Maximum displayable frame rate on a LCD

[Webby]

Well I have been thinking about this since we all Hijacked the 2ms or 5ms thread (http://forums.hexus.net/graphics-car...ainst-5ms.html) and now with Agents thread about perceivable and accepted frame rates (http://forums.hexus.net/graphics-car...imum-game.html).

So I thought about why at lower resolutions an LCD monitor will allow higher refresh rates for example my 22" panel will run at 1680*1050 at 60Hz but will run at 1440*900 at 75Hz, obviously this is not related to pixel response times as the pixels will not react faster just because less are being used (or without 1:1 pixel mapping the same number of pixels with the image stretched).

So I decided it must be a bandwidth issue, so checking Wikipedia I discovered that single link DVI (what any monitor up to 1920*1200 will be using) has a bandwidth of 3.96 Gb/s (http://en.wikipedia.org/wiki/Digital_Visual_Interface) so this led me to how much data does a pixel require? This obviously depends on what colour level you are using, since most people like to work at the maximum which is "called" 32bit colour (16.7 million colours) this is in fact 24bit colour (2^24 = 16.7million) the remaining 8 bits are used to carry other information. So now for a bit of maths...

A 1920*1200 resolution has 2304000 pixels each requiring 32 bits of information per frame.
32bits = 4 bytes
2304000 pixels * 4 bytes = 9216000 bytes per frame
9216000 bytes = 9000 kB
9000kB = 8.79 MB
So a 1920*1200 frame requires the transfer of 8.79 MB

Looking at the bandwidth of single link DVI 3.96 Gb/s which is 0.495 GB/s = 506.88 MB/s

So at a resolution of 1920*1200 you can only send 506.88/8.79 = 57.7 frames/second not quite 60Hz
1680*1050 = 6.73MB/Frame = 75.3 frames/second
1440*900 = 4.94MB/Frame = 102.6 frames/second

So as you can see at high resolutions using single link DVI you are limited to how many frames can be displayed by the bandwidth of the interface, theoretically as you drop down the resolutions your frames go up.

The burning question now is though does an LCD when running at 1680*1050 set to 60Hz actually display 60 refreshes a second or if the data is available would it refresh 75 times per second?

I throw the floor open for suggestions and thoughts!

[muddyfox470]

Well done Sherlock Seems convincing to me. Glad I've got my 22incher then.

Also I'd like to thank you for your well laid out post - it's not often you see many posts that are "thought-through" mine included

[Ed Omega]

I did know the exact answer to this in detail about 2 years ago, but numerous sleeps have faded my memories.

Suffice to say, all TFT screen run at approx 60Hz when set at their native resolution 9I think this might even be an XP thing - like I say, gray matter gone mouldy). If you reduce your resolution on the desktop, it will open up higher refresh rates to you, which you can then use in your game at that lower resolution. If (like most people) you are running your desktop at its native resolution, you cant usually alter the refresh rate in the game above 60Hz (tho please don't take that as an absolute - I just haven't come across it)

I haven't seen the threads on 2ms vs 5ms, but I have been told by a friend who is opthalmic surgeon and somewhat knowledgable in these area's that the human eye can barely discern below 12ms, some people with exception eye response (there is a posh word for that, but it had far to many letters in it for me to remember) can discern down to about 10ms. This is of course assuming a true 10 to 12ms (what ever that may be defined as this week!)