GeForce/TNT Image Quality Problems Due to RFI Filtering

Ultimo aggiornamento: Friday, 3 May, 2002 18:32

Some new info regarding the effectivity of different modifications
24-Sep-2000  - "The Fix" -section updated

Not happy with the image quality of your TNT/GeForce -videocard?
The cause might be inproperly designed RF filter -- many nVIDIA -based cards have this problem.

Contents: The Problem

The Fix

The Problem

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.

    The Fix
    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 or here

    GeForce2 GTS/MX boards
    by nVIDIA reference design

    On the left you see a closeup near the VGA-connector of a typical GeForce 2 GTS/MX card. There has been reports that many of these cards suffer from inferior image quality.

    The biggest difference for Pixelview GF2MX seems to be the protection diodes, located left from the rightmost inductors. They are black cases with 3 leads each. These won't affect the image quality and they protect the nVIDIA GPU from voltage spikes, so I advice to leave them alone.

    Modifications on these cards can be done same way as to Pixelview GF2MX described above. The part numbers will of course be different. Just use the relative locations of the parts marked on the pictures as a guide.

    Note that if you in this case bypass both of the inductors with single wire, you are degrading the protection provided the diodes a bit. This can be prevented by bypassing each inductor individually. But then again, some cards don't have the protection diodes at all so this may not be necessary.

    Hercules 3D Prophet II MX

    There has been this fuzzy image problem with this card too, but in somewhat lesser quantity than the others. I previously recommended this board for the best image quality for a GF2MX board, but now I have to say that if 2D image quality is important to you, the only way is to first test the card before you buy it.

    The filter in this card is completely at the left side from of protection diodes.

    Other videocards (nVIDIA or any)

    If you have experiences modifying some other card(s), send me the info and I'll put it here.