Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Patent
1992-07-17
1994-06-14
Issing, Gregory C.
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
315386, 315367, 348100, 348173, G09G 104, H01J 2970, H04N 530, H04N 336
Patent
active
053213394
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to high power cathode ray tubes used for scanning an image. One example of such a CRT is that used in a flying spot telecine in which a film is sequentially scanned by a scanning raster produced by the deflection coils of the CRT.
BACKGROUND TO THE INVENTION
A problem with flying spot telecines and cathode ray tubes in general is burn. This is caused by the flying spot moving too slowly or stopping altogether at a point on the CRT face. The result is that the phosphor on the screen at this point is burnt which seriously degrades the response of that part of the screen.
We have appreciated that there are a number of types of fault or operating conditions which could lead to phosphur burn and which it would be desirable to protect against in high power cathode ray tubes such as telecine flying spot scanners.
A first situation occurs where there is a high concentration of beam energy on a single spot or scan location for a very short time; that is a few microseconds. A second situation is where there is a high concentration of beam energy in certain areas over longer periods of time; that is, a few milliseconds.
SUMMARY OF THE INVENTION
The present invention in its various aspects aims to identify situations where burn might occur and to take action to prevent damage to the CRT face.
In one aspect, the invention monitors the frequency with which a given scan address or scan area is addressed during a scan line and takes action if this exceeds a maximum value. In another aspect the beam current is set according to the frequency with which areas are addressed during a frame.
The invention is defined by the claims to which reference should be made.
The first burn condition mentioned above is likely to be caused by a fault condition where the line scan stops, but could also be caused by particular scan effects which slow the spot or cause it repeatedly to traverse a given scan address. With an EHT voltage of 25 KV and a beam current of 300 uA i.e. a power of 7.5 watts, it has been shown that damage will occur if the tube has not been blanked within 20 uS.
The second condition could happen due to a fault where the frame scan collapses but the line scan still runs. In this case the beam current can remain on for a few frames before damage is likely. 100-200 ms is the maximum time the beam can continue for before burn damage occurs. The second situation can also arise in a continuous motion telecine where the film is being run at a particular range of speeds where the film motion is equal or approximately equal to the frame scan; this is equivalent to a frame collapse. This is permissible if the applicants burn reducing system known as SCANTRACK (TM) is operating but if it were to fail, or the user were to disable the SCANTRACK burn reduction system then protection is required with 100-200 ms. The SCANTRACK burn reduction system reduces burn in continuous motion telecines in which the same area of the CRT faceplate would usually be scanned by the raster. Instead, the raster is slowly moved up and down the CRT face to reduce exposure to the raster at any one location. The movement is compensated for by the servo system and the raster moves with a period of about 2 minutes in the vertical scan direction.
Large concentrations of beam energy which could lead to burn if not controlled can be produced by the use of scan effects. The nature of the concentration is unpredictable but falls in two categories.
The first is similar to a line collapse or partial collapse where the spot stops moving or slows down below a safe velocity for part of the line. This must be protected against within a few uS, preferably 2 uS.
The second category is similar to a frame collapse where parts of the tube face are being scanned many times. This condition, like the second condition described above is permissible if SCANTRACK (TM) is operating but the uses may disable scantrack. Moreover, SCANTRACK (TM) does not operate when the film is stationary.
In a preferred embodiment of the invention
REFERENCES:
patent: 4198661 (1980-04-01), Gatten et al.
patent: 4677351 (1987-06-01), Brust et al.
patent: 4722005 (1988-01-01), Ledenbach
patent: 4792733 (1988-12-01), Redfern
patent: 5068647 (1991-11-01), Hinton et al.
Patent Abstracts of Japan, vol. 13, No. 581 (E-865) Dec. 21, 1989 & JP, A, 1243674; Sep. 28, 1989.
Issing Gregory C.
Rank Cintel Limited
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