DVI to VGA quality?

[Skinjob]

I'm building a new box and was considering a video card with dual DVI as I'd ultimately like to have dual 19" LCD pannels. However, I'm going to be stuck with an analog CRT for a while. Do DVI to VGA adapters really work? Is the quality the same as a normal analog connection?

Thanks.

 

[SynthDude2001]

Originally posted by: Skinjob
I'm building a new box and was considering a video card with dual DVI as I'd ultimately like to have dual 19" LCD pannels. However, I'm going to be stuck with an analog CRT for a while. Do DVI to VGA adapters really work? Is the quality the same as a normal analog connection?

Thanks.

Yes, the adapters work just fine. What actually happens is that the DVI connections are DVI-I, meaning that they carry both the analog and the digital signal - so all the adapter does, is convert the analog signal that's already there, into the standard 15-pin D-sub connector format.

 

[Skinjob]

Cool. Thanks for the info. That's what I thought was going on. I just hadn't heard from anyone who's actually used one.

 

[BenSkywalker]

Is the quality the same as a normal analog connection?

No, although at lower resoltutions and settings you would be hard pressed to notice a difference. Start pushing it to 1600x1200 or higher and there is an easily noticeable quality dropoff.

 

[Matthias99]

Originally posted by: BenSkywalker

Is the quality the same as a normal analog connection?

No, although at lower resoltutions and settings you would be hard pressed to notice a difference. Start pushing it to 1600x1200 or higher and there is an easily noticeable quality dropoff.

The only difference, physically, between a DVI-I and D-SUB connection is that with DVI-I->VGA, you have to move some of the pins around with an adapter plug/cable. There's no reason there should be an "easily noticeable quality dropoff", unless the VGA output of the card is substandard to begin with or the adapter cable is defective.

It's no different electrically than, say, going through a KVM switch.

 

[uOpt]

Actually I have seen some reports of external DVI->VGA adapters being better than the onboard analog.

Sometimes card vendors srew up something in the analog port. If that happens with an external converter you just get a different one for $7.

 

[BenSkywalker]

There's no reason there should be an "easily noticeable quality dropoff", unless the VGA output of the card is substandard to begin with or the adapter cable is defective.

I have yet to see any DVI-VGA adaptor that didn't clearly degrade IQ quality at higher settings compared to using the straight VGA out. My monitor has dual inputs- I can directly compare them at the exact same settings- DVI adaptors destroy IQ particularly at higher settings. I should qualify this by saying I've only seen it on about a dozen different boards- all of them ATi and nVidia, no Matrox parts.

It's no different electrically than, say, going through a KVM switch.

Absolutely correct, and KVMs have a horrific impact on IQ. If you can't see it using one, daisy chain half a dozen KVMs together sometime and even the legally blind should be able to see the difference- although all you need to do is set your monitor to something like 1920x1440@85Hz and you should have no problem at all seeing the impact(although it is quite noticeable running 1600x1200@85Hz on every part I've seen).

 

[Matthias99]

Originally posted by: BenSkywalker

There's no reason there should be an "easily noticeable quality dropoff", unless the VGA output of the card is substandard to begin with or the adapter cable is defective.

I have yet to see any DVI-VGA adaptor that didn't clearly degrade IQ quality at higher settings compared to using the straight VGA out. My monitor has dual inputs- I can directly compare them at the exact same settings- DVI adaptors destroy IQ particularly at higher settings. I should qualify this by saying I've only seen it on about a dozen different boards- all of them ATi and nVidia, no Matrox parts.

It's no different electrically than, say, going through a KVM switch.

Absolutely correct, and KVMs have a horrific impact on IQ. If you can't see it using one, daisy chain half a dozen KVMs together sometime and even the legally blind should be able to see the difference- although all you need to do is set your monitor to something like 1920x1440@85Hz and you should have no problem at all seeing the impact(although it is quite noticeable running 1600x1200@85Hz on every part I've seen).

I will admit I don't run 1600x1200 resolutions on my desktop (my monitor has issues with it at refreshes over 60Hz), but I can't see any difference between straight VGA and DVI-I->VGA (on my 9800Pro) in games or on the desktop at 1600x1200@60Hz. I can't speak to higher resolutions than that, but I'll take your word for it that there's some sort of issue (at least for you).

I was also talking about *one* KVM, not six, if that wasn't clear. :disgust:

 

[Peter]

I have done this quite often. As has been said, the DVI-VGA adapters are just mechanical adapters that change from one kind of plug to the others. The analog VGA signal is right there on the DVI-I plug on your graphics card.

I have yet to see a DVI-VGA adapter that degrades signal quality - but I have seen plenty of graphics cards that have poor analog signal quality on the DVI-I to begin with. So if you're experiencing a difference in quality between the native VGA 2nd head and the adapted 1st head, then it's most probably because the card designers didn't pay attention to the quality of the analog signal on the DVI plug.

 

[BenSkywalker]

I was also talking about *one* KVM, not six, if that wasn't clear.

If they don't impact signal integrity it wouldn't make the slightest bit of difference if you used a hundred of them.

I have done this quite often. As has been said, the DVI-VGA adapters are just mechanical adapters that change from one kind of plug to the others.

A KVM is essentialy the same, when you are dealing with bandwidth on the level that it takes to drive a display at higher settings any sort of mechanical adapters without signal boosting/filtering is going to have a negative impact on signal integrity.

 

[Peter]

There's a difference here: DVI-VGA cables are just an extra inch of cable and an extra connector pair. KVM switches have to pass the signal either through a mechanical switch or an electrical circuit. That'll mess with it a LOT more than the DVI adapter does.

 

[blafarm]

Interesting thread.

Since most video cards featuring DVI ports ship with at least one DVI -> VGA adapter - I would assume that both analog and digital signals are available at that port - yet I have never seen a "Y" adapter (for lack of a better description) that splits off the digital signals to a another DVI connector - and splits off the analog signals to a 15 pin VGA connector.

Has anyone ever seen such an animal - or is there something wrong with my logic that both signals can be used at the same time?

 

[L00PY]

Though it's been implied, I thought I should point out that you need to make sure that the dual DVI card you plan on buying has a DVI-I connector. If they have DVI-D connectors, you won't be able to use your CRT.

As for Y-cables, I suspect that while you might be able to find a cable like that, both signals can't be used at the same time, (unless it's a specially designed graphics card).

 

[FrustratedUser]

Originally posted by: SynthDude2001

Originally posted by: Skinjob
I'm building a new box and was considering a video card with dual DVI as I'd ultimately like to have dual 19" LCD pannels. However, I'm going to be stuck with an analog CRT for a while. Do DVI to VGA adapters really work? Is the quality the same as a normal analog connection?

Thanks.

Yes, the adapters work just fine. What actually happens is that the DVI connections are DVI-I, meaning that they carry both the analog and the digital signal - so all the adapter does, is convert the analog signal that's already there, into the standard 15-pin D-sub connector format.

This is true. The output is not worse than a normal VGA output because the two are EXACTLY THE SAME SIGNAL.

 

[Peter]

Originally posted by: blafarm
Interesting thread.

Since most video cards featuring DVI ports ship with at least one DVI -> VGA adapter - I would assume that both analog and digital signals are available at that port - yet I have never seen a "Y" adapter (for lack of a better description) that splits off the digital signals to a another DVI connector - and splits off the analog signals to a 15 pin VGA connector.

Has anyone ever seen such an animal - or is there something wrong with my logic that both signals can be used at the same time?

You can't, simply because DVI-I is defined to be either-or. Various things keep you from using both, from just one DDC channel on the plug all the way down to the graphics engine not having two sets of image generators feeding the one plug.

 

[BenSkywalker]

There's a difference here: DVI-VGA cables are just an extra inch of cable and an extra connector pair.

Missed replying to this previously. You are putting in place an extra connector- another fault point in terms of easily introduced noise. Set up any remotely decent CRT and set if for 2048*1536@85Hz and use the dual input options and check it for yourself. With the amount of bandwidth you are dealing with it is very easy to introduce noise.

 

[Pete]

Originally posted by: FrustratedUser

Originally posted by: SynthDude2001
Yes, the adapters work just fine. What actually happens is that the DVI connections are DVI-I, meaning that they carry both the analog and the digital signal - so all the adapter does, is convert the analog signal that's already there, into the standard 15-pin D-sub connector format.

This is true. The output is not worse than a normal VGA output because the two are EXACTLY THE SAME SIGNAL.

The mere presence of another converter on the chain introduces potential signal quality degradation, no? It's not like the converters are gold-capped or "oxygen-free" copper.

The DVI-VGA adapter on my 9700P introduces noticable--if not especially objectionable--ghosting. The DB15 connector doesn't exhibit ghosting, but it does offer some nice rolling lines at certain res/refresh combos.

Unless you're paying for Matrox, it's a potential IQ minefield out there. I'd be curious to know if Matrox's converters are higher-quality than anyone else's, or if it's just the inherent analog signal that's at fault. Only way to find out is to compare with a CRT that has DVI-A connector.