How does more RAMDAC affect visual quality?

[Biggs]

Does a video card with more RAMDAC automatically translates to better image quality?

 

[jimmygates]

I believe the with a higher ramdac, you can get higher resolution/refresh rates out of your video card. I could be wrong...but I'm sure if I am, LXi will correct me..


-Jimbo

 

[Czar]

The Ramdac only effects the highest resolution awailable, the quality of the ramdac is much more important. Lookee here, a G400 with 300mhz Ramdac has higher 2d quality than a Voodoo3 3500 with a 350mhz Ramdac.

 

[peemo]

It's not just the RAMDAC you have to worry about. There has been a lot of discussion about nVidia 2D quality. See this article and many posts on this BBS:

GeForce/TNT Image Quality Problems Due to RFI Filtering

 

[Oyeve]

BBS? Damn, I havent heard that in a while. I feel so old.

 

[Goi]

A video card only has 1 RAMDAC, unless it has support for multi-display configurations using discrete RAMDACs(such as the Matrox G450). I believe you are referring to the RAMDAC speed, in which case the higher the speed, the higher the refresh rate a video card can support at any particular resolution. This is especially important at high resolutions(1600x1200 or more) since more bandwidth is required. A faster RAMDAC isn't indicative of better image quality, it only allows higher refresh rates. However, higher refresh rates may lead to reduced eye stress, which may be perceived as "better image quality", even though the displayed image may be the same, just refreshed more frequent. Also, video cards with good image quality tend to have faster RAMDACs too.

To correct Czar, RAMDAC speed is pretty much independent of resolution support. Resolution support is strictly only a factor of memory size. They are correlated, in that you don't want to display a 1600x1200 screen at 20Hz, but not directly dependent, in that you can do so if you wish to(or rather, if the video card manufacturer so desires).

 

[Czar]

Thanks for correcting me, so very true. Ramdac only affects refresh rates.

 

[Biggs]

So how d'you do the math? What is the maximum refresh rate for a card with a 300MHz RAMDAC? 360Mhz RAMDAC? Thanks.

 

[Mark R]

The theoretical minimum speed can be calculated as follows:

Minimum RAMDAC speed = horizontal res x vertical res x refresh rate

e.g.

1600 x 1200 x 75 = 144,000,000 Hz (144 MHz).

 

[Dulanic]

The main point is the RAMDAC can still affect picture quality. The main reason being is even though the RAMDAC supports a certain res/refresh by the time the signal goes through the RF filters etc it loses alot of bandwith, that is one of the reason some people will mod a GeForce if they run it at very high res. They will take out part of the RF filter because by the time the signal goes through the RF filter it loses some bandwith and it can cut down on picture quality at high res.

 

[Goi]

The formula given above is very theoretical, but doesn't really apply in real life, in that a 144MHz RAMDAC cannot support a refresh rate of 75Hz at 1600x1200. I'm not sure of the real world formula, but I know its dependent on either the horizontal or vertical refresh rate.
Also, correct me if I'm wrong, but I don't believe the RF filters are part of the RAMDAC, but are external to it. So saying that the RAMDAC affects image quality because the signal that comes through the RF filters are modified from the original signal, isn't very accurate.

 

[Moohooya]

A RAMDAC is a Random Access Memory Digital to Analog Converter.

Its job is to convert digital bits to the analog signal for your monitor. The speed of the RAMDAC tell you how many pixels per second it can spew out.

When calculating what speed RAMDAC is required for a resolution you need to know much more that the resolution and refresh rate. The electron gun travels back and forth across the screen and up and down. Only during a portion of its travels will it be turned on and electrons shot out. The required speed I believe is

refresh rate * (H. front porch pixels + H. sync pixels + H. back porch pixels + H. resolution pixels ) * (V. front porch lines + V. sync lines + V back porch lines + V resolution lines)

If you want to know what all that means, I believe there are sereral Linux FAQs out there that go into lots of detail. They are old, but the technology is the same, just higher numbers these days.

The front and back porch and sync could well eat up 30% of the time, so an approx RAMDAC speed would be

X * Y * refresh * 1.3

If you have a must have resolution, use 1.5 to be safer when buying a video card.

For most people, any recent video card will easily out perform their monitor. Don't worry about speed, but quality. If you can find a monitor for under $1,000 that will outperform a $80 or more video cards RAMDAC I'd be surprised.

Moohoo