Method and apparatus for decoding digital signal

An apparatus for decoding a digital signal, comprising:

• received signal image generation means that receives a multiple-bit digital signal that including information to be transmitted and arrays bit strings of the received multiple-bit digital signal to thereby generate a received signal image as a two-dimensional image;
• spatial frequency filtering means that arrays all types of predetermined proper signals used for signals transmitted and bit strings of patterns including errors each derived from each proper signal to thereby generate a different two-dimensional image, disposes a set of received signal patterns, each comprised of an arbitrary proper signal and a group of two-dimensional images of patterns having the arbitrary proper signal added with an error, in a state that enables identification of each proper signal to thereby generate a received signal pattern image, uses optical signal processing to evaluate a coefficient of correlation between the received signal image from the received signal image generation means and the received signal pattern image to obtain a correlation projection image in which depth and brightness intensity distribution is proportional to the coefficient of correlation;
• and
• decoding processing means that extrapolates transmitted proper signals from a region that includes a maximum point of the depth and brightness intensity distribution appearing in the correlation projection image based on a correspondence between the region and the set of received signal patterns in the received signal pattern image and identifies transmitted information from the extrapolated proper signals.