First post, by elianda
I would like to discuss some general problems that TFTs and VGA to HDMI/DVI Converters show with analog VGA Signals.
So default text mode is 720x400 70 Hz. The 720 horizontal originates from 80 chars with 9 pixels each.
Here the resolution and the mode matches.
However other low res graphics resolutions like 320x200 are also output as 720x400 70Hz mode.
Any device that samples this mode to a digital representation samples 720 pixels. The result is that a checkerboard pixel pattern in 320x200 gets a moiree effect, where each 4th pixel is in the sampled 720 resolution 3 pixels wide.
The checkerboard pattern is then AABBAABBBAABBAABBBAABBAABBBAABBAABBB and so on. (notice the triple BBBs)
The title screen of Keen5 is very suited to check . See attached photo, where the checkerboard pattern has pixels with uneven width.
This is a very general problem where the sampling is incorrect and an artificial moiree effect is introduced. Since a TFT sees only the analog 720x400 mode, it can not know that the displayed resolution might be e.g. 320x200.
From our tests, general TFTs and converters:
If there is a 400 lines mode then it is sampled with 720 samples.
If there is a 480 lines mode then it is sampled with 640 samples.
In case the displayed resolution deviates or is a non-integer of it then the moiree effect comes up.
It also does not help to have a TFT with a high resolution because the effect is already introduced when the original signal is sampled. High TFT resolution blows the moiree effect just up when scaling.
You might want to check your device if you can set the number of sample points to 640 for these modes.
For most of the low res graphics modes sampling with 720 samples is wrong.
There are some eceptions, which is text mode and e.g. from the Quake 1 mode selection modes like 360x350 .
Remember that Commander Keen shows a red tint on some TFTs?
The cause is that Commander Keen uses a cyan border color and some TFTs use the overscan color for white balance.
Good thing is Commander Keen offers a cheat code to change border color:
In the game press A+2+Enter to activate cheat mode then press F10 and B to change the border color.
If you have a TFT that shows a red tint you can now get any other wrong color tint until you set the border to black.
There is also a TSR from wbc here that fixes that: Commander Keen problem!
However, this means that all programs that change the border color are affected. If the TFT does not do this per frame but per scanline and one has VGA rasterbars in the border area one could thing of new demo effects 😉
At least it is not an issue of the graphics card.
Someone has seen the red tint effect also on VGA converters?
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