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Automatic Brightness Control for CRT

IP.com Disclosure Number: IPCOM000043409D
Original Publication Date: 1984-Aug-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 3 page(s) / 54K

Publishing Venue

IBM

Related People

Ling, CJ: AUTHOR [+2]

Abstract

A method for the automatic brightness control of monochrome or color CRT displays is described. The CRT beam current is measured during an all white line every picture frame, and a feedback loop sets the bias level for the CRT cathode to give the required brightness. This improves long term drift and makes color balance set up easier, although spot size will increase with CRT aging. Fig. 1 shows the basic control circuit. The video waveform to be applied to the cathode is capacitively coupled from the output of the video amplifier to the CRT cathode by C1. The DC bias of the cathode is provided by the resistor R1 charging C1 to a DC level set by the amplifier output voltage. At this point diode D1 conducts, clamping the cathode bias voltage. When a negative-going pulse turns on the CRT gun, the beam current charges C1.

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Automatic Brightness Control for CRT

A method for the automatic brightness control of monochrome or color CRT displays is described. The CRT beam current is measured during an all white line every picture frame, and a feedback loop sets the bias level for the CRT cathode to give the required brightness. This improves long term drift and makes color balance set up easier, although spot size will increase with CRT aging. Fig. 1 shows the basic control circuit. The video waveform to be applied to the cathode is capacitively coupled from the output of the video amplifier to the CRT cathode by C1. The DC bias of the cathode is provided by the resistor R1 charging C1 to a DC level set by the amplifier output voltage. At this point diode D1 conducts, clamping the cathode bias voltage. When a negative-going pulse turns on the CRT gun, the beam current charges C1. During every positive-going transition this is discharged into capacitor C2 which is much greater than C1. If a line is continuously illuminated across the screen, such as for an operator dividing line, the amount of charge held in C1 at the end of the line is proportional to the brightness level.

When C1 discharges into C2 at this point, the small resistor R2 measures the current in C2. By connecting this to the input of the amplifier controlling the DC potential of the cathode a feedback loop can be set up. If the inverting input is now connected to a DC reference voltage as in Fig. 1, the amplifier will act so as to keep the average voltage developed across R2 equal to the reference voltage. The required function, however, is to keep the maximum voltage developed across R2 during all white line proportional to the reference voltage. This can be accomplished with either a sample-and-hold circuit activated at this time every frame or with the peak detecting circuit, shown in Fig. 2, which has a l...