If you have VSync disabled, the two cards will be rendering different frames some of the time. This makes the interleaved alternate scanlines very apparent, particularly in scenes with lots of horizontal motion.

For best image quality, have VSync enabled when using SLI. You generally only want to disable it if you're running benchmarks.

Don't think it is V-sync related. Scanlines are very apparent even with static imagery, like a main menu. Not in all games though.

Doesn't seem to be anything wrong with the cards individually either, as they work fine swapping the cables between the different cards.

Hmm, I found a post suggesting that this could be down to the framebuffer on each card being synchronised to a different RAMDAC clock, even though only one RAMDAC is used for video output in SLI- Re: 3Dfx Voodoo 2 PCB SLI Bridge / SLI Cable

All part of the charm of SLI, I guess!

Both cards must be either 12MB or 8MB. If the cards have different memory sizes, they can be swapped. If the first card is 8MB, the second one will only use 8MB. If the first card is 12MB, the driver will think that the second one also has 12MB and the screen will show interlacing and even places with unfilled textures. You also need to check the SLI cable. If the cards and SLI cable are working properly, the image should be excellent without visible lines, even with fast movements.

My only experience with SLI is using the two 12MB diamond cards I originally bought new back in the day (and still have) but I have never noticed any visible scanlines with this setup

They are a matching pair, both labled as: PCI 3DFXV2 Rev : A1
No idea what brand though.

I played around with things some more, and it seems to have something to do with screen resolution. For whatever reason, 640x480 will not play nice. 800x600 and 1024x768 look perfectly fine, not the slightest hint of a scan-line.

640 x 480 will have scanlines visible on most CRTs, except on small (15" and less) or low end things with a coarse dot pitch.

Is one noticeably darker than the other when it's by itself?

Tiido wrote on 2025-02-05, 08:01:

640 x 480 will have scanlines visible on most CRTs, except on small (15" and less) or low end things with a coarse dot pitch.

Like I said, the picture is completely fine when running off just one Voodoo 2.

myne wrote on 2025-02-05, 08:09:

Is one noticeably darker than the other when it's by itself?

Not as far as I can tell, and they match up just fine at any higher resolution.

A photo of the effect would be helpful

Are you sure it's not an artifact of upscaling the resolution on the screen? (in other words, is it an lcd)

Tiido wrote on 2025-02-05, 08:48:

A photo of the effect would be helpful

I've tried, but unfortunately the cameras noise filter and JPG compression destroys the effect. I can only describe it as "enhanced scan-lines".

I did a little more methodical testing though by manually setting the white balance and locking down the shutter speed and ISO and switching around outputs on the cards, and if the camera is correct it seems the one card outputs a "slightly" brighter image to the monitor, at resolutions 640x480 and below. So I guess that could probably account for the noticable scan-lines.

Why the anomaly does not occur at higher resolutions, I couldn't even guess.

myne wrote on 2025-02-05, 10:08:

Are you sure it's not an artifact of upscaling the resolution on the screen? (in other words, is it an lcd)

I believe I mentioned i was using a CRT in my original post. 😉

Why do you make the resolution 640x480 for Voodoo2 SLI?
To increase performance?
I just tried 3DMark-99 on Asus P2B/PIII-500/256MB Voodoo2 SLI
Game1,Game2:
640x480 - 4556
800x600 - 4541
1024x768 - 3841
In Quake2 and UnrealGold 640x480 I saw interlacing only on some fast-moving textures.
I think making the resolution 640x480 for Voodoo2 SLI is not that critical.

Postman5 wrote on 2025-02-05, 12:04:

Why do you make the resolution 640x480 for Voodoo2 SLI? To increase performance? I just tried 3DMark-99 on Asus P2B/PIII-500/256 […]
Show full quote

Why do you make the resolution 640x480 for Voodoo2 SLI?
To increase performance?
I just tried 3DMark-99 on Asus P2B/PIII-500/256MB Voodoo2 SLI
Game1,Game2:
640x480 - 4556
800x600 - 4541
1024x768 - 3841
In Quake2 and UnrealGold 640x480 I saw interlacing only on some fast-moving textures.
I think making the resolution 640x480 for Voodoo2 SLI is not that critical.

Because Need For Speed 2: SE doesn't support any other resolution. 😄

I didn't see any interlacing in the game itself, but on the yellow panel that moves to the left after pressing "RACE" there are indeed jags on the border

FIXED IT!!! 🥳

Remembered i've had some difficulties in the past with oxidation/contacts on the PCI slots on this old motherboard. So I tried reseating and re-connecting the cards a couple of times, and it cleared right up!

Still don't know why only the lower resolutions were affected though, weaker signal? 🤔

Digital signals are either good enough to reliably read a 1 or 0, or shit crashes constantly.

So... No.

But you pulled the analogue vga cable out, and reinserted it.
That's more likely to make an observable change.

Doubtful, since I've already been pulling it out and reinserting it like a dozen times. And as mentioned, the issue was exclusive to SLI-mode.

Maybe some SMD component with a crack or soldering defect. You can lightly tap on the working boards with a pencil or a wooden stick and see if there is any change in the image. This way you can even localize the defect.