Legibility for alpha-mosaic characters

Details Legibility for alpha-mosaic characters Legibility for alpha-mosaic characters

H - Electricity

04

N

375/12, 350/34

H04N 7/00 (2006.01) H04N 5/44 (2006.01) H04N 5/57 (2006.01)

Patent

CA 1174387

-13- RCA 76,460 ABSTRACT OF THE DISCLOSURE In add-on alpha-mosaic character (teletext) decoders, the character signals pass through the limited bandwidth chroma channel of the TV receiver. For certain color combinations and signal durations this reduces legibility and contrast due to pulse stretching and amplitude reduction. The present invention determines if a pulse is less than a predetermined pulse width and has an amplitude difference with respect to a preceding pulse of less than a selected amount. If both conditions are met, then a luminance signal pulse is generated to substitute for the original pulse, the generated pulse having a large amplitude difference with respect to the preceding pulse for improved contrast. If the generated pulse would exceed the white level or go below the black level, then a pulse of opposite polarity with respect to the first generated pulse is generated to cancel the first generated pulse but still result in a large amplitude difference with respect to the preceding pulse.

396351

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