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KVM switches: which ones degrade VGA the least?

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First post, by keenmaster486

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I bought a Belkin Omniview 8 port rack mount KVM switch from eBay the other day. This one is really nice because it has both serial and PS/2 ports for the mouse, and is able to emulate either one for any connected system no matter which type of mouse you have plugged in.

Using all high quality VGA cables, the signal coming out of it is good enough to look fine on a CRT, but when captured with OSSC and an HDMI capture card, it becomes clear that there is significant "ghosting" going on - the colors smear to the right. There are also vertical lines through the image that shouldn't be there, and the image is too bright. This is all using 320x200 resolution and compared to the raw output from the computer, which looks good on both the CRT and my capture setup.

This is coming from a Western Digital ISA VGA card in a 386.

I've tried swapping out cables, eliminating all other possibilities, and yes, it's the KVM.

Do they all do this? Are there any that don't? I really like that serial/PS2 emulation feature of the OmniView, so it would be nice to get one that has that.

Anyone have any tips for me?

World's foremost 486 enjoyer.

Reply 1 of 3, by Tiido

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Almost all the KVMs I have seen use similar kind of switching mechanism using two transistors (one for termination when input isn't selected so video card still detects "monitor connected" and other to pass signal through) and it will produce an impedance mismatch as there isn't 75ohm termination anymore and transistor base isn't an uniform load as freq rises.

One expensive Adder branded thing uses CMOS switches similar to Voodoo cards and it produced less ghosts and reflections but the switches don't have high enough bandwidth for 1600x1200 and 1920x1440 I routinely pass to my monitor, leading to soft blurry image, especially when refresh rate increases (while direct connection to monitor is nice and clean). Fortunately stacking those switch chips can help as it does with Voodoo cards.

I did find out that cables do play a major role, and that my cheapest looking noname cables produced least image degradation compared to the most expensive ATEN brand ones I had around. Some other good looking cables were producing really awful output with many reflections. I did try to take photos but nothing showed up on the photos, it is mighty hard to photograph a kinescope 🤣

It is in my todo list to make a "proper" KVM but that todo list is endless already and I still lack means to actually produce anything due to my recent move to another country...

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Reply 2 of 3, by Unknown_K

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The top brand name ones with OEM cables seem to do fine unless you need bleeding edge resolution and refresh rates outside of their design. VGA is analog so you have to deal with noise if you run cables close to noisy devices or just have cheap cables.

I suspect whatever you are using to convert VGA to HDMI for the capture device is screwing up your video not the KVM.

Collector of old computers, hardware, and software

Reply 3 of 3, by keenmaster486

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Unknown_K wrote on 2023-04-24, 22:27:

The top brand name ones with OEM cables seem to do fine unless you need bleeding edge resolution and refresh rates outside of their design. VGA is analog so you have to deal with noise if you run cables close to noisy devices or just have cheap cables.

I suspect whatever you are using to convert VGA to HDMI for the capture device is screwing up your video not the KVM.

My capture setup works perfectly when I cut the KVM out of the chain though. It’s an OSSC which is as close to gold standard as we have right now.

World's foremost 486 enjoyer.

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