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Filter to Remove Spurious Colors From Computer-Generated Composite Video Images

IP.com Disclosure Number: IPCOM000040589D
Original Publication Date: 1987-Dec-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 1 page(s) / 11K

Publishing Venue

IBM

Related People

Blue, L: AUTHOR [+4]

Abstract

Computer-generated composite video displays of text and graphics have performance problems which result from contradictions between the requirements for such images and those of NTSC composite video encoding. The filter described here alleviates one of these consequences, namely spurious coloring of monochrome (black and white) text and graphic items. The problem of spurious coloring is caused by the fact that pixels small enough to be useful for computer displays have frequency components in and above the frequency range reserved in NTSC for chroma information. These components are incorrectly interpreted by the display as color instead of luminance information.

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Filter to Remove Spurious Colors From Computer-Generated Composite Video Images

Computer-generated composite video displays of text and graphics have performance problems which result from contradictions between the requirements for such images and those of NTSC composite video encoding. The filter described here alleviates one of these consequences, namely spurious coloring of monochrome (black and white) text and graphic items. The problem of spurious coloring is caused by the fact that pixels small enough to be useful for computer displays have frequency components in and above the frequency range reserved in NTSC for chroma information. These components are incorrectly interpreted by the display as color instead of luminance information. Comb filters cannot work with non-interlaced computer displays because the chroma and luminance signals are not frequency interleaved as they are in standard NTSC encoding. Practically all computer composite video signals are non-interlaced due to flicker problems. A band-stop filter centered at 3.58 MHz (in the case of NTSC) and with a bandwidth of 1 MHz acting on the luminance signal before it is mixed with the chrominance signal removes those frequency components of the luminance signal which are in the frequency range normally decoded as chroma, thereby reducing or eliminating spurious coloring. The filter must act only on the luminance portion of the signal; if it affected the chroma portion, then all color would be remo...