Unusual monitor behavior? (refresh rate question)

[GanChan]

I bought a new Viewsonic VA903b to use with my G4 Mac Mini. I set it to use the display profile for this model, at 75 Hz, using a VGA cable with Apple's DVI adaptor. While generally the picture was good, characters seemed a little darker/heavier, and the last degree of sharpness seemed missing from the images, compared to my HP Pavilion LCD monitor. Also, when I first turned the unit on I saw a tiny but noticeable "data shimmer" on vertical edges, which seemed to settle down after 30 minutes or so but never entirely disappeared. I thought my new monitor was a lemon!

Then I reset it to 60 Hz, and suddenly it looks fabuluous, with no vertical simmering, perfect sharpness, etc. So obviously, if it looks and behaves great now there's no need to return it. Still...

What gives? i thought the rule of thumb was that 75 Hz was the way to go, and that a slower refresh rate such as 60 Hz was usually the cause of these kinds of problems -- not the solution to them!

Does my computer, monitor, or cable live in Bizarro-land or something?

 

[mkrishnan]

My guess would be that you're experiencing something akin to beating -- that is, you're providing a new signal at a different rate than the LCD actually does refreshes, and so it is confused about what to put on some pixels during each of its own refreshes. The LCD persists -- the transistors stay in a color state until the next color is put up -- but it's possible it's misinterpreting information it's receiving at 75Hz and so it's changing some pixels more often than it needs to, leading to the shimmers on edges.

Probably 60Hz just happens to jibe better with the actual way the screen refreshes.

 

[Angrist]

The monitor should have documentation about the range of refresh rates that it accepts. If you try to drive it at too high of a rate it'll look like trash.

 

[phidauex]

EDIT: Not contradicting the previous posters, they are correct. I'm just giving a little more info.

With CRT monitors, it is advantageous to run them at the highest refresh rate that you possibly can. This is because the image is actually a dot sweeping the screen, and each pixel becomes dim between sweeps. If the refresh rate is too low, you notice each sweep as a flicker. Not cool. Above about 75hz, it looks much clearer, and I prefer 85hz if I can do it.

LCDs on the other hand, keep their pixels lit between refreshes. Your eye can't distinguish between an image changing 60 times a second and one changing 75 times a second, the only reason we push refresh rates that high is to keep flicker on CRTs down.

The LCD can really only switch it's pixels at one speed. If you feed it a faster or slower refresh rate, it will work, since it is designed to be friendly, but it has to match on the fly, which can create the artifacts you are seeing.

Modern Multisync CRT monitors don't have a native resolution, or a native refresh rate. Thats why they are called multisync. But LCDs have a native resolution and a native refresh rate. If you deviate from these, it will do its best to show you a picture, since, like I said, it is trying to be friendly. But it won't look its best.

So look in the book, or the product specs, and set the resolution and refresh to the native values. It seems 60hz is the native refresh, since it looks good at that speed. Leave it there. That seems slow if you are used to CRTs, but its fine for LCDs, since they don't flicker in the same way.

Anyway, sorry for the computer lesson. Your monitor is fine, and you seem to have found the solution by yourself, so enjoy!

Peace,
Sam

 

[GanChan]

Well, now it's acting funny again!

The computer had been asleep for several hours, and when I woke it up, the monitor showed horrible wavering and shimmering. It's pretty much settled down now, several minutes later, though even now I still see a little isolated pixel action here and there. Presumably in a few more minutes it will go back to looking great again.

But is this normal behavior????? do I need to swap out this monitor for a different one, or do monitors act all glitchy and weird when they first receive a signal? Is it a voltage thing? The manual says absolutely NOTHING about this monitor except for how to plug it in and turn it on....