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Calculating Video Bandwidth/Signal Frequency

Discussion in 'General Hardware' started by TIGR, Mar 7, 2011.

  1. TIGR

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    Trying to clear up a number of things here.

    I want to determine the exact meaning of the listed bandwidth of products like this (lists 400MHz bandwidth/2048x1536 @ 72Hz). Other products (example) list a "pixel frequency". Still elsewhere I've seen "pixel bandwidth" and "signal frequency".

    So firstly, are the terms "pixel bandwidth", "pixel frequency", "signal bandwidth", and "signal frequency" interchangeable in the context of analog video signal transmission?

    Intuitively, I thought video signal bandwidth would be simply pixels multiplied by refresh rate. This page, however, says the actual calculation is [(pixels * refresh rate)/2]3.

    Thus, the signal frequency of 72Hz 2048 x 1536 video would be calculated as:

    [(2048 * 1536 * 72)/2]3 = 340MHz

    ....which is closer to the listed 400MHz than the calculation I intuitively expected:

    2048 * 1536 * 72 = 227MHz

    ....but still significantly off.

    So secondly, where does the extra 1.5x factor (result/2*3 from this page again) come from?

    Thirdly, why does it still not match the numbers provided for the first product I linked to? Is it just that 2048x1536 @ 72Hz is the highest "relatively standard" display configuration that doesn't exceed 400MHz? Or are they actually using a different calculation? Because the way I'm calculating it, 400MHz could handle 2048x1536 @ 84Hz.

    I just want a solid understanding of video signal bandwidth as I frequently use custom resolutions and refresh rates, so want to fully understand what these products can handle.
     
  2. Easy Rhino

    Easy Rhino Linux Advocate

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    400MHz is most likely simple marketing. my guess is that there is no standard refresh rate between 72hz and 120hz. so obviously a 120hz refresh rate would significantly reduce resolution is could handle and it most likely is not meant to handle that high refresh rate in the first place.
     
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  3. Easy Rhino

    Easy Rhino Linux Advocate

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    hrm, that is confusing. does this help?

    http://www.csgnetwork.com/videosignalcalc.html

    my guess is that since the calculations are considered conservative a more reasonable number would be closer to but not higher than 400MHz when you take into account filtering and noise? again, just a guess.
     
    Last edited: Mar 7, 2011
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  4. Easy Rhino

    Easy Rhino Linux Advocate

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    yea report back. that is interesting since i dont have much knowledge in the field of electrical engineering.
     
  5. TIGR

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    Reporting back. A user at OCForums (where I also posted this query) posted the following response yesterday (link):

    Generally I wouldn't revive an old thread like this but it's a good explanation for anyone else who comes searching for answers in the future.
     
  6. Aquinus

    Aquinus Resident Wat-man

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    400Mhz is the maximum frequency of the DAC on the video card. Consider for a moment that VGA (that is what you're talking about,) uses an analog signal and the card needs something to make that signal, which the DAC is used for. Not running it at 100% most likely is more to keep the DAC within safe limits within the realm of what it can do which is why you won't see it get all the way to 400mhz output. All in all, as video cards stop supporting VGA I think we will see video cards start ditching the DAC all together which I can't wait for. :p

    Edit: Just noticed that this is an old thread. WHY YOU REVIVE IT?! :eek:
     

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