DVI to VGA Question

Discussion in 'Mac Basics and Help' started by MM07, Nov 15, 2008.

  1. MM07 macrumors 6502a

    Joined:
    Feb 10, 2008
    #1
    I just got a new 19" monitor for my MacBook a few weeks ago. Since I was in a hurry I bought the VGA to mini DVI Apple adapter. Things have been working out great so far.

    I then realized that my new LCD has a DVI input. So, I bought a DVI to VGA adapter and a DVI cord.

    This is how it goes:

    DVI cable into LCD. DVI cable to DVI side of adapter. VGA side of adapter plugged into Apple VGA to mini-DVI adapter to MacBook.

    But it doesn't work. I know I should of bought the DVI to mini DVI, but I didn't look to see that my new LCD had DVI. ;)

    Any ideas on why this won't work?

    TIA
     
  2. dukebound85 macrumors P6

    dukebound85

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    #2
    you cant down scale to vga and then upscale back to dvi without using a analog digtal coverter (such as this). even if you could, the quality in this "setup" would be worse than the vga alone

    just use vga input or get the mini dvi to dvi dongle
     
  3. MM07 thread starter macrumors 6502a

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    Feb 10, 2008
    #3
    I appreciate it. I guess I'll have the spend another $19.

    Is it really worth it to use DVI as compared to VGA for this set up? Would I really notice a difference?
     
  4. dukebound85 macrumors P6

    dukebound85

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    #4
    well i cant really tell a difference between my two monitors (one using vga and one using dvi)
     
  5. mkrishnan Moderator emeritus

    mkrishnan

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    Location:
    Grand Rapids, MI, USA
    #5
    Agreed -- the difference between VGA (or even component for HT apps) and DVI/HDMI is minimal in most situations.

    Downscaling is the wrong way to think of it.... it has nothing to do with resolution or sampling frequency.

    What's happening is this:

    The mini-DVI port on the Mac carries a digital signal (DVI-D) and an analog one (DVI-A). Together, these are known as DVI-I. When you convert mini-DVI to VGA, you lose the DVI-D signal. Your display is probably designed for cost savings to only accept DVI-D on the DVI port, even though it can obviously accept the analog signal when it comes in the VGA port (this is pretty common on displays). So when you convert back to DVI, even though the DVI cable can carry both the A and D signals, it's only carrying the A signal, because the D was lost when you converted to VGA.

    This is essentially the same reason that Mini-DVI to DVI to VGA doesn't work -- the Apple Mini-DVI to DVI cable only outputs the DVI-D signal, so there's nothing to feed into the VGA cable.
     
  6. MM07 thread starter macrumors 6502a

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  7. dukebound85 macrumors P6

    dukebound85

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    #7
    i agree, downscale is not the right term. although correct me if im wrong, but if you had some dac's and adc's it would work right?
     
  8. mkrishnan Moderator emeritus

    mkrishnan

    Joined:
    Jan 9, 2004
    Location:
    Grand Rapids, MI, USA
    #8
    Huh, maybe... it would be a major pain, but possibly. And plus it probably would lead to some signal degradation. The adapters aren't converting back and forth between the analog signal and the digital one. They're both being produced at the card and just passed through.

    The small amount of signal degradation when you use VGA or component arises because you start with a digital device (video card), convert to analog, transmit, and then reconvert to digital so that it can be displayed on the inherently digital LCD or Plasma screen. But it's pretty minimal. Nonetheless, if you do an extra round of conversion, I guess it'll be a little worse.
     
  9. SnowLeopard2008 macrumors 604

    SnowLeopard2008

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    Location:
    Silicon Valley
    #9
    I'd return the first adapter and exchange for the mini-DVI to DVI. It's the adapter I have for my 24" Samsung. Just tell them that your display only had DVI and you thought it had VGA.
     

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