vga analog output accuracy measuring

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vga analog output accuracy measuring

Postby arncht » 2019-5-09 @ 13:46

hi

is there any util/tool for measuring of the output quality?

if not... :) i have an idea with the capture card and software. i can generate an image with random pixels (with a specified resolution and color depth) or just use a shot from a game, then i load this on the retro computer.
after to capture the image, we could measure many things:
* color accuracy
* brightness checking
* pixel perfect checking
* and changing of the input dac settings - would it possible to calibrate the card - maybe automatically

with this solution for a specified case (eg 320x200 vga, or windows 1024x768) would it be possible to find the best output quality of the:
* card
* cable
* and the best input settings for the card
Socket 3 VLB @ 1994.10
Socket 5 @ 1995.05
Socket 7 @ 1997.09
Slot 1 @ 2000.05
Socket A @ 2003.06
Socket 775 @ 2006.11 - Core 2 Extreme QX6700, Geforce 8800 GTX, Sound Blaster X-Fi Fatal1ty
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Re: vga analog output accuracy measuring

Postby arncht » 2019-5-09 @ 19:25

very-very experimental, i integrated to my capture tool. 100% = perfect image :) actually it calculates continuously the accuracy of the capture.

the funniest, it shows between difference between two cables! not big difference, but 99.99% vs 99.95% at 800x600. i am sure at bigger resolution shows more.
Attachments
sharpness_3.png
sharpness_2.png
sharpness_1.png
Socket 3 VLB @ 1994.10
Socket 5 @ 1995.05
Socket 7 @ 1997.09
Slot 1 @ 2000.05
Socket A @ 2003.06
Socket 775 @ 2006.11 - Core 2 Extreme QX6700, Geforce 8800 GTX, Sound Blaster X-Fi Fatal1ty
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Re: vga analog output accuracy measuring

Postby arncht » 2019-5-09 @ 19:34

oh yes... this is 1024x768 between the two vga cable :) 100 vs 99%. I will do later more detailed tests, if the theory works well.
Attachments
1024 2.png
1024 3.png
Socket 3 VLB @ 1994.10
Socket 5 @ 1995.05
Socket 7 @ 1997.09
Slot 1 @ 2000.05
Socket A @ 2003.06
Socket 775 @ 2006.11 - Core 2 Extreme QX6700, Geforce 8800 GTX, Sound Blaster X-Fi Fatal1ty
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Re: vga analog output accuracy measuring

Postby Error 0x7CF » 2019-5-09 @ 22:04

This would be a pretty good way to definitively show how much the Voodoo VGA passthrough degrades quality.
Old precedes antique.
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Re: vga analog output accuracy measuring

Postby elianda » 2019-5-10 @ 07:49

My Align tool for the Epiphan cards has such features included:

Image

You see black level noise min/max/StdDev, 50% grey level StdDev
Grey bar linearity and stepping (in graph) channel resolved
Estimated sample position on pixel in second graph for a black and white pixel, estimated video bandwidth from 20% to 80% intensity threshold (tunable).
Estimated HSync edge precision.

Is that what you are looking for?
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Re: vga analog output accuracy measuring

Postby arncht » 2019-5-10 @ 08:37

yeah, similar like that... it looks really awesome :) did you do cable tests? i am looking for a good analog vga cable.

i think the datapath api is not too detailed for that, i have to choose a practical way to filter the image problems. eg some videocard looks sharper, but the solid areas are not too homogeneous. i want numbers about the signal loss and i also want to do a quality test for the output filtering / scaling.
Socket 3 VLB @ 1994.10
Socket 5 @ 1995.05
Socket 7 @ 1997.09
Slot 1 @ 2000.05
Socket A @ 2003.06
Socket 775 @ 2006.11 - Core 2 Extreme QX6700, Geforce 8800 GTX, Sound Blaster X-Fi Fatal1ty
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Re: vga analog output accuracy measuring

Postby elianda » 2019-5-10 @ 09:30

I talked to Oerg866 and we think that the same should also be possible with the DataPath cards.

I did some cable tests and you can see the difference. However it is always the combination of card + cable that matters. I think it has to do with impedance matching of both. So one cable might be good for one card but bad for another.
Later PCI cards seem to have been manufactured more precise.

To get usable values you have to optimize the parameters of the capture card before of course. The StdDev of ~1 for grey as seen in the upper picture is already very good considering that the results are always folded with the precision of the capture card.
Retronn.de - Vintage Hardware Gallery, Drivers, Guides, Videos. Now with file search
Youtube Channel
FTP Server - Driver Archive and more
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Re: vga analog output accuracy measuring

Postby arncht » 2019-5-12 @ 17:41

i did more "real world" comparison (it compares in every sec). Left side is the difference to the reference image, right is the capture view.
Attachments
shot2.png
shot.png
Socket 3 VLB @ 1994.10
Socket 5 @ 1995.05
Socket 7 @ 1997.09
Slot 1 @ 2000.05
Socket A @ 2003.06
Socket 775 @ 2006.11 - Core 2 Extreme QX6700, Geforce 8800 GTX, Sound Blaster X-Fi Fatal1ty
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Re: vga analog output accuracy measuring

Postby bestemor » 2019-5-19 @ 15:31

arncht wrote:.... i am looking for a good analog vga cable.

http://www.vogons.org/viewtopic.php?t=25581
(bottom comments)

If you are still looking, I suppose those IXOS ones would be really nice, if you are able to actually get them shipped to you in Austria...(!) They are extremely thick, compared to any other cables I've seen... With no need for any ferrite chokes.
(eBay number: 381669780610)

https://www.amazon.co.uk/IXOS-XHV625-HD ... B002WTE0NU
or directly:
https://www.audiovisualonline.co.uk/pro ... -cable-3m/
https://www.hificables.co.uk/product/16 ... -cable-3m/
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Re: vga analog output accuracy measuring

Postby Jo22 » 2019-5-19 @ 16:52

^Where are the ferrite clamps to block coat waves ? :confused:

Edit: Never mind, saw the links. I'm sceptical that shielded coax is enough.
In my CB radio days I used comparably thick RG58U cables that still had coat wave issues.
Making coaxial rings with the help of ferrite cores greatly reduced noise and other issues.

Edit: In English language, the issue is called "Sheath current" it seems.
https://en.wikipedia.org/wiki/Sheath_current
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In what to one race is no time at all, another race can rise and fall..." - The Minstrel

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Re: vga analog output accuracy measuring

Postby retardware » 2019-5-19 @ 17:36

...and don't forget the DAC rating.
If this is an ISA VGA, you might have a DIP DAC.
These exist in 35, 50, 66 and 80MHz versions (usually visible in the end of the type number).
But there are also bad manufacturers, so there are variations too.
Quite often back then I took a good DAC from a bad VGA to improve display quality even of cheapo cards with cheapo DAC.
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Re: vga analog output accuracy measuring

Postby Jo22 » 2019-5-19 @ 18:23

^It's also worth to note that VGA cards (or video devices in genral) were the victim of local RFI/TVI regulations.
If memory serves, some organizations like FCC or DBP didn't allow high bandwiths (~200MHz was the limit ?).
So if a DAC was performing too good, either the VGA card or the whole PC wouldn't get an approval (remember FTZ/BZT stickers ? ;) ).
"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

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