Browse Prior Art Database

Modulator Driver for CRT Display With Distortion Correction

IP.com Disclosure Number: IPCOM000035171D
Original Publication Date: 1989-Jun-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 2 page(s) / 50K

Publishing Venue

IBM

Related People

Evans, PJ: AUTHOR [+2]

Abstract

An east-west modulator driver for a horizontal CRT scan circuit based on an available audio power amplifier chip is disclosed. The circuit incorporates trapezoidal correction. To compensate for different vertical frequencies, switched attenuators are used on the amplifier feedback path.

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Modulator Driver for CRT Display With Distortion Correction

An east-west modulator driver for a horizontal CRT scan circuit based on an available audio power amplifier chip is disclosed. The circuit incorporates trapezoidal correction. To compensate for different vertical frequencies, switched attenuators are used on the amplifier feedback path.

Color monitor CRTs inherently suffer from pin-cushion distortion. This is normally corrected at the top and bottom of the CRT, i.e., north-south, by yoke design. The distortion either side ie east-west, is normally corrected using a modulation waveform in the form of a parabola. There are two methods of generating this waveform 1) by sampling the vertical current waveform and transforming it to a parabolic voltage using a function generator chip or, 2) sampling differentially the parabolic waveform which occurs across the vertical DC blocking capacitor, C1 in the schematic diagram. This latter technique requires no extra components, but does have three disadvantages: 1) it requires a differentially connected amplifier to sample the voltage, 2) it is slightly asymmetric due to the vertical flyback occurring mostly in the first part of the vertical video blanking period, and 3) for multi-mode monitors, the parabolic voltage varies inversely proportional to vertical scan frequency. Described here is a circuit employing an audio power amplifier, in this case an LM380, as a power operational amplifier which overcomes the three disadvantages of the method of generating the parabolic waveform, and also provides sufficient power output to drive the east-west modulator directly. The east-west modulator may be of either type, i.e., transductor or d...