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Line Pairing and Flicker Reduction in Interlaced Cathode Ray Tube Displays

IP.com Disclosure Number: IPCOM000041658D
Original Publication Date: 1984-Feb-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 63K

Publishing Venue

IBM

Related People

Beeteson, JS: AUTHOR [+2]

Abstract

Interlace is a technique used to reduce video data rates in visual display units employing cathode ray tubes without producing excessive flicker. The technique of interlace works well with characters that are reasonably well balanced, i.e., characters that contain roughly equal amounts of data on each field. However, many characters are inherently flicker prone, because they contain either all or most, of their information on one field only. Examples are minus sign, underscore, L, E, etc. There are character sets that can reduce this effect in some cases but not all. As well as flicker, the effect of field imbalance tends to induce line pairing and tearing. This is because heavy loading on one field causes progressive EHT (extra high tension) drop down the loaded field, but not on the unloaded field.

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Line Pairing and Flicker Reduction in Interlaced Cathode Ray Tube Displays

Interlace is a technique used to reduce video data rates in visual display units employing cathode ray tubes without producing excessive flicker. The technique of interlace works well with characters that are reasonably well balanced, i.e., characters that contain roughly equal amounts of data on each field. However, many characters are inherently flicker prone, because they contain either all or most, of their information on one field only. Examples are minus sign, underscore, L, E, etc. There are character sets that can reduce this effect in some cases but not all. As well as flicker, the effect of field imbalance tends to induce line pairing and tearing. This is because heavy loading on one field causes progressive EHT (extra high tension) drop down the loaded field, but not on the unloaded field. This EHT difference results in scan sensitivity differences between fields, and the visual effect is line pairing and tearing. This article describes a technique to reduce all three effects (line pairing, line tearing and flicker) while remaining independent of specific character sets. Fig. 1 shows the present interlace of known CRT display. An analog skip pulse produces an unscanned vertical gap of 4 lines (i.e., 2 per field) between each character row. Fig. 2 shows the proposed new scheme. Here, alternate rows start with different fields, i.e., row 1 starts with an odd field, row 2 starts with an even field, row 3 starts with an odd field, etc. If data is written on every row in the normal (Fig. 1) interlace scheme, then flicker and line pairing can be caused if there is a preponderance of data on one field. The new scheme automatically balances such data. A stress case can still be produced with the new scheme, but only with data written on alternate rows. Thus, only half th...