AMD Radeon HD 7990: Hardware & Specs
The Radeon HD 7990 uses two Tahiti XT2 GPU chips, similar to the GPUs used to power the Radeon HD 7970 GHz Edition. There are 2048 processing units per GPU and the clock speed is 1 GHz. However, the Turbo mode seen in the 7970 GE isn't available here, so the clock speed stays fixed in games (it could be boosted by up to 50 MHz in the 7970 GE).
The graphics memory seems to be based on the GHz Edition specs too, as the Radeon HD 7990 gets 3 GB of GDDR5 per GPU. The card therefore has 6 GB of memory in total, but only 3 GB is actually available to use because data is duplicated in each block of RAM. Like in the single-GPU card, the memory is clocked at 1.5 GHz.
SIZE & NOISE
The Radeon HD 7990 is a pretty big graphics card—it's almost 31 cm long! Not all PC cases will be able to accommodate this beast of a card, which is a whole 3 cm longer than the GTX 690 / Titan and 3.5 cm longer than the Radeon HD 7970 / 7950.
AMD was keen to highlight the onboard cooling system (among other things) when presenting the Radeon HD 7990. The system comprises a plate heatsink plus two radiators with aluminium fins criss-crossed by four heat pipes. That little lot is topped off with three axial fans. This set-up is supposed to keep things nice and cool while also keeping noise to a minimum—even more so than with the GeForce GTX Titan, apparently, which already does an excellent job.
In practice, it's not quite that impressive. It's true that this graphics card is very quiet indeed when idle. In a tower case with just a low level of sound-insulation it's hard to make out any noise at all from the fans. In fact, noise levels were similar to those seen with the Titan graphics card in our test set-up.
One radiator per GPU and a plate heatsink for the memory chips.
It's a different story when you start gaming, however. When all three fans fire up at once there are obvious consequences—noise output rises sharply. At full pelt, the 7990 falls part way between a GTX Titan and a GTX 690. The system is therefore no longer quite so discreet, but AMD has still made good progress here, managing to keep things quieter than with the Radeon HD 7970 GHz.
We noticed a slightly more irritating noise coming from the card's power module. This only kicks in when gaming, but the noise has a directional and quite piercing tone. Some users may not hear this at all—it depends on the kind of tower case you use and on your position in relation to the case. Still, it's a bit disappointing to see a very high-end graphics card making such an annoying noise.
With two GPUs at work, it's no surprise to see that power use is pretty high, ranging from 415 to 450 watts for our entire test PC. That's much more than the GTX Titan, which only used up to 290 watts in the same test set-up. Still, it's a lot less power-hungry than the GTX 690 (another twin-GPU card), which was pushing 650 watts.
At idle on the Windows desktop, our test PC used around 72 watts—like the GTX 690. This fell to 61 watts when the screen dropped onto standby. Both of those readings are slightly higher than the Radeon HD 7970 GHz, which used 56 watts and 42 watts in the same conditions.
The drop in power use when the screen goes on standby is no doubt due to the ZeroCore Power function, which more or less switches off the GPU when the screen is out of use. What's more, AMD has extended this function in the 7990, switching off the second GPU when the card has no 3D processing tasks at hand.
PERFORMANCE IN GAMES
With this twin-GPU graphics card, AMD clearly harbours ambitions to claim the top spot with the fastest graphics card on the market. And the Radeon HD 7990 did indeed outperform its competitors when we tested it with 14 different games. On average, it proved 12% faster than the GeForce GTX Titan and 9% faster than the GeForce GTX 690.
Average general performances.
See all test results and compare graphics cards in the Face-Off.
The average difference in performance compared with a Radeon HD 7970 GHz Edition, however, was only 38%, because some of the games we use in our test procedure are CPU limited. With games that aren't CPU limited, the difference between the two cards reached up to 95%.
In conclusion, there's no doubt that the Radeon HD 7990 beats its competitors on performance. The gamble therefore seems to have paid off for AMD. However, using a twin-GPU doesn't come without consequences, and—once again—drivers and updates will need to be quick off the mark as and when new games are released to make sure users don't face problems (hangs, crashes, etc.). As a card that won't leave you much change from £800, the Radeon HD 7990 is on par price-wise with the GeForce GTX Titan, a model that's admittedly a little slower, but which has a better finish, isn't as noisy and only uses one GPU. Pay your money, take your choice.
Graphics Card Performance Index Table