Referring back to FIG. 4A, when the boundary line 410 is far from the origin point, a relatively large number of the high frequency components are included in the output image, the quality of the output image is improved, and the bitrate of the output image also increases. Thus, when a difference between the current bitrate and the target bitrate is relatively small, a frequency filter having the boundary line 410 far from the origin point as in FIG. 4A may be used.

FIG. 4C shows an example of a frequency filter corresponding to a case where the difference between the current bitrate and the target bitrate is relatively small.

Referring to FIG. 4C, a boundary line 412 is far from the origin point when comparing with the case of FIG. 4B. When the above frequency filter is applied to the magnitudes of one or more frequency components, the output image includes ten frequency components, which will be described later.

The above frequency filters may be generated in advance according to the difference between the current bitrate and the target bitrate, and may be stored in the memory 140. In other words, the memory 140 stores one or more frequency filters, and the controller 130 may select an appropriate frequency filter from the one or more frequency filters stored in the memory 140, according to the difference between the current bitrate and the target bitrate, and may apply the selected filter to the magnitude of the frequency component.