated NCS signal values based on results from the compare operation.

20. An apparatus for searching for an interpolated time lag representative of an audio signal pitch period, the method using a correlation-based signal derived from an audio signal swd(n), the correlation-based signal having N p local peaks at corresponding known time lags k p (j), where j=1, 2, . . . N p , each of the N p local peaks being near a corresponding one of interpolated correlation-based peaks, each of the interpolated correlation-based peaks corresponding to an interpolated time lag, comprising:

a first module for determining if any of the time lags k p (j) are within a predetermined time lag range, the predetermined time lag range including a time lag representative of a past pitch period of a past portion of the audio signal;

a second module for comparing the interpolated peaks corresponding to the time lags determined to be within the predetermined time lag range; and

a third module for selecting the interpolated time lag corresponding to a largest interpolated peak among the interpolated peaks compared by the second module.