Encoding method, decoding method, encoder, decoder, program and recording medium

A decoding method comprising:

• a long-term prediction information decoding step of receiving a time-domain pitch period code which is output from an encoder, and decoding the received time-domain pitch period code to obtain a time-domain pitch period L;
• a period converting step of obtaining, as a converted interval T1, a sample interval in an N-points frequency-domain sample string, the sample interval corresponding to the time-domain pitch period L, receiving a first frequency-domain pitch period code which is output from the encoder, decoding the received first frequency-domain pitch period code to obtain a multiple value indicating how many times a first frequency-domain pitch period T is greater than the converted interval T1, and obtaining, as the first frequency-domain pitch period T, the converted interval T1 multiplied by the multiple value;
• and
• a frequency-domain-pitch-period-based decoding step of receiving a code string which is output from the encoder, and decoding the code string by a decoding method in which a first sample group of all or some of one or a plurality of successive samples including a sample corresponding to the first frequency-domain pitch period T in the N-points frequency-domain sample string and one or a plurality of successive samples including a sample corresponding to an integer multiple of the first frequency-domain pitch period T in the N-points frequency-domain sample string is obtained by decoding processes according to a first criterion corresponding to magnitudes of amplitudes or estimated magnitudes of amplitudes of samples included in the first sample group and a second sample group of samples in the N-points frequency-domain sample string that are not included in the first sample group is obtained by decoding processes according to a second criterion corresponding to magnitudes of amplitudes or estimated magnitudes of amplitudes of samples included in the second sample group, to obtain and output the first sample group and the second sample group of the N-points frequency-domain sample string, wherein the first sample group is a part of the N-points frequency-domain sample string.

An encoder comprising:

• a period converter receiving a time-domain pitch period L corresponding to a time-domain pitch period code of an audio signal in a given time period, obtaining, as a converted interval T1, a sample interval in an N-points frequency-domain sample string, the sample interval corresponding to the time-domain pitch period L, and outputting the time-domain pitch period code to a decoder;
• a frequency-domain pitch period analyzer receiving the N-points frequency-domain sample string derived from the audio signal in the given time period, choosing a first frequency-domain pitch period T from among a plurality of candidates including integer multiples U×T1 of the converted interval T1, where U is an integer in a predetermined first range, the first frequency-domain pitch period T being a pitch period in the N-points frequency-domain sample string derived from the audio signal, obtaining a first frequency-domain pitch period code indicating how many times the first frequency-domain pitch period T is greater than the converted interval T1, and outputting the first frequency-domain pitch period code to the decoder;
• and
• a frequency-domain-pitch-period-based encoder encoding a first sample group of all or some of one or a plurality of successive samples including a sample corresponding to the first frequency-domain pitch period T in the N-points frequency-domain sample string and one or a plurality of successive samples including a sample corresponding to an integer multiple of the first frequency-domain pitch period T in the N-points frequency-domain sample string in accordance with a first criterion corresponding to magnitudes of amplitudes or estimated magnitudes of amplitudes of samples included in the first sample group and encoding a second sample group of samples in the sample string that are not included in the first sample group in accordance with a second criterion corresponding to magnitudes of amplitudes or estimated magnitudes of amplitudes of samples included in the second sample group, to obtain a code string, and outputting the code string which is obtained by encoding the first sample group and the second sample group to the decoder, wherein the first sample group is a part of the N-points frequency-domain sample string.

A decoder comprising:

• a long-term prediction information decoder receiving a time-domain pitch period code which is output from an encoder, and decoding the received time-domain pitch period code to obtain a time-domain pitch period L;
• a period converter obtaining, as a converted interval T1, a sample interval in an N-points frequency-domain sample string, the sample interval corresponding to the time-domain pitch period L, receiving a first frequency-domain pitch period code which is output from the encoder, decoding the received first frequency-domain pitch period code to obtain a multiple value indicating how many times a first frequency-domain pitch period T is greater than the converted interval T1, and obtaining, as the first frequency-domain pitch period T, the converted interval T1 multiplied by the multiple value;
• and
• a frequency-domain-pitch-period-based decoder receiving a code string which is output from the encoder, and decoding the code string by a decoding method in which a first sample group of all or some of one or a plurality of successive samples including a sample corresponding to the first frequency-domain pitch period T in the N-points frequency-domain sample string and one or a plurality of successive samples including a sample corresponding to an integer multiple of the first frequency-domain pitch period T in the N-points frequency-domain sample string is obtained by decoding processes according to a first criterion corresponding to magnitudes of amplitudes or estimated magnitudes of amplitudes of samples included in the first sample group and a second sample group of samples in the N-points frequency-domain sample string that are not included in the first sample group is obtained by decoding processes according to a second criterion corresponding to magnitudes of amplitudes or estimated magnitudes of amplitudes of samples included in the second sample group, to obtain and output the first sample group and the second sample group of the N-points frequency-domain sample string, wherein the first sample group is a part of the N-points frequency-domain sample string.