Hi,
I am wondering why some high end graphic card offer peak memory bandwidth as high as 2x256 GB/s !!
I am confuse because most motherboard memory bus speed is much much below that, e.g., my Intel DP35DP could only achieve max 8GB/s.
I am not sure what is the highest memory speed today but i believe its not over 30GB/s. So, the question is, do one need to pay top dollar for such cards?
If you have notice, the relationship between CPU and RAM is pretty different from GPU and VRAM. When you buy RAM, say DDR3, there are weird numbers like PC8500 or PC16000. This number roughly refers to the peak bandwidth, PC8500 = 8.5Gb/sec and PC16000 = 16Gb/sec.
The exact formula is
Peak Bandwidth = (Bus Width) x (Bus Speed x Operations/Clock Cycle)
or
Peak Bandwidth = (Bus Width) x (Data Rate)
Now VRAM runs at a much data rate and much wider bus width. For example, HD5850, 256 bit, 4000Mhz memory clockrate. That is
(256bit/8) X 4Ghz = 32 byte X 4Ghz = 128GB/sec
GT300 was said to have 512 bit interface, that means 512bit bus width, so in theory:
(512bit/8) X 4Ghz = 64 byte X 4Ghz = 256GB/sec
If 2 VRAM running in parallel, then it becomes
2x256GB/sec
However, the real spec of fermi is not out yet.
Another thing is, i saw many reviews that run several games and showed the frame per second capabilities of the tested card. High end cards could achieve over 100fps !! But our PC monitor mostly refresh at 75Hz or below. So, once again, do one need to pay top dollar for such cards?
I am posting this because i am not sure what i am writing above so please be kind to me.
Thanks.
Most LCD support up to 60Hz because at 60FPS, human eye will see the picture as continuous, meaning you will not feel shuttering. However, that doesn't mean you can't detect differences between 60FPS from 120FPS if the LCD actually support 120Hz. Now of course, any games above 50FPS is very smooth already and everything above 60FPS+ is only meaningful for benchmarking as most LCD can show it anyways. Yet no games can maintain a steady FPS throughout the game, as sometimes FPS will dip to say 20 FPS and you will feel shuttering, or lagging.
However, 3D scene with shuttering glasses divides FPS by half, one picture to one eye per cycle(Hz), that means FPS is cut by half. In other words, 60 FPS = 30 FPS in 3D, and people feel dizzy due to the shuttering with normal LCD, and 120Hz LCD is needed for 3D.
Now new LCD is coming out supporting up to 120Hz, allowing games to produce 120FPS. At that rate, each eye will still see 60FPS, producing smooth scene for each eye, or smooth scene in 3D. That means, 100FPS vs 60FPS is no longer a benchmark difference, but a difference that you can actually see.
As to whether or not you should spend big bucks on heavy guns depend on you. Newer games are more demanding, and high-end video cards can still play newer games with decent FPS 2-3 years down the road with most things max out. If 30-40 FPS is okay for you, then buy mid-high range video card and replace it with a new one when you can no longer maintain those FPS with new games. If you have a high resolution 120" LCD and 3D is your cup of tea, then high-end video card is what you are looking for.