Apparatus for reducing quantization noise in image signals

An apparatus for decoding a discrete cosine transform coded image signal, which includes means for inversely quantizing the discrete cosine transform coded image signal to generate discrete cosine transform coefficients by using a quantization parameter representative of a quantization step size used in a quantization process of the discrete cosine transform coded image signal, means for converting the discrete cosine transform coded coefficients into a reconstructed image signal, said reconstructed image signal having a frequency domain quantization noise, and a frame memory for storing the reconstructed image signal to be displayed, characterized in that said apparatus further includes an image restoration circuit comprising:

• first conversion means for converting the reconstructed image signal into its frequency domain image signal including image signal phase components and image signal amplitude components thereof;
• predictive quantization noise generation means for responding to the quantization parameter and for generating a predictive quantization noise having a known probability density function;
• second conversion means for converting the predictive quantization noise into its frequency domain noise including noise phase components and noise amplitude components;
• means responsive to the image signal amplitude components and the noise amplitude components for generating compensated amplitude components representative of the difference between the image signal amplitude components and the noise amplitude components, and
• inverse conversion means responsive to the image signal phase components and the compensated amplitude components for converting them to restore the image signal, free of the quantization noise, in the time domain to thereby improve the quality of the displayed image.