Encoding method, decoding method, encoder apparatus, decoder apparatus, and recording medium for processing pitch periods corresponding to time series signals

1. An encoding method comprising:

(A) a step of obtaining pitch periods corresponding to time series signals included in a predetermined time interval; and

(B) a step of outputting a code corresponding to the pitch periods;

wherein the step (B) comprises a step of outputting the code obtained by encoding the pitch periods expressed at a first quantization resolution when an index that indicates a level of stationarity of the time series signals does not satisfy the condition that indicates high stationarity, and

of outputting the code obtained by encoding the pitch periods expressed at a second quantization resolution when the index satisfies the condition that indicates high stationarity; and

the second quantization resolution is higher than the first quantization resolution.

2. The encoding method according to claim 1 ,

wherein the step (A) further comprises a step of obtaining a quantized pitch gain corresponding to the time series signals;

the index includes the quantized pitch gain or a value corresponding thereto; and

the condition that indicates high stationarity includes a condition in which the quantized pitch gain or the value corresponding thereto is larger than a specified value.

3. The encoding method according to claim 1 ,

wherein the step (A) further comprises a step of obtaining a vector-quantized gain code corresponding to a combination of a quantized pitch gain corresponding to the time series signals or a value corresponding to the quantized pitch gain, and a quantized fixed-codebook gain corresponding to the time series signals or a value corresponding to the quantized fixed-codebook gain;

the index includes the vector-quantized gain code; and

the condition that indicates high stationarity includes a condition in which the vector-quantized gain code corresponds to a combination of a quantized pitch gain that is larger than a specified value or a value that corresponds to the quantized pitch gain and that is larger than the specified value, and the quantized fixed-codebook gain or the value corresponding thereto.

4. The encoding method according to claim 1 ,

wherein the step (A) further comprises a step of obtaining a quantized pitch gain corresponding to the time series signals and a quantized fixed-codebook gain corresponding to the time series signals;

the index includes the quantized pitch gain or a value corresponding thereto, and the quantized fixed-codebook gain or a value corresponding thereto; and

the condition that indicates high stationarity includes a condition in which the ratio of the quantized pitch gain or the value corresponding thereto to the quantized fixed-codebook gain or the value corresponding thereto is larger than a specified value.

5. The encoding method according to claim 1 ,

wherein the step (A) further comprises a step of obtaining a vector-quantized gain code corresponding to a combination of a quantized pitch gain corresponding to the time series signals or a value corresponding to the quantized pitch gain, and a quantized fixed-codebook gain corresponding to the time series signals or a value corresponding to the quantized fixed-codebook gain;

the index includes the vector-quantized gain code; and

the condition that indicates high stationarity includes a condition in which the vector-quantized gain code corresponds to a combination of a quantized pitch gain or a value corresponding thereto, and a quantized fixed-codebook gain or a value corresponding thereto where the ratio of the quantized pitch gain or the value corresponding thereto to the quantized fixed-codebook gain or the value corresponding thereto is larger than a specified value.

6. The encoding method according to claim 1 ,

wherein the step (A) further comprises a step of obtaining a quantized pitch gain corresponding to the time series signals and a quantized fixed-codebook gain corresponding to the time series signals;

the index includes the quantized pitch gain or a value corresponding thereto and the quantized fixed-codebook gain or a value corresponding thereto; and

the condition that indicates low stationarity includes a condition in which the quantized pitch gain or the value corresponding thereto is smaller than a first specified value and the quantized fixed-codebook gain or the value corresponding thereto is smaller than a second specified value.

7. The encoding method according to claim 1 ,

wherein the step (A) further comprises a step of obtaining a vector-quantized gain code corresponding to a combination of a quantized pitch gain corresponding to the time series signals or a value corresponding to the quantized pitch gain, and a quantized fixed-codebook gain corresponding to the time series signals or a value corresponding to the quantized fixed-codebook gain;

the index includes the vector-quantized gain code; and

the condition that indicates low stationarity includes a condition in which the quantized pitch gain corresponding to the vector-quantized gain code or the value corresponding to the quantized pitch gain is smaller than a first specified value and the quantized fixed-codebook gain corresponding to the vector-quantized gain code or the value corresponding to the quantized fixed-codebook gain is smaller than a second specified value.

8. The encoding method according to claim 1 ,

wherein the step (A) further comprises a step of obtaining a vector-quantized gain code corresponding to a combination of a quantized pitch gain corresponding to the time series signals or a value corresponding to the quantized pitch gain, and a quantized fixed-codebook gain corresponding to the time series signals or a value corresponding to the quantized fixed-codebook gain;

the index includes the vector-quantized gain code; and

the encoding mode is switched according to the vector-quantized gain code while referencing a table in which each vector-quantized gain code is associated with a resolution used to express a pitch period and/or a pitch period encoding mode.

9. The encoding method according to claim 1 ,

wherein the index includes an index that indicates the ratio of the magnitude of the time series signals to the magnitude of prediction residuals obtained by applying linear prediction analysis to the time series signals; and

the condition that indicates high stationarity includes a condition in which the index that indicates the ratio of the magnitude of the time series signals to the magnitude of the prediction residuals obtained by applying linear prediction analysis to the time series signals is larger than a specified value.

10. The encoding method according to claim 1 ,

wherein the index includes the magnitude of the difference between a value corresponding to a pitch period in a time interval included in the predetermined time interval and a value corresponding to a pitch period in a past time interval before the time interval included in the predetermined time interval; and

the condition that indicates high stationarity includes a condition in which the magnitude of the difference between the value corresponding to the pitch period in the time interval included in the predetermined time interval and the value corresponding to the pitch period in the past time interval before the time interval included in the predetermined time interval is smaller than a specified value.

11. A decoding method comprising:

receiving of a code corresponding to a predetermined time interval; and

decoding a pitch code included in the code to obtain the pitch periods corresponding to the predetermined time interval;

wherein the pitch code corresponding to the pitch periods is decoded with a decoding mode that obtains each of the pitch periods expressed at a first quantization resolution, when an index that indicates a level of stationarity, the index being included in or obtained from the code corresponding to the predetermined time interval, does not satisfy the condition that indicates high stationarity;

the pitch code corresponding to the pitch periods is decoded with a decoding mode that obtains each of the pitch periods expressed at a second quantization resolution, when the index satisfies the condition that indicates high stationarity; and

the second quantization resolution is higher than the first quantization resolution.

12. The decoding method according to claim 11 ,

wherein the index includes a quantized pitch gain or a value corresponding thereto; and

the condition that indicates high stationarity includes a condition in which the quantized pitch gain or the value corresponding thereto is larger than a specified value.

13. The decoding method according to claim 11 ,

wherein the index includes a vector-quantized gain code corresponding to a combination of a quantized pitch gain or a value corresponding thereto, and a quantized fixed-codebook gain or a value corresponding thereto; and

the condition that indicates high stationarity includes a condition in which the vector-quantized gain code corresponds to a combination of a quantized pitch gain that is larger than a specified value or a value that corresponds to the quantized pitch gain and that is larger than the specified value, and the quantized fixed-codebook gain or the value corresponding thereto.

14. The decoding method according to claim 11 ,

wherein the index includes a quantized pitch gain or a value corresponding thereto, and a quantized fixed-codebook gain or a value corresponding thereto; and

the condition that indicates high stationarity includes a condition in which the ratio of the quantized pitch gain or the value corresponding thereto to the quantized fixed-codebook gain or the value corresponding thereto is larger than a specified value.

15. The decoding method according to claim 11 ,

wherein the index includes a vector-quantized gain code corresponding to a combination of a quantized pitch gain or a value corresponding thereto, and a quantized fixed-codebook gain or a value corresponding thereto; and

the condition that indicates high stationarity includes a condition in which the vector-quantized gain code corresponds to a combination of a quantized pitch gain or a value corresponding thereto, and a quantized fixed-codebook gain or a value corresponding thereto where the ratio of the quantized pitch gain or the value corresponding thereto to the quantized fixed-codebook gain or the value corresponding thereto is larger than a specified value.

16. The decoding method according to claim 11 ,

wherein the index includes a quantized pitch gain or a value corresponding thereto, and a quantized fixed-codebook gain or a value corresponding thereto; and

the condition that indicates low stationarity includes a condition in which the quantized pitch gain or the value corresponding thereto is smaller than a first specified value and the quantized fixed-codebook gain or the value corresponding thereto is smaller than a second specified value.

17. The decoding method according to claim 11 ,

wherein the index includes a vector-quantized gain code corresponding to a combination of a quantized pitch gain or a value corresponding thereto, and a quantized fixed-codebook gain or a value corresponding thereto; and

the condition that indicates low stationarity includes a condition in which the quantized pitch gain corresponding to the vector-quantized gain code or the value corresponding to the quantized pitch gain is smaller than a first specified value and the quantized fixed-codebook gain corresponding to the vector-quantized gain code or the value corresponding to the quantized fixed-codebook gain is smaller than a second specified value.

18. The decoding method according to claim 11 ,

wherein the index includes a vector-quantized gain code corresponding to a combination of a quantized pitch gain or a value corresponding thereto, and a quantized fixed-codebook gain or a value corresponding thereto; and

the decoding mode is switched according to the vector-quantized gain code while referencing a table in which each vector-quantized gain code is associated with a resolution used to express a pitch period and/or a pitch period decoding mode.

19. The decoding method according to claim 11 ,

wherein the index includes an estimated value of prediction gain calculated by using linear prediction coefficients obtained from the code or coefficients corresponding to the linear prediction coefficients; and

the condition that indicates high stationarity includes a condition in which the estimated value of prediction gain is larger than a specified value.

20. The decoding method according to claim 11 ,

wherein the index includes the magnitude of the difference between a value corresponding to a pitch period in a time interval included in the predetermined time interval and a value corresponding to a pitch period in a past time interval before the time interval included in the predetermined time interval; and

the condition that indicates high stationarity includes a condition in which the magnitude of the difference between the value corresponding to the pitch period in the time interval included in the predetermined time interval and the value corresponding to the pitch period in the past time interval before the time interval included in the predetermined time interval is smaller than a specified value.

21. An encoder which obtains pitch periods corresponding to time series signals included in a predetermined time interval; and outputs a code corresponding to the pitch periods;

wherein the code obtained by encoding the pitch periods expressed at a first quantization resolution is output when an index that indicates a level of stationarity of the time series signals does not satisfy the condition that indicates high stationarity, and

the code obtained by encoding the pitch periods expressed at a second quantization resolution is output when the index satisfies the condition that indicates high stationarity; and

the second quantization resolution is higher than the first quantization resolution.

22. A decoder which receives of a code corresponding to a predetermined time interval; and decodes a pitch code included in the code to obtain the pitch periods corresponding to the predetermined time interval;

wherein the pitch code corresponding to the pitch periods is decoded with a decoding mode that obtains each of the pitch periods expressed at a first quantization resolution, when an index that indicates a level of stationarity, the index being included in or obtained from the code corresponding to the predetermined time interval, does not satisfy the condition that indicates high stationarity;

the pitch code corresponding to the pitch periods is decoded with a decoding mode that obtains each of the pitch periods expressed at a second quantization resolution, when the index satisfies the condition that indicates high stationarity; and

the second quantization resolution is higher than the first quantization resolution.

23. A non-transitory computer readable recording medium having stored therein a program causing a computer to execute processing of the encoding method comprising:

(A) a step of obtaining pitch periods corresponding to time series signals included in a predetermined time interval; and

(B) a step of outputting a code corresponding to the pitch periods;

wherein the step (B) comprises a step of outputting the code obtained by encoding the pitch periods expressed at a first quantization resolution when an index that indicates a level of stationarity of the time series signals does not satisfy the condition that indicates high stationarity, and

of outputting the code obtained by encoding the pitch periods expressed at a second quantization resolution when the index satisfies the condition that indicates high stationarity; and the second quantization resolution is higher than the first quantization resolution.

24. A non-transitory computer readable recording medium having stored therein a program causing a computer to execute processing of the decoding method comprising:

receiving of a code corresponding to a predetermined time interval; and

decoding a pitch code included in the code to obtain the pitch periods corresponding to the predetermined time interval;

wherein the pitch code corresponding to the pitch periods is decoded with a decoding mode that obtains each of the pitch periods expressed at a first quantization resolution, when an index that indicates a level of stationarity, the index being included in or obtained from the code corresponding to the predetermined time interval, does not satisfy the condition that indicates high stationarity;

the pitch code corresponding to the pitch periods is decoded with a decoding mode that obtains each of the pitch periods expressed at a second quantization resolution, when the index satisfies the condition that indicates high stationarity; and the second quantization resolution is higher than the first quantization resolution.