Block noise reducing apparatus

1. An apparatus for reducing block noise in an input video signal resulting from compressively encoding an original video signal rectangular-block by rectangular-block to get an encoded digital video signal, and expansively decoding the encoded digital video signal rectangular-block by rectangular-block, comprising:

first means for calculating first and second differences in level represented by the input video signal at first and second inter-block boundaries adjoining opposite edges of every rectangular block respectively;

second means for calculating a corrective value at every pixel in the rectangular block from a position of the pixel and the first and second differences calculated by the first means according to a predetermined equation;

third means for adding the corrective value calculated by the second means to a level represented by the input video signal to get a first output video signal;

fourth means for comparing the first and second differences calculated by the first means with a predetermined threshold value to decide whether or not block noise occurs at the first and second inter-block boundaries;

a filter for filtering the input video signal to get a filtered video signal; and

fifth means for selecting one of (1) the first output video signal generated by the third means and (2) the filtered video signal generated by the filter in response to results of the comparing by the fourth means to get a second output video signal;

wherein the predetermined equation is designed so that levels represented by the first output video signal at respective pixels in the rectangular block will be on a line given by a predetermined function of pixel position and connecting a first point in a range corresponding to the first difference calculated by the first means and a second point in a range corresponding to the second difference calculated by the first means; and

wherein the filter comprises a low pass filter.

2. An apparatus for reducing block noise in an input video signal resulting from compressively encoding an original video signal rectangular-block by rectangular-block to get an encoded digital video signal, and expansively decoding the encoded digital video signal rectangular-block by rectangular-block, comprising:

first means for calculating first and second differences in level represented by the input video signal at first and second inter-block boundaries adjoining opposite edges of every rectangular block respectively;

second means for calculating a corrective value at every pixel in the rectangular block from a position of the pixel and the first and second differences calculated by the first means according to a predetermined equation;

third means for adding the corrective value calculated by the second means to a level represented by the input video signal to get a first output video signal;

fourth means for comparing the first and second differences calculated by the first means with first and second predetermined threshold values to decide conditions of block noise at the first and second inter-block boundaries;

a filter for filtering the input video signal to get a filtered video signal; and

fifth means for selecting one of (1) the first output video signal generated by the third means, (2) the input video signal, and (3) the filtered video signal generated by the filter in response to results of the comparing by the fourth means to get a second output video signal;

wherein the predetermined equation is designed so that levels represented by the first output video signal at respective pixels in the rectangular block will be on a line given by a predetermined function of pixel position and connecting a first point in a range corresponding to the first difference calculated by the first means and a second point in a range corresponding to the second difference calculated by the first means; and

wherein the filter comprises a low pass filter.