Matthew Hagerty

Good looking video: flat screen vs CRT

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Does anyone have ideas, suggestions, opinions, etc. on getting good video from a flat screen vs. a CRT?

In this case I'm talking about driving a VGA signal from an FPGA (which is rather easy). Since flat screens have fixed pixels sizes and resolution, I find I can't expand the display to the borders like I can with a CRT. I end up with "pixel noise" on the flat screens, but on the CTR the display is beautiful. I can correct this by adding borders to the display and ensuring the horizontal timing falls on a multiple of the flat screen's resolution, but that does not maximize the display area, i.e. like watching a movie with letter-box.

The native video resolutions I'm working with will always be low compared to modern systems, i.e. 340x280, etc. and probably never over 640x480. However, using modern monitors is desirable since they are cheap and available.

Thoughts?

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To get a perfect picture on an LCD you have to be bang-on the frequency in the VESA standard. For 640x480 you need 25.175MHz.

 

A better option would be to use 800x600 @60Hz, as it needs 40.0MHz.

 

Mike

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Getting the frequency right is not really my problem. In my first design the original resolution of the system was 256x192 pixels. I ran the monitor at 640x480 and just doubled the pixels for a 512x384 display with 64-pixel borders left/right and 48-pixel borders top/bottom. While the pixels look fine, this does not maximize the screen area (and people asked how they could get the display to fill the screen).

When I did a later design I decided to just change pixel colors at the original resolution during the horizontal trace. On a CRT this filled the screen nicely and the pixels look very nice. However, on an LCD (or LED) display I'm getting artifacts since a pixel color may change halfway through an LCD's element. The horz/vert scan rates are spot on, but I was treating the horizontal sweep as a true analog raster as it really is on a CRT.

I can't come up with a good way to address the problem on an LCD and I was wondering if anyone else had done anything similar or solved this problem? This is one case where a CRT is still superior to an LCD/LED display.

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If you play with the fine tuning on the LCD you will be able to make it look good - try a set of fine black/white stripes.

 

It is because the LCD's DACs are sampling at 25.125MHz, not the 25.0MHz that you are sending the pixel, given you virtical bands bands of miss-sampled pixels.

 

The only way I could solve it was to either fine-tune the screen, or to use a "bang-on" pixel clock.

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