Hitherto, audio signal coding methods for compressing the amount of data of an audio signal have been developed. As one of such coding methods, High-Efficiency Advanced Audio Coding (HE-AAC) is known. This coding method has been standardized as MPEG-2 HE-AAC and MPEG-4 HE-AAC by the Moving Picture Experts Group (MPEG). In HE-AAC, the low frequency band (low frequency components) of an audio signal is coded in accordance with an Advanced Audio Coding (AAC) method, whereas the high frequency band (high-frequency components) of an audio signal is coded in accordance with a Spectral Band Replication (SBR) method. In the SBR method, each frame of an audio signal is divided into a plurality of time-frequency domains, and auxiliary information or the like for reproducing high-frequency components by reproducing corresponding low frequency components on the basis of the signal power within each time-frequency domain are calculated as SBR data. Then, an SBR parameter is coded. This time-frequency domain is called a grid.
In the SBR method, if the time length of a grid is too long with respect to the temporal change of an audio signal, the electric power of the audio signal is averaged in the grid, and thereby the information indicating the temporal change is lost. As a result, the reproduction sound quality of the coded audio signal deteriorates. There is a case where, in particular, as a result of sound in a certain time period being affected by sound later than that sound, sound that differs from the original sound is produced. Such a phenomenon is called a pre-echo. In Japanese National Publication of International Patent Application No. 2003-529787, a technology is disclosed in which a highly transient sound, such as attack sound, is detected with respect to each channel of an audio signal, and a grid is set so that the time resolution increases with respect to the highly transient sound. Such a transient portion of sound is called a transient.
Furthermore, in Japanese Laid-open Patent Publication No. 2006-3580, a technology has been disclosed in which when it is determined that the degree of similarity of a plurality of channels of an audio signal is high, a grouping of frequency data such that an audio signal is frequency-converted in the time direction or in the frequency direction is performed in common with respect to a plurality of channels.