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Line Pairing Correction By EHT Compensation

IP.com Disclosure Number: IPCOM000052998D
Original Publication Date: 1981-Aug-01
Included in the Prior Art Database: 2005-Feb-12
Document File: 2 page(s) / 36K

Publishing Venue

IBM

Related People

Morrish, AJ: AUTHOR

Abstract

This article describes a technique for the use of existing pin cushion correction circuitry to compensate for EHT sag in interlaced displays and the correction of line pairing by EHT compensation.

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Line Pairing Correction By EHT Compensation

This article describes a technique for the use of existing pin cushion correction circuitry to compensate for EHT sag in interlaced displays and the correction of line pairing by EHT compensation.

To reduce the visual effect of flicker from a CRT display when low refresh rates are used, the data to be presented is displayed in an interlaced form. However, if the video content varies from field to field as in the case of underscoring and most capital letters, then line pairing occurs due to variations in the EHT voltage. The usual method of correcting this is to lower the EHT supply impedance using a separate regulator or a capacitor across the EHT, which is ineffective and expensive.

An alternative method is to vary the scan current to give constant deflection; as this is inversely proportional to the square root of the EHT, the scan current must be varied accordingly, i.e.: See original. where I = scan current E = E(o) + delta E = EHT voltage See original.

This can be achieved by modulating the scan waveform directly with a voltage derived from the existing EHT bleed resistor network by using a small sense resistor, and then adding this to an equal amount of unmodulated scan waveform, as in the figure. Such modulation circuitry exists in many displays for high quality pin cushion correction, and it is very inexpensive but effective to use this to also correct for EHT variations.

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