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Of course, this is quite interesting. By adding more bandwidth we can get higher frame rates, athough its more than likely we have to get above the 100 MHz difference for the 6 quad GeForce 7800GTX. Time to see if our speculation is correct. Let's see just how much performance increase we can squeeze out of core and memory clock changes. All the numbers below are in percent, not frame rates.


We can see that clock changes can offer higher gains than having more quads, ranging from 3 to 12 percent. However, there's a caveat to using higher clocks. Now, this is quite hard to visualize, but remember that we need to have at least a 100 MHz difference between core and memory clocks. We can see that's true even for clock increases, but there's more to it than that.

With 6 quads, increasing core clock from 350 to 400 MHz has about the same increase as increasing memory clock from 450 to 500 MHz (or 900 to 1000 MHz effective). However, at higher clocks, we have to use a higher base memory clock to get as much increase as with the core. Increasing core clock from 400 to 450 MHz offers the same increase as increasing memory clock from 600 to 650 MHz (or 1200 to 1350 MHz effective).

If we look at the results with 4 and 5 quads, the situation is similar. With 4 quads, we gain pretty much the same by increasing the core from 350 to 400 MHz as we did with increasing the memory from 450 to 500 MHz (or 900 to 1000 MHz effective). As we hit 500 MHz, we have to increase the memory clock from at least 600 to 650 MHz (or 1200 to 1300 MHz effective). So, having less quad actually means less strain on bandwidth, but it's not really a plus point since we already need higher memory clocks to compensate in the first place.

So, in that respect, the GeForce 7800GTX 512 MB can be thought of as the upper most limit of what the GeForce 7 can do. There's no point to using even higher clocks except in some extreme situations, since we will most likely gain very little benefit from it.

What have we learned?

On a mostly texture dependent game like Quake 4, having more quads does help performance with the GeForce 7 series (somewhat). However there are limits to this for several different reasons. One, having more quads means higher bandwidth is required to maintain optimal usage. The solution may seem simple enough - simply add more bandwidth, in this case by increasing the memory clock. We can do that, but then we will ran into the second problem - with higher clocks an even higher increase in memory clock is needed - that can range from 100 MHz to 300 MHz (or more if you can get high enough) on very high clocks. A more efficient approach to adding more quads or ROPs (since we're talking about AA and AF performance) is to clock the card higher, but eventually you'll ran into the same problem. Using higher and higher clocks is not always possible, just like in our case.

Even if you managed to clock the card high enough to get the most out of all 6 quads, most of the performance increase you get will go to 'improve' anisotropic filtering performance and not AA. So, running with AA and AF will always be slightly slower than say, AA and trilinear, even at very high clocks, AF's penalty is very small, maybe around 3 - 5 percent overall. So, that's the most we can get out of more quads and higher clocks. Although nominal frame rates should be higher with higher clocks, the penalty for AA will remain around 20 %.

There's no point of having more quad, or more precisely, ROP units if you don't have the bandwidth to feed them all, particularly for texture dependent games. You can compensate the lack of bandwidth with higher memory clocks, but there's a limit to that. Compression and caching may help some, but not in the raw bandwidth department - these features will only allow the card to make optimal use of the bandwidth that's available. Sans a new, wider memory controller, the next generation of cards will likely not offer much improvement in texture dependent games than the current generation of cards.

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