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irregular rectangles in OSD

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I am drawing a white rectangle , a grey rectangle and then a white rectangle stack on top of each other on DVD recorder's OSD.

so one white rectangle on top, grey in middle and one more white at bottom. Rectangle widths are same and is about 586 pixels.

When I see this on a 14' CRT TV, I see that vertical edges of white rectangle are getting distorted and rectangle looks like a trapezium. This problem is not observed on flat and LCD TV. I guess CRT TV is doing some thing wrong because of colorings used in OSD. grey rectange in between seems to be the cause of the problem. Do you have any idea about this problem?




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It's probably due to the beam current variation, and poor regulation of the EHT supply inside the CRT TV, particularly if it is a cheap model.


Most CRTs use a common supply for the electron gun and EHT supplies. When an area of white is on-screen this requires a higher beam current (more electrons to fire at the screen). This higher current usually causes a small voltage drop on the supply, leading to a lower EHT at the grid. The electrons therefore pass more slowly across the deflection field and get bent out a bit more, giving the distortion that you see.


The supplies can probably not cope well with the variation in beam currents, giving you the problem. This is inherent to CRT technology although better monitors generally have better regulation. Flat panels are completely different so don't suffer.


I know this is a bit technical, but if you understand how a CRT works this should make some sense. If you don't then I can't explain it better, and the short version is it is a common problem with cheap CRT monitors.



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