VOGONS


First post, by GabrielKnight123

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I have 3 ISA video cards and I'm using an LCD 16x9 1920 x 1080 monitor and I've noticed some faint vertical lines when I load the game Wolfenstein but in dos and other full screen colour programs its fine, on one of the cards a Trident TVGA9000B 512KB its much worse so I thought it might be the monitor so I tried an old LCD 4x3 monitor (I cant remember its max resolution) and its the same. I looked in various system tools like Aida16, MSD and Sysinfo and they say its using video mode 3, number of columns 80 and number of rows is 25 would this be why it has trouble with an LCD monitor and instead should be run on a CRT monitor or is it that they are just dying? I know the monitors are working fine especially with PCI video cards.

I put two pictures one bad and one faint although it might be hard to see the faint lines but on the monitor its noticeable.

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Reply 1 of 6, by clb

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Here is another thread of the topic: Re: Weird lines with Trident 8900CL ISA VGA card

This artifact is referred to as the "jailbar" effect. The issue occurs to my understanding when the LCD display circuitry incorrectly locks on to the horizontal total clock count of the video signal. This effect can be reproduced for example with the OSSC scaler by deliberately setting the horizontal total clocks count to a slightly incorrect amount. When the correct horizontal clock count is in use, the artifact will magically disappear.

For example, here is a 640x480 resolution picture from OSSC when incorrect horizontal line counts are used, and for reference a digital DVI-D output of the same image.

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Reply 2 of 6, by root42

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In my experience the jailbars also heavily depend on the type of RAMDAC used on the card and if the RAMDAC is actually properly connected to the VGA socket. Some cheaper boards save on noise suppression, which was pretty invisible on CRTs, but TFTs will make it very noticable.

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Reply 3 of 6, by GabrielKnight123

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Ok thanks guys I dont use ISA video cards often as my 486DX2 uses a VLB card and it doesnt show up on a 16x9 LCD I just keep a few in case I'm testing 286 boards or anything without VLB

Reply 4 of 6, by Jo22

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Some ISA VGA cards are simply incorrectly jumpered, as well.
There's usuall a DIP switch on the back which determines the monitor type.
The default setting is a plain, generic IBM compatible VGA monitor, I think.
Other types like PS/2, XGA or NEC Multisync use modern timings or more accurate timings.
It's worth a try. LCDs can't be damaged, after all.

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Reply 5 of 6, by keenerb

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GabrielKnight123 wrote on 2023-07-26, 09:21:

I have 3 ISA video cards and I'm using an LCD 16x9 1920 x 1080 monitor and I've noticed some faint vertical lines when I load the game Wolfenstein but in dos and other full screen colour programs its fine, on one of the cards a Trident TVGA9000B 512KB its much worse so I thought it might be the monitor so I tried an old LCD 4x3 monitor (I cant remember its max resolution) and its the same. I looked in various system tools like Aida16, MSD and Sysinfo and they say its using video mode 3, number of columns 80 and number of rows is 25 would this be why it has trouble with an LCD monitor and instead should be run on a CRT monitor or is it that they are just dying? I know the monitors are working fine especially with PCI video cards.

I put two pictures one bad and one faint although it might be hard to see the faint lines but on the monitor its noticeable.

It's an LCD thing. Mine does it on a lot of games as well, including Darklands. Once I realized it wasn't a problem with the graphics card or RAM and is simply some strange artifact of the LCD/VGA interaction I stopped worrying about it and went back to playing games.

Reply 6 of 6, by maxtherabbit

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clb wrote on 2023-07-26, 10:20:

Here is another thread of the topic: Re: Weird lines with Trident 8900CL ISA VGA card

This artifact is referred to as the "jailbar" effect. The issue occurs to my understanding when the LCD display circuitry incorrectly locks on to the horizontal total clock count of the video signal. This effect can be reproduced for example with the OSSC scaler by deliberately setting the horizontal total clocks count to a slightly incorrect amount. When the correct horizontal clock count is in use, the artifact will magically disappear.

For example, here is a 640x480 resolution picture from OSSC when incorrect horizontal line counts are used, and for reference a digital DVI-D output of the same image.

This is correct, and to add to it:

The reason the monitor samples this mode incorrectly is that the total line count and horizontal refresh rate between the 720x400 text mode and the 320x200 (640x400) graphics mode are identical. The only difference is the pixel clock (900px/line vs 800px/line) which the display has no means to "detect". So LCD monitors choose to sample correctly for sharp text. This can all be sidestepped by adequately oversampling the signal which is why not all LCDs show the issue.

Then there is also a totally different potential source of jailbars: high frequency noise present in the video signal. Most LCDs have a LPF in the input stage to address this though.