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Solved: Switched to dvi-d and now only 60hz.

Discussion in 'Hardware' started by Dude500, Oct 7, 2008.

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  1. Dude500

    Dude500 Thread Starter

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    I switched from dsub to dvi-d for my monitor, and added a secondary monitor. My monitor is a 19" lcd, i had it running at 75hz but now it will only let me do 60hz. it is at max resolution. Is dvi only capable of 60hz or what.
     
  2. fairnooks

    fairnooks Banned

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    It's not really analogous on a LCD monitor compared to the old CRT monitor since LCD remains at a constant luminance until it is affected to change by the refresh rate. So instead of needing to be updated 60 times a second or whatever to maintain an image, the screen pixels only change on a need to change basis which might skip hundreds or thousands of refresh signals before one affects some pixels. So you can see that refresh rate is far less important for an LCD screen--its how fast it can change once it gets the signal to change that's important and when response time was commonly 25 milliseconds many people could see some smearing and bluring and ghosting in gaming when the action was very fast.

    In some small percentage of activity I suppose the change could be so fast that there would be a benefit to a higher refresh rate on LCD, just doesn't seem to be any great call for it on a consumer or technical basis though I think there is some movement to or experimentation with 120hz refresh rates. Perhaps response time is getting so good its warranted or maybe there's another reason for that.

    I think the higher compatible refresh rates on non-DVI hookups is a legacy effect from analog, whereas digital doesn't have that legacy.
     
  3. fairnooks

    fairnooks Banned

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    From Tweakguides: explains it better than I did and indicates there is even more legacy effects from using CRTs than I thought;

    Response Time vs. Refresh Rate



    Ok, so if an LCD monitor doesn't refresh itself many times a second, why does an LCD monitor still require a particular refresh rate setting in Windows? We previously discussed the fact that refresh rates are something only a CRT monitor needs because of the way it physically operates. Well it appears that LCD monitors need to emulate a refresh rate in Windows primarily for compatibility purposes with games and hardware. Games, Windows and your graphics card are all still designed around composing individual frames in the frame buffer, and sending these whole frames to your monitor one by one, with the timing for buffer flipping typically based on Vertical Blank Intervals - all things which were originally required for CRT monitors. Therefore LCD panels have to try to operate on the same basis, despite the fact that they don't have the same physical limitations of a CRT.



    Let's look at an LCD's theoretical refresh rate, based on its response time rating. Consider the example of an LCD monitor nominally rated at an 8ms response time. Given 8 milliseconds is 8/1000ths of a second, in one full second it can refresh all the pixels on the screen (if necessary) 1000/8 = 125 times, which makes it equivalent to a 125Hz refresh rate. Yet no 8ms LCD monitor allows you to set a refresh rate even remotely close to this in Windows, nor do even 4ms LCD monitors. There are two main reasons:



    1. Connection Limitations: A single DVI digital connection is like an Internet connection, it has limited bandwidth for digital graphics data; not enough to allow higher than 60Hz refresh rate at full 24bpp Color Depth for all resolutions, so typically all resolutions on LCD monitors using DVI are capped at 60Hz. Some LCD monitors using DVI do allow higher refresh rates, though the absolute maximum possible is 85Hz at 1280x1024. If you use a VGA analog connector instead, you can often select a refresh rate higher than 60Hz on an LCD, though again nowhere near the theoretical refresh rate limit based on your response time, partly because of the reason below.



    2. Monitor Limitations: LCD manufacturers want to ensure that their monitors function satisfactorily in all situations, particularly since they often overstate response times. So typically they set the maximum supported refresh rates on their monitors such that they are relatively conservative and can meet the challenge of refreshing the entire screen as often as required in any situation without any ghosting. Furthermore, setting too high a refresh rate on an LCD, even if it's available, can actually result in problems in certain games and applications due to timing issues. So for reliability and compatibility purposes, LCD refresh rates are not as high as they could theoretically be.



    As you can see, there are a few factors involved in why your LCD monitor may not provide a refresh rate as high as you might expect given its response time. The underlying reason however is that current graphics software and hardware is designed around compatibility with CRT monitors, and as such, LCD monitors are limited in some respects by having to emulate the same process.



    Refresh Rate Issues on LCDs



    An LCD monitor does not flicker, so for the purposes of reducing eye strain it doesn't really matter what refresh rate is chosen. The default of 60Hz in Windows is perfectly fine in that regard. In fact when using a DVI connection, you may be restricted to a 60Hz refresh rate regardless of resolution, so you may not have any choice. An LCD using a VGA connection however usually gives you the choice of a higher refresh rate, but most manufacturers may still recommend 60Hz as the most stable and compatible choice.



    The main reason why the refresh rate on an LCD may matter in gaming is because of VSync - which is discussed in greater detail in the Vertical Synchronization section. The simple fact of the matter is that LCD monitors have to work on the basis of receiving new frames of information from a graphics card's frame buffer like a CRT would: i.e, during the VBI. So when VSync is disabled the graphics card will sometimes race ahead and when the LCD monitor indicates it is ready for a new frame during the blanking interval, the graphics card may provide a partially new frame overlapping an older one, just like it would for a CRT. An LCD monitor will then display this just the same way a CRT monitor would, resulting in visible tearing. The alternative of enabling VSync can resolve this, but in turn can reduce FPS to a fraction of the refresh rate. The lower your refresh rate, the greater the performance drop, which is why a 60Hz refresh rate on an LCD may be problem.



    Therefore LCD monitors, despite not actually physically working on the same basis as a CRT, wind up being bound by the same limitations and problems - minus the flicker - because they operate in a software environment originally designed with CRTs in mind.
     
  4. Dude500

    Dude500 Thread Starter

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    thanks good info
     
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