Browse Prior Art Database

White Background Level Control Circuit

IP.com Disclosure Number: IPCOM000036986D
Original Publication Date: 1989-Nov-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

Kobayashi, M: AUTHOR [+2]

Abstract

Disclosed is a technique to control the white background level by keeping the white balance when adjusting a brightness and contrast control potentiometer for a color CRT display monitor.

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White Background Level Control Circuit

Disclosed is a technique to control the white background level by keeping the white balance when adjusting a brightness and contrast control potentiometer for a color CRT display monitor.

The figure shows a conceptual structure of the technique. In the case of a color CRT display monitor, the same video amplifier circuit are provided for red, blue and green channels. One channel of a video amplifier circuit consists of a preamplifier 1, a clamp circuit 2, an output amplifier 3, a voltage feedback circuit 4, a clamp amplifier 5 and a sub-gain potentiometer, such as 6 for the red channel.

The preamplifier 1 is a non-inverted amplifier receiving a maximum 0.7 V peak-to-peak of an analog video signal from a logic circuit with AC coupling. An output gain of this preamplifier 1 can be controlled by an external DC voltage which is indicated as Vgain. For example, in a red channel, DC voltage of Vgain is adjustable by sub- gain potentiometer 6 independently. By using these sub- gain potentiometers 6, 7 and 8, white balance and maximum luminance of a display screen area is adjusted to obtain the specified value of the monitor. A contrast potentiometer 10 is an operator-accessible adjustment to obtain adequate luminance level of a screen according to user's preference. In adjusting the contrast potentiometer 10, Vgain of each channel is controlled to keep the voltage ratio constant.

At the clamp circuit 2, the black level of the output video signal from the preamplifier 1 is clamped to a DC voltage from the clamp amplifier 5, and the DC clamped video signal is applied to the output amplifier 3.

The output amplifier drives a cathode of CRT directly, through a buffer stage, or through a buffer and DC shift circuit stage for cut- off adjustment. On the other hand, the...