VOGONS

What causes noise when setting the VGA palette?

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First post, by root42

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Lots of games and demos use palette fading techniques for either fading in and out images, or for color cycling and similar effects. Some VGA cards will display noise at the top of the screen. One example is shown below. I have two ET4000 cards and one displays more noise than the other. My Trident 9000 cards have almost no noise in these scenarios. What causes the noise and can I "fix" the card (e.g. replace the RAMDAC), or is it inherent in its design?

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Reply 1 of 2, by Tiido

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This is a problem of using single port memory in the RAMDAC, the same lines that are used to read the palette are also the lines used to write the data. When data is written the video output signals are briefly disconnected so that the data can enter the palette, and show some fixed color or previously read color or color being written etc. which causes the artifacts.
Better things use dual port memory which allows simultaneous read and write, so the display output can continue while new data is being written.

Only fix is using a better RAMDAC chip, most of them are pin and function compatible though I don't know if any actually is immune to the problem and there may be BIOS issues when higher color depths are concerned as setting up those sometimes differs between chips.

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Reply 2 of 2, by root42

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Interesting. My ET4000 uses an ADV476 but I have an 80MHz INMOS available. I could try that...

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