VOGONS


First post, by Deksor

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Hi !

As some of you may have noticed, the video quality of old video cards can vary to pretty good to absolutely mediocre.

But is it just "how it is" or can it be fixed ?

There are two major things involved into generating an image on these old video cards and that's the RAMDAC and the analogue circuit itself.

As root42 has shown here https://m.youtube.com/watch?v=9gROqRlejwA, changing the RAMDAC proves to make some changes in the way the card reacts to graphical effects and other things

For the analogue circuit it depends of the card and I don't have the knowledge to figure out what may or may not be wrong but I know I have a pretty good specimen for experimenting.

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Here's a trident tvga8800br which must have one of the worst video quality I've ever seen on a video card.
Not only it suffers from various video artifacts, but there are also graphical glitches caused by the ramdac itself.

And I think that if I can get this one to look decent and share it with you, any card with a removable ramdac could be improved.

Some of the colors were simply not stable in the game pinball fantasies with the original ramdac (bt Bt476KP50).
By swapping the ramdac for another one salvaged from another card this problem was fixed.

However there are other problems left.
In some graphic modes there are jailbars and the brightness flickers a lot.

Obviously I tested another video card (I even used the same ramdac) and there was no such thing, so nothing in my setup is to blame except something in the card itself which must be in the analogue circuit.

To be fair I think I'm going to make a ramdac comparison first to check which model(s?) are the best.

I know the original chip used by IBM was an inmos chip, but having multiple sources to replace vintage chips will definitely help with the availability.

For the analogue circuit, I don't know. My knowledge in electronics is pretty basic
One thing I noticed looking at the trident card is that there are no capacitors connected to the rgb output whereas one of my old S3 cards does have three little ceramic caps. Maybe adding caps there would help with the flicker and the jailbars ? Maybe not ? Looking at the datasheet of the RAMDACs, none of the circuits I've seen on my video cards seem to match so I don't know how it's really supposed to be. I'll need help for that part.

I can't shoot photos right now to illustrate the things I've been dealing with so far but I will do later so stay tuned 😉

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Reply 1 of 12, by Tiido

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To improve analog video quality, you can remove all inductors and capacitors from between RAMDAC output and VGA connector, providing straight unfiltered signal path. Sometimes there are resistors and diodes but those need to stay in place. This can make blurry looking image nice and sharp again and certainly will make a difference at higher resolutions.

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Reply 2 of 12, by kdr

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Deksor wrote on 2021-04-29, 23:45:
Here's a trident tvga8800br which must have one of the worst video quality I've ever seen on a video card. Not only it suffers f […]
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Here's a trident tvga8800br which must have one of the worst video quality I've ever seen on a video card.
Not only it suffers from various video artifacts, but there are also graphical glitches caused by the ramdac itself.

And I think that if I can get this one to look decent and share it with you, any card with a removable ramdac could be improved.

Some of the colors were simply not stable in the game pinball fantasies with the original ramdac (bt Bt476KP50).
By swapping the ramdac for another one salvaged from another card this problem was fixed.

Nice work!

There must be a reason that so many of the early 16-bit SVGA cards feature a socketed RAMDAC.

As I understand it, the speed of the RAMDAC and the speed of the video memory clock are the two factors that dictate how fast you can run the dot clock. When you did the swap, did you end up using a RAMDAC specced for a higher frequency?

What are you using to display the video output? I've had cards that look awful on LCD panels but which are flawless when driving an authentic SVGA CRT.

SVGA cards are often running the highest frequency clocks in an entire 486 system. Have you checked the tantalum capacitors on the card to make sure they're within spec? Glitches on the power supply to the RAMDAC could certainly produce all kinds of weird artifacts.

Reply 3 of 12, by Deksor

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Thanks !

Interesting tips about the sharpness. Though this card isn't particularly blurry, it's just looking bad for other reasons (jailbars and flickering).

This also explains why the schema I've seen in many RAMDAC is quite different (maybe replicating that circuit would lead to the best quality ?)

I haven't checked the tantalum caps more than "at least they don't explode"

Yes the RAMDACs used were faster (original is 50mhz, swapped ones were 66 and 80mhz) but I don't think this is truly important for the quality. It's more for the maximum resolution and refresh rate than quality I think (problems I've seen were more related to palette operations than actual output quality, at least on this card).

I do use a LCD as video output but that's on purpose. I think it's possible to make them look good anywhere 🙂

I'll use a CRT as well to see how the image looks on there but even if it'll probably look better I don't expect it to look really that good.

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Reply 4 of 12, by Deksor

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Here's some footage I've taken from this video card so you can see the glitches :
https://youtu.be/0xNpb7vTQmk

The text mode is ok though

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However the flickering is present even in text mode and as I used the computer, the video signal went more and more dim. I don't know what's up with that card, I checked the tantalum caps and they mostly looked fine (one was 50µf instead of 22 for some reason but I don't see how that's supposed to make a difference in the brightness of the signal ?)

Here you can also see the "jailbar" effect that's only seen in the game (which is in high res mode by the way)

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Actually most of the artifacts are only seen in high res mode.
The issues with the colors is only seen in the game, not in text mode (being a VGA card from 1989 with only 128K of RAM maybe they didn't expect it to access these modes ?) and they're fixed using a different RAMDAC.

The issue with the jailbars is present whatever the RAMDAC is.

And finally the flicker and image becoming more and more dim is present everywhere, no matter what the RAMDAC is.

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Reply 7 of 12, by canthearu

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I looked a little into that on a S3 VLB 964 graphics card.

It looks like the RAMDAC on this one produced it's own ramdac Vref. I was hoping to boost the signal slightly to maybe allow LCDs to get a better lock on the signal to reduce the jailbar effects, but I'd have to mangle the card considerably to disconnect the internal vref and supply a new one.

Reply 8 of 12, by weedeewee

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Deksor wrote on 2021-05-03, 10:30:

Maybe, I don't know. How can I check that ?

mmmh, from the datasheet, the 28pin package only has an IREF pin.
https://www.datasheets360.com/pdf/-1791301427568322710
Just a guess though, oscilloscope the signal ?

Reply 9 of 12, by Deksor

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The problem is, if that frequency is the same as the one rated on the ramdac (50mhz ?) Then I can't probe it as my scope maxes out at 20MHz.

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Reply 10 of 12, by Tiido

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The jailbars effect on LCDs and other digital devices is supposed to be due to mismatch between pixel clock the LCD expects and what is actually used by the card. Many of these cards have PLLs that do not produce exact VGA/SVGA etc. spec clocks, some have no PLL at all and create the pixel clocks by dividing some value they have around and fudge timings so that good resolution is achieved. It is why many cards have those wonky refresh rates at various resolutions.

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 11 of 12, by weedeewee

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Deksor wrote on 2021-05-03, 11:39:

The problem is, if that frequency is the same as the one rated on the ramdac (50mhz ?) Then I can't probe it as my scope maxes out at 20MHz.

erhm... since it's current reference, or voltage reference, I doubt there will be any frequency component to it. I just mentioned oscilloscope because it gives an easy continuous visualisation of the signal.

Reply 12 of 12, by maxtherabbit

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Tiido wrote on 2021-05-03, 13:23:

The jailbars effect on LCDs and other digital devices is supposed to be due to mismatch between pixel clock the LCD expects and what is actually used by the card.

Correct, but the problem is not necessarily that the cards are outputting an off-spec signal. The pixel clocks between 720x400 (VGA Text mode) and 320x200 (double-scanned low res graphics mode) are different despite the line counts being identical. Nearly all LCD monitors sample at 900px/line to get sharp text in text mode, but you then end up with jailbars on DOS games which use the low res graphics mode - since it should be sampled at 800px/line and the monitors have no way to distinguish which mode is being used due to identical line counts.