Question about VGA and DVI

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Ozkar
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Question about VGA and DVI

Post by Ozkar » Sun Apr 01, 2007 6:42 pm

I have a Dell monitor which accepts DVI and VGA inputs. I also have a video card which has DVI and VGA outputs. I want to connect a HDTV cable box to the DVI input of the monitor so that I can enjoy HDTV. However, that leaves the VGA input of the monitor free so that I can use it with my computer. The problem is that I don't want to use the VGA output of my video card to the VGA input of my monitor because I would be wasting the DVI output of my video card. I assume DVI is better than VGA, so would it be ok to use a cable and some kind of adapter to connect the DVI output of the video card to the VGA input of the monitor? Would I get the same image quality as using direct DVI to DVI?

qviri
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Post by qviri » Sun Apr 01, 2007 6:53 pm

The DVI port on your video card carries both a DVI (digital) and a VGA (analog) signal. If you use the DVI port with a DVI->VGA adapter, you are simply getting the analog signal without any benefits of digital signalling that DVI uses.

There won't be any difference in quality between using the VGA port directly and a DVI port with a DVI->VGA adapter. The choice could be that of convenience -- the DVI is likely the "primary" port so it could require a bit less configuration, on the other hand, the adapter adds slightly to the mess behind the computer.

The quality may be worse than compared to the DVI port being connected directly to the monitor's DVI input. Whether it is and by how much largely depends on your video card.

Varun
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Post by Varun » Sun Apr 01, 2007 8:35 pm

Just connect straight from the VGA output of your video card to the VGA input of the monitor. The quality differences would be so slight that you would never notice it.

The main advantage with DVI is that you never have to adjust the geometry of your monitor.

len509
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Post by len509 » Sun Apr 01, 2007 11:51 pm

Varun wrote:The quality differences would be so slight that you would never notice it.

The main advantage with DVI is that you never have to adjust the geometry of your monitor.
I disagree. DVI quality is much better than VGA. Text and graphics are more sharp and crisp. But you should try it out yourself. You probably need a switchbox if you want to use your 2 DVI inputs on one monitor.

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Post by nutball » Mon Apr 02, 2007 12:17 am

Varun wrote:The quality differences would be so slight that you would never notice it.
len509 wrote:I disagree. DVI quality is much better than VGA.
It does rather depend on the quality of the monitor. I've seen DVI input on some (cheap) monitors which looks worse than VGA on others (some of which were also cheap). It may well have been that the bad DVI examples were using the analogue signal carried over DVI-I rather than the digital, it's hard to tell I suppose.

But in general terms DVI is more reliably going to give you the best you can get out of any given monitor than VGA would on the same monitor.

nick705
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Re: Question about VGA and DVI

Post by nick705 » Mon Apr 02, 2007 1:51 am

Ozkar wrote:I have a Dell monitor which accepts DVI and VGA inputs. I also have a video card which has DVI and VGA outputs. I want to connect a HDTV cable box to the DVI input of the monitor so that I can enjoy HDTV. However, that leaves the VGA input of the monitor free so that I can use it with my computer.
I'd do it the other way around, assuming your cable box has a VGA output - the (sometimes) lower quality of the VGA connection won't matter anything like as much for watching video as it would for typical PC use.

Depending on the cable box's capabilities, the DVI input might be wasted on it anyway - if it can't do 1:1 pixel mapping for instance at your monitor's resolution there's really not much to be gained.

Ozkar
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Post by Ozkar » Mon Apr 02, 2007 5:29 am

The monitor is a Dell UltraSharp Widescreen 2007WFP. The Dell website doesn't mention anything about HDTV support on this monitor. However, I have seen a lot of online reviews and they all say that this monitor supports HDTV.

I like the idea of a DVI switch. I didn't even know they existed.

Ozkar
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Post by Ozkar » Mon Apr 02, 2007 5:37 am

The other option I was considering was using a HDTV capture card. I would connect the cable box to the capture card, and the DVI from the video card to the DVI of the monitor. This set up would be perfect because I could use my PC as a PVR. However, I haven't found a good HDTV capture card that works with my HDTV cable box.

floffe
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Post by floffe » Mon Apr 02, 2007 5:44 am

When it comes to DVI vs VGA, my monitor (Hitachi CML174SXW bought in autumn 2003) shows a huge difference, but my friend's Samsung (17", not sure what model) barely shows any difference. I only noticed mine after I forgot my DVI cable at a LAN party, and had to use VGA for a couple of days until I got it back. I will never try that again, at least not on this one.

And HDTV capture cards as a rule do not work with cable boxes, there are a few exceptions where the cable box has Firewire out, but then you have no use for a capture card anyway... Current HDTV cards are OTA or in some cases unencrypted cable (QAM).

nick705
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Post by nick705 » Mon Apr 02, 2007 8:16 am

Ozkar wrote:The monitor is a Dell UltraSharp Widescreen 2007WFP. The Dell website doesn't mention anything about HDTV support on this monitor. However, I have seen a lot of online reviews and they all say that this monitor supports HDTV.

I like the idea of a DVI switch. I didn't even know they existed.
Well, the 2007WFP has a native resolution of 1680x1050, so 720p or 1080i/p output from your cable box would have to be scaled to fit anyway. I really don't think you'd gain much if anything by using DVI over VGA in those circumstances.

Another potential gotcha is the Dell's aspect ratio of 16:10 as opposed to 16:9 of HDTV - if you connect the cable box directly, any scaling will have to be done by the monitor itself rather than a PC graphics card, and it may want to stretch the input to fill the whole screen (rather than keeping the correct aspect ratio with narrow black bars at the top and bottom). I'm not sure if the 2007WFP does this, but it's quite a common problem using PC monitors with non-PC sources, and you might want to check how the Dell handles it.

Varun
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Post by Varun » Mon Apr 02, 2007 7:48 pm

len509 wrote:
Varun wrote:The quality differences would be so slight that you would never notice it.

The main advantage with DVI is that you never have to adjust the geometry of your monitor.
I disagree. DVI quality is much better than VGA. Text and graphics are more sharp and crisp. But you should try it out yourself. You probably need a switchbox if you want to use your 2 DVI inputs on one monitor.
I disagree with you. I also prefer DVI but the difference is not huge by any means. Maybe the brand of monitor makes a difference but at work with a dual monitor setup of DVI/VGA the difference is really almost nothing. You would need clones of both right next to each other to notice a difference.

It's very subjective I guess but to me the difference is nothing to get worked up about. If I needed the DVI port on my LCD and had to hook my computer up with VGA I wouldn't care.

niels007
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Post by niels007 » Mon Apr 02, 2007 11:54 pm

VGA doesn't have to be worse but it can be. DVI is pretty much always good. In some bad cases, especially with higher reslutions like the monitor in this topic, the vga card but especially the cable can, when crap, cause some blurring / ghosting.

If you're going to watch TV and use the same screen for PC use, I'd say use DVI for pc and VGA for TV. HDTV is a bit overhyped; most content doesn't even have enough quality to do it justice, plus the chances of it downscaling/upscaling somewhere are great so really the VGA cable won't cause any noticable further degrading.

jhhoffma
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Post by jhhoffma » Tue Apr 03, 2007 5:23 am

What are we arguing here? That DVI is better than VGA for fixed-pixel displays? Of course it is. You pull out the step of DAC-ADC between the vid card and display. That's like saying optical out from a DVD/CD player isn't better than analog inputs.

If an LCD monitor has a less than stellar image via DVI compared to VGA it's due to one of two reasons: one, the monitor does not scale well or has other design deficiencies (bad color, brightness, response time), in other words, it's junk. Two, the video card is not outputting an appropriate signal. This was investigated a while back (maybe ~2yrs) by TomsHardware when DVI was starting to become the main connector. What happened would be the DVI output would not have the specified voltage throughout the signal or the timing would be off, resulting in artifacts, blurry images, bad color, etc, depending on how the display compensated for these problems.

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