| Many (or all?) nVIDIA GeForce and TNT -based cards have the above circuit
onboard. It is a low pass filter and it's purpose is to reduce RFI emissions.
In many cases it's manufactured with low quality components and / or designed
badly. If 'misdesigned' it basically stops most of the high frequency information
necessary to produce, for example, sharp text at high resolutions and refresh
rates.
To produce decently sharp image at 1600x1200@75Hz you need at least
150MHz of videobandwidth. That is half of the pixelclock frequency, called
also as RAMDAC speed. However, on some videocards the RF filter on the
VGA outputs (diagram above) is designed so that it restricts the videobandwidth
to even as low as 60...70MHz. This is unacceptable because these videocards,
in case of GeForce2 GTS / MX, are advertised to have 350MHz RAMDACs and
capable of resolutions of 2048x1536 at refresh rate of 75Hz. |
You can use this table as a guideline what
should be possible with you monitor and videocard. These numbers are approximate
only, they may vary ±10% depending on operating system and the videocard
used.
The videobandwidth of your monitor is often
found in the technical specifications part of your monitors manual.
Videobandwidth of a videocard is generally
not told to the public - because it would tell the customer too much about
the real quality of the product, and that is something the marketing people
don't like us to know... You should except that a videocard has bandwidth
something near half of the maximimum RAMDAC frequency. |
| Resolution |
Refresh Rate |
Video Bandwidth |
RAMDAC clock |
| 800x600 |
75Hz |
40MHz |
80MHz |
| 800x600 |
85Hz |
45MHz |
90MHz |
| 800x600 |
100Hz |
50MHz |
100MHz |
| 1024x768 |
75Hz |
60MHz |
120MHz |
| 1024x768 |
85Hz |
70MHz |
140MHz |
| 1024x768 |
100Hz |
80MHz |
160MHz |
| 1280x1024 |
75Hz |
100MHz |
200MHz |
| 1280x1024 |
85Hz |
115MHz |
230MHz |
| 1280x1024 |
100Hz |
130MHz |
260MHz |
| 1600x1200 |
75Hz |
145MHz |
290MHz |
| 1600x1200 |
85Hz |
165MHz |
330MHz |
|
If you have a blurry image on resolutions that
should be crystal clear on your monitor:
Check that your monitor has enough videobandwidth
Make sure your vga cable is sufficiently high quality
Test the system without any additional devices on the vga cable -- connect
the cable from your monitor directly to your videocard
If possible, verify that your monitor and cabling is up to task by trying
some other videocard that is known to have sharp picture quality (matrox
G200/G400 for example)
|
| If according to these tests the cause of blurriness
is the nVIDIA -based videocard I advice you to return it, or at least complaint
about it, to the manufacturer.
The following contains some guidelines for fixing the problem yourself
by modifying the videocard. |
Prolink Pixelview GeForce 2 MX
| Please read this through also in case of other cards, it contains some
necessary information. |
|
GeForce2 MX nVIDIA reference based boards.
These include: Asus V7100, Gainward CardExpert GeForce2 MX, Leadtek
Winfast GeForce2 MX, Sparkle SP6800 GeForce2 MX and many others.
Elsa Gladiac MX has also similar filter configuration. |
|
GeForce2 GTS nVIDIA reference based boards.
| These include: Asus V7700, Chaintech GeForce2 GTS 64MB, Creative Labs
3D Blaster Annihilator 2, Elsa Gladiac GeForce2 GTS, Guillemot Hercules
3D Prophet II, Leadtek WinFast GeForce2 GTS, MagicPro GeForce2 GTS 64MB
and many others. |
|
Hercules 3D Prophet II MX
| This one differs a bit from the reference design, but can be modified
as well. |
|
|
| WARNING : the following
contains instructions to modify low-pass filtering of some videocards.
This requires soldering of Surface Mount Devices. If you haven't done SMD-work
before don't practice with your videocard - the lessons might be expensive!
Have an experienced person to do it for you with correct equipment if you
have the slightest doubt about this.. or don't do it all.. Modifying your
videocard may destroy it and void any warranties. Modified videocard may
also cause radio frequency interference (RFI) exceeding your local regulations.
I take no responsibility of any damage caused. |
|
|
| Prolink Pixelview GeForce
2 MX |
| Below you see a closeup near the VGA connector of the Prolink Pixelview
GeForce 2 MX. This card, in my experience, suffers very badly about image
blurriness above 1024x768@75Hz, which is equivalent of videobandwidth of
only ~65MHz. The card (like any other MX card) has a 350MHz RAMDAC, requiring
at least 175MHz of video BW to usable sharp image quality on all supported
resolutions.
Many users have experienced similar problems, in varying degree, also
with other GF2MX cards like Asus7100, Elsa Gladiac MX, Hercules Prophet
II MX and so on. GeForce 2 GTS cards generally have better videobandwidth
than the GF2MX ones, but this fix will most likely improve also the image
quality of GTS-cards, at least in resolutions of 1600x1200 and up. |
|
| The videobandwidth is limited by the components indicated in above
image. Removing the capacitors, marked with red, and shorting the inductors
(soldering a piece of wire between the ends), marked with blue will disable
this filter totally. Doing so improved image quality dramatically in my
case. I left the rightmost capacitors in place (3 of them) .. removing
them may improve the picture in the case of long monitor cabling.
The part numbers on this card are:
1st row: C84, L10, C85, L11, C86
2nd row: C101, L12, C102, L13, C103
3rd row: C111, L14, C112, L15, C113 |
| I removed C84, C101, C111, C85, C102, C112 and shorted L10, L12, L14,
L11, L13, L15 and left C86, C103, C113 in place. With this modification
the videobandwidth, estimated visually on monitor, is at least over 120MHz
giving a sharp image at 1280x1024@85Hz. I Can't test any higher because
my monitor is the limiting factor above this but I guess that 1600x1200@75Hz
should not be bad (It would require VideoBW around 145MHz). This mod caused
no RFI problems for me. I have a radio (with antenna), a TV/VCR set and
image scanners under 2 meters distance from the computer and they all work
as well as before.
addenum: I've also removed the rest of those capacitors,
resulting significant image quality improvement at high resolutions such
as 1280x1024@85Hz. |
| If you like to try the mod in phases do the following, testing the
card for image quality between each stage:
remove C84, C101, C11
short L10, L12, L14
remove C85, C102, C112
short L11, L13, L15
remove C86, C103, C113
|
Or if you like to tryout a quick test:
Use 3 wires, as short as possible, to connect
leftmost pad of L10 to rightmost pad of L11
leftmost pad of L12 to rightmost pad of L13
leftmost pad of L14 to rightmost pad of L15
After this you can go on by removing C84, C101, C111 and then C86,
C103, C113. And if you wan't, also C85, C102 and C112. If you _really_
don't like to remove any components from your videocard it is quite safe
to leave all the capacitors in place.
Some people tell me they have had success also using conductive paint
to short these inductors. Just put a layer of that on top of the inductor
so it covers the soldering areas of the inductor at both ends.
Shorting the inductors will skip the filter, but the capacitive loading
on the RAMDAC output will be higher - that will most likely continue to
have some negative effect on image quality.
Note that the modification is not nearly as effective if the capacitors
are left in place compared to that they are removed.
If all of the 9 capacitors are removed and the inductors left untouched
the image quality will most likely to be much better compared to just shorting
the inductors and leaving the capacitors in place. |
| Other GF2 MX -cards (and even other nNVIDIA based cards) are often
quite similar to this one. The parts to modify might not be exactly on
the same location, but a skilled person should find them easily...
Note: The really correct way to do this mod should
be to find new component values for the L and C of the filter in order
to make it cut-off frequencies just above the videobandwidth you will use.
This would maximize the image quality and give lowest possible RFI. They
should have done so already in the factory....
If you are really into hardware modifications, you can convert this
card into a Quadro2 MXR, See here |
|
