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Cathode Ray Tube Horizontal Processor Using Inexpensive 555 Timers

IP.com Disclosure Number: IPCOM000035151D
Original Publication Date: 1989-Jun-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 2 page(s) / 64K

Publishing Venue

IBM

Related People

Andres, JL: AUTHOR

Abstract

Disclosed is a circuit for preventing drift of alpha-numeric characters on a CRT screen. It keeps a constant phase relationship between the horizontal synchronism of a logic driver and the flyback pulse. The circuit uses well known 555 integrated circuit timers.

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Cathode Ray Tube Horizontal Processor Using Inexpensive 555 Timers

Disclosed is a circuit for preventing drift of alpha-numeric characters on a CRT screen. It keeps a constant phase relationship between the horizontal synchronism of a logic driver and the flyback pulse. The circuit uses well known 555 integrated circuit timers.

IC1 is an astable oscillator synchronized by the external horizontal sync pulses. It free runs in the absence of the sync pulses at a frequency that should be lower than the external sync in order for both signals to be synchronized.

IC2 is a monostable. It works as a PWM (pulse width modulator). The phase difference between the voltage ramp formed by R4, C4 and R3, C3 modulates the pulse width at the output pin (output). The trailing edge of this pulse triggers the second monostable, IC3, to give a constant pulse width output to drive the horizontal line transistor Q3 properly. This pulse causes the initiation of the flyback pulse and the reset of the voltage ramp in R4, C4.

(Image Omitted)

The R3, C3 ramp is triggered by the horizontal sync. When the crossover between these ramps occurs, the IC2 output goes low. The duration of this pulse represents the phase difference. Any variation in the flyback interval initiation will be translated into a different pulse width that will try to compensate this variation. The slope of the R3, C3 ramp sets the desired phase.

For example, suppose the flyback pulse is delayed due to temperature change a...