Method to control CRT phosphor aging

Details Method to control CRT phosphor aging Method to control CRT phosphor aging

1999-11-29

2002-03-19

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Cathode ray tube circuits

Cathode-ray deflections circuits

C315S364000, C315S367000, C348S123000

Reexamination Certificate

active

06359398

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to methods and apparatus for use in conjunction with display device. Most particularly, these inventions relate to methods and apparatus for uniformly aging one or more cathode ray tubes (CRT's) within a display device, such as a television.
BACKGROUND OF THE INVENTION
All televisions utilize one or more cathode ray tubes (CRT's) to transform video information into light rays for display on the television screen. This is typically accomplished by exciting the phosphor material on the CRT with a scanned beam of electrons, causing the phosphor material to output light. Typically, over time, usage of the CRT causes aging or reduction of light output from the CRT phosphor material. When the entire nominal scan area of the CRT phosphor is excited (i.e., the area of the CRT phosphor corresponding to the geometry of the television screen), the CRT phosphor material is uniformly aged, resulting in an unnoticeable uniform reduction of light from the CRT. Under such circumstances, this phenomenon is typically not problematic. When only a portion of the nominal scan area of the CRT phosphor is excited over a period of time, however, the CRT phosphor is non-uniformly aged, resulting in a noticeable non-uniform reduction of light from the CRT.
This problem typically arises when video signals formatted with one aspect ratio are displayed on a television formatted in another different aspect ratio. The aspect ratio of a television picture image is a ratio of horizontal length to vertical length, expressed in relative units. Standard video signals, such as NTSC and PAL video signals, are formatted with a 4:3 aspect ratio (i.e., 1.33 aspect ratio), whereas non-standard video signals, such as HDTV video signals, are formatted with an aspect ratio greater than the standard 4:3 aspect ratio. For example, an HDTV video signal is typically formatted with a 16:9 aspect ratio (i.e., 1.77 aspect ratio). Modern cinematographic theater movies, not made expressly for conventional television, are typically films with aspect ratios greater than 1.33, typically ranging between 1.65 and 2.35.
When standard video signals are displayed on a standard television screen (i.e., a television screen having a 1.33 aspect ratio), the picture image appears on the entire television screen. As long as these standard video signals are displayed on a standard television screen, the associated CRT or CRT's are uniformly illuminated, and thus uniformly aged. When a non-standard video signal, such as an HDTV video signal, is displayed on a standard television screen, the picture image appears on the middle horizontal region of the television screen and black bars appear on the respective top and bottom horizontal regions of the television screen. As a result, a region of the CRT that would normally be illuminated in response to a standard video signal is not illuminated in response to the non-standard video signal.
Thus, without correction, display of a non-standard video signal on a standard television screen over an extended period of time causes the phosphor material disposed on a primary region of the CRT (represented by a middle horizontal region of the nominal CRT scan area) to age more than the phosphor material disposed on a secondary region of the CRT (represented by respective top and bottom horizontal regions of the nominal CRT scan area). When a standard video signal is then displayed on a standard television screen on which non-standard video signals have been displayed over an extended period of time, the top and bottom horizontal regions of the television screen will be distinctly brighter than the middle horizontal region of the television screen. These distinctions will appear as two respective spatial transition lines between the top and middle horizontal regions of the television screen and between the bottom and middle horizontal regions of the television screen. A similar phenomenon occurs when a standard video signal is displayed on a non-standard television screen for an extended period of time, causing the phosphor material on the middle vertical region of the nominal scanning area of the CRT to age more than the phosphor material disposed on the respective left and right vertical regions of the nominal scanning area of the CRT. Most viewers will complain about this phenomenon, which may require a service call to replace the CRT or CRT's. If the television is a projection television (PTV), which typically includes three color CRT's, this service call may cost $500 or more.
One method used to reduce this non-uniform CRT phosphor aging problem involves slowly moving the electron scanning angle over time, so that the spatial transition lines are blurred. Another method involves pre-aging the CRT phosphor with white video, which takes advantage of the fact that the problem of non-uniform CRT illumination lessens with time. These methods, however, only reduce the problem of non-uniform CRT illumination, and do not necessarily eliminate the problem. Still another method involves illuminating the respective top and bottom horizontal regions of the nominal CRT scan area with gray video when a non-standard video signal is displayed on a standard television screen, or illuminating the left and right vertical regions of the nominal CRT scan area with gray video when a standard video signal is displayed on a non-standard television screen. This method, however, requires the luminous intensity of the gray video to match an average luminous intensity of the program video. This estimation is not perfect, resulting in a reduced, but still present, non-uniform aging of the CRT.
Accordingly, an improved method and apparatus for uniformly aging the phosphor material on a CRT is desired.
SUMMARY OF THE INVENTION
Methods and apparatus are utilized for uniformly aging the phosphor material on a CRT. The method serves to uniformly age a CRT irrespective of the aspect ratio in which a received external video signal is formatted. The CRT may be located in a television, such as a projection television, a computer monitor, or any other device that uses a CRT to transform a video signal into light output.
A preferred method includes receiving an external video signal that includes a signal aspect ratio that is different from the aspect ratio of the television screen. By way of non-limiting example, if the CRT has an aspect ratio of 4:3, illumination of CRT at a 16:9 aspect ratio (resulting from the illumination of the CRT in response-to a non-standard video signal containing video material formatted with a 16:9 aspect ratio, such as HDTV video material) will cause a primary region of the CRT, represented as a middle horizontal region, to be illuminated, and a secondary region of the CRT, represented as respective top and bottom horizontal regions, to be non-illuminated. By way of further non-limiting example, if the CTR has an aspect ratio of 16:9, illumination of CTR at a 4:3 aspect ratio (resulting from the illumination of the CRT in response to a standard video signal containing video material formatted with a 4:3 aspect ratio, such as NTSC video material) will cause a primary region of the CRT, represented as a middle vertical region, to be illuminated, and a secondary region of the CRT, represented as respective left and right vertical regions, to be non-illuminated.
To facilitate uniform aging of the CRT, a CRT age equalization video signal is derived from the external video signal. The equalization video signal is based on the signal aspect ratio of the external video signal, such that the secondary region of the CRT, which is non-illuminated in response to the external video signal, is illuminated in response to the equalization video signal. Preferably, the derivation of the equalization video signal is accomplished by duplicating of mirroring a portion of the external video signal corresponding to the portion of the primary CRT region adjacent to the secondary CRT region. In this manner, matching of the illumination intensities of the respective pri

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