1. A method for signal processing, comprising the steps of: calculating a power spectrum of an input image signal; selecting a power spectrum approximate to the calculated power spectrum from a plurality of power spectra stored in a first filter storage portion; selecting a first filter corresponding to the selected power spectrum from a plurality of first filters stored in the first filter storage portion; calculating a S/N (signal-to-noise ratio) of the input signal; selecting a S/N approximate to the calculated S/N from a plurality of S/Ns stored in a second filter storage portion; selecting a second filter corresponding to the selected S/N from a plurality of second filters stored in the second filter storage portion; creating a third filter by adding up the selected first and second filters; and convolving the input signal using the created third filter.