CRT display anti-burn circuit

• US 4,127,796 A
• Filed: 05/23/1977
• Issued: 11/28/1978
• Est. Priority Date: 05/23/1977
• Status: Expired due to Term

• Current Assignee: Hughes Aircraft Company (Rtx Corporation)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: Henderson, Alan R.
• Primary Examiner(s): Blum, Theodore M.
• Application Number:
  US05/799,804
• Time in Patent Office: 554 Days
• Field of Search: 315/393, 315/394, 315/395, 358/220
• US Class Current: 315/395
• CPC Class Codes: H04N 3/20 Prevention of damage to cathode-ray tubes in the event of failure of scanning

Abstract

Damage to the phosphorescent screen of a cathode ray tube (CRT) by an electron beam is minimized by preventing the display of a stationary image on the screen. This is accomplished by superimposing or adding to each of the normal electron beam deflecting voltages an additional triangular offset waveform, with the combined effect of the offset waveforms being to constantly move the entire electron scan pattern over an infrequently recurring path at a slow enough rate to be imperceptible to the viewer.

Claims

1. In a cathode ray tube display system the combination comprising:

(a) a cathode ray tube having a sealed envelope which includes a faceplate having an inner surface coated with a phosphorescent compound;

(b) means in said envelope for generating an electron beam and projecting it at said coated surface;

(c) first and second means for deflecting said electron beam across said faceplate, said first and second means imparting movement to said electron beam in mutually orthogonal directions in response to deflection signals applied thereto;

(d) first and second means for individually applying to said first and second deflecting means respectively, first and second periodic deflection signals; and

(e) means for generating, and individually adding to said first and second deflection signals respectively, first and second periodic out-of-phase offset signals, said first and second out-of-phase offset signals being of relatively low frequency and small magnitude with respect to the resultant of said first and second periodic deflection signals whereby the deviation of said beam due thereto is not perceptible to the human eye.

2. The combination of claim 1 characterized further in that said offset signals differ in phase by a uniformly changing amount.

3. The combination of claim 2 characterized further in that both of said offset signals have triangular waveforms whose frequencies differ by an amount which is between 2 and 10% of their mean frequency.

4. The combination of claim 1 characterized further in that said means for generating and adding said offset signals include a pair of individual triangular waveform signal generators.

5. The combination of claim 4 characterized further in that said waveform generators have identical circuit configurations and identical nominal values for corresponding components, the nominal random variation between the values of corresponding components of said waveform generators being sufficient to effect a frequency difference between the waveforms generated by them.

6. The combination of claim 3 characterized further in that said frequency difference is between 2% and 10% of the mean frequency of the waveforms generated by them.

7. The combination of claim 1 characterized further in that said means for generating and adding includes a single triangular waveform generator and a phase shifter connected to receive the output of said generator and operative to produce a second triangular waveform output whose phase varies from the output of said generator by a continually and uniformly varying amount.

8. A method of preventing burn-in of a CRT having a screen across which a cathode ray beam is scanned by means of vertical and horizontal beam deflectors under the influence of horizontal and vertical deflection signals applied to said deflectors comprising the steps of:

(a) adding to the horizontal deflection signal being applied to said horizontal beam deflector a first periodic offset signal; and

(b) adding to the vertical deflection signal being applied to said vertical beam deflector a second periodic offset signal whose phase is different from that of said first offset signal so that the traverse of said screen by said cathode ray beam is deviated at a sufficiently slow rate and over a sufficiently small distance that said deviation is not perceptible to the human eye.

9. The method of claim 8 characterized further in that said second offset signal has a phase which differs from that of said first offset signal by a continually varying amount.

10. A method of preventing burn-in of a CRT having a screen across which an electron beam is scanned by means of vertical and horizontal beam deflectors under the influence of horizontal and vertical deflection signals applied to said deflectors comprising the steps of:

(a) displacing the pattern traced by said beam under the influence of said deflection signals over a serpentine path at a rate and over a distance which in combination renders said displacement imperceptible to the human eye during normal viewing.