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Thread: Minimum dot clock, Radeon HD3200 (780G), TV SCART VGA cable

  1. #1
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    Nov 2008
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    Default Minimum dot clock, Radeon HD3200 (780G), TV SCART VGA cable

    Hello,
    I have the Gigabyte MA78GM-S2H motherboard with an integrated 780G chipset
    and Radeon HD3200 graphic card. The card work perfectly under Linux with both fglrx and open ati drivers. However, I would also like to connect a TV through a
    VGA<->SCART cable (http://www.mythtv.org/wiki/index.php/RGB_Scart). The following Modeline works well with an old graphics card (ATI Mach64) in the same computer:


    ModeLine "720x576@25i" 13.5 720 732 795 864 576 581 586 625 interlace -hsync -vsync

    so that I know that the modeline and cables are OK. However, for the Radeon HD3200, I am having problems setting the dotclock to a sufficiently low value. The fglrx driver refuses to do it (saying that the dot clock is too low), the open ati driver
    pretends to honour the "ForceMinDotClock" but probably does not set the frequencies correctly because the TV cannot sync, I only get moving bands.

    Do you know if that is a real hardware limitation that the Radeon HD3200 cannot generate such low dot clock frequencies? Or is it just software? Would you know of some other way to circumvent it?

    I eventually want to buy a HD TV with a HDMI input but at the moment I would like to use our old TV, so I need the low-resolution / low-frequency TV modes. And the old Mach64 seems to be too slow for full-screen video.

    Thank you very much for your help,

    Jan

  2. #2
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    Nov 2008
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    Default

    I am running my HD3200 using a VGA->SCART adapter connected to my TV. I had to fix two things to get it running:

    1. I changed the modline settings from -hsync -vsync to +hsync +vsync

    2. I used a more advanced VGA->SCART adapter I had build for a nvidia card. I never managed to get the simple ATI VGA->SCART converter to work.


    This is my ModeLine (but most examples you find on the internet works if you change -sync to +sync)


    ModeLine "768x576pali" 14.76 768 789 858 944 576 580 583 625 +hsync +vsync interlace


    What is your result running fgl_glxgears and glxgears on HD3200? My results are around 300 for both, which I think is to low. I am running the latest driver from ATI.

    /Melfon

  3. #3
    Join Date
    Nov 2008
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    Default

    Thank you. I will try your modeline when I get to my HTPC computer.
    So you are using the fglxrx drivers? Can you tell me which version?

    Yes, I have also built the more advanced interface with a transistor.

    I do not remember the exact glxgears values but it was around 2000fp/s
    (for the default window size).

    Jan

  4. #4
    Join Date
    Sep 2006
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    Default

    Try one of the modelines from:

    EDID Modelines Direct From Your TV/Monitor (The Collection).

    http://www.phoronix.com/forums/showthread.php?t=13931

    I found two, although you probably want the line for an interlaced screen:

    # 720x576 @ 50 Hz (EDID) HSync: 31.25 kHz
    ModeLine "720x576" 27.00 720 732 796 864 576 581 586 625 -HSync -VSync

    # 720x576 @ 50 Hz (EDID) HSync: 15.625 kHz
    ModeLine "720x576" 13.50 720 732 795 864 576 580 586 624 -HSync -VSync Interlace
    Last edited by Jade; 11-24-2008 at 08:06 AM.

  5. #5
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    Nov 2008
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    Default

    Thank you for all the modelines. Just to clarify things:

    - I cannot get a modeline directly from my TV, because it is an old TV which I want to connect through a VGA<->SCART cable. The TV has no digital interface (EDID).

    - my primary problem is not an incorrect modeline, as I have verified with another card and driver that the modeline works.
    My primary problem is that the ATI fglrx driver refuses such a low dot clock and the open source (radeon) seems to ignore the ForceMinDotClock directive.

    Jan

  6. #6
    Join Date
    Sep 2006
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    Default

    Quote Originally Posted by kybic View Post
    My primary problem is that the ATI fglrx driver refuses such a low dot clock and the open source (radeon) seems to ignore the ForceMinDotClock directive.

    Jan
    The drivers are often "legalistically" sabotaged.

    For instance the nvidia driver used to work with some screen sizes that were not a multiple of 8. Then, someone invented the rule that screen sizes that were not a multiple of 8 were not allowed and the driver was redesigned so that it would not work for screen sizes that were not a multiple of 8.

    Same thing here. The low dot clock only became a problem when someone decided to make it a problem. Its now a feature of the driver.

    Ignoring the ForceMinDotClock directive is another "feature." You may find that the directive name has changed, so that this "feature" seems more natural.

  7. #7
    Join Date
    Jun 2008
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    Default

    Hi,

    I don't think that the fglrx driver has the "forcemindotclock" option.

    The opensource driver does have this option. However, not all ATI cards support this possibility.

    By the way, does interlace work for you ? It seems to be broken in the most recent version of the opensource ati driver.

    Greetings,
    Steven

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