Following on from Tahiti XT (7970) comes the Tahiti Pro (7950). The small difference in names does make a significant difference to performance however, and you should do your homework on exactly what you want out of your gaming PC or workstation rig before deciding on which Tahitian is the right one for you.
For the 7950 AMD has cut back the GCN units from 32 to 28, which may not sound like a whole lot. But Each GCN unit contains 64 Stream Processors, so the difference is actually 256 Stream Processors. The 7970 houses a whopping 2048 Stream Processors, while the 7950 “only” houses 1792 Processors. We aren’t yet sure if AMD has laser-cut these extra GCN units off, or if they are in fact only disabled through BIOS (like the previous generation of HD6950 2GB cards). We hope the latter is true, as the cards both share the same 3GB 384-bit memory bus. This means in theory the 7950 can be unlocked to 7970 performance as long as AMD has left the extra GCN units on the die. Only time will tell if that is the case, as custom BIOS code needs to be written (or you need to flash an existing 7970 BIOS and hope for the best), and at the moment we’re not really willing to brick any of our cards in order to conduct this experiment - maybe in a few weeks.
Texture units have also been reduced, sporting 112 compared to the 7970s superior 128. However, this doesn’t seem to have a huge effect on texture fill rates when clock speeds are matched – it falls behind the 7970 only in resolutions of 2560x1400 and higher. For this reason the 7950 and 7970 will have extremely comparable performance on your average 1920x1080 monitor when compared with identical core and memory clock cycles.
In terms of stock clocks it isn’t really a shock to see the 7950 being held back in order to keep the gap between it and the 7970 look more substantial. A tame 800MHz core clock is what you should find on the majority of early-to-market cards, with overclocked editions finding their way to market shortly after. As for memory clocks, we see a similar story, with the clock being held back to 1250MHz, instead of the 1375Mhz clock found on the 7970. This ensures greater performance on the 7970 at higher sample rates and resolutions, despite both cards sharing the same 384-bit Memory Bus.
After we overclock the 7950, we quickly see that this card is an extremely capable unit, almost mirroring the 7970 perfectly. Memory clocks as high as 1800MHz were achieved, along with core clocks as high as 1220MHz. This is lower than the maximum 7970 clock we managed (1350Mhz), but considering the lower heat output and the smaller price tag we think this card is definitely something for NVIDIA and its current offerings to fear. Also, we will likely see some highly-tweaked PCB designs in the future, giving the 7950 an extra shot in the arm, and further improving its value for money in the overclocking arena.
When we asked around for a few cards to take a look at, ASUS and XFX were kind enough to ship over their custom-cooled models, though both are still using the AMD reference board layout. This means that the significant differences between the two models are component choice, BIOS, cooling and warranty. Everything else remains identical, and should be remembered when we compare their overclocking results. We are simply comparing them to give a rough indication of what you can expect from the reference boards.
As for PCB design and power requirements, the PCB is exactly the same length as the 7970 (leads us to believe they are identical) and the only difference is the dual 6-pin power connectors as opposed to the 6+8 pin found on the 7970. Apart from that there is no discernible difference; even the display outputs are identical.
As it stands the two cards reviewed today are the only examples of custom-cooled 7950s on the market, and therefore should be compared. They both have strengths and weaknesses; though we do think there is a clear and decisive winner.
ASUS have no doubt made a good cooler in the Direct CU II. It has a nice heat sink with enough copper heatpipes to quickly and effectively remove heat from the GPU, memory chips and any critical PWM. The triple-slot cooler means there is room for thicker fan blades, moving more air over the heat sink fins and therefore cooling the card faster. Unfortunately this is where our initial problem lies. The card doesn’t need to be triple slot, and it doesn’t need to be as big as it is. ASUS has wasted a good couple of cubic inches behind the rear display bracket. This is due to the cooler looking like it has been recycled from either their HD 6970 or GTX 580 range.
The reference PCB this time around sits the GPU roughly 2cm further back on the board than previously found on the ASUS HD 6970 DCU II. This has forced ASUS to mount their cooler roughly 2cm further back on the PCB, giving the cooler a nasty (and unnecessary) 3.6cm overbite. This kind of waste cannot afford to be on cards the size of the 7950, and reeks of laziness and over excitement to be among the first to market with a custom-cooled solution. This card could be a 2-slot solution if better engineered, but unfortunately ASUS has opted for style over substance.
Suspecting that this may be a short term solution in order to get a 7950 Direct CUII to market in time for launch, we asked ASUS about the cooler. The response we received was that “It is the final heatsink design for 7950 DCII”, which is somewhat disappointing considering the high regard in which we hold this product line.
XFX on the other hand has done its homework with its Double Dissipation cooler. It has managed to fit an equivalent (possibly even greater) surface area of aluminium and copper on the card for cooling along with a similar dual-fan design. Along with this XFX has wasted no room, kept the card in a dual-slot configuration, built a sturdy and (if we don’t mind saying so) beautiful shroud for the card. It glows with creativity and innovation, especially considering they were able to get these cards to us over a week before product launch!
In all honesty the only cards we would be buying at this point in time are the XFX series. Not only are they nice to look at, they have provided both style and substance - Something that is not commonly found at the launch of brand new GPU families.
Overclocking results
XFX 7950 Black Edition
Stock: 900/1375
Overclock: 1220 / 1800
ASUS 7950 Direct CU II
Stock: 900 / 1375
Overclock: 1180 / 1490
7950 Benchmark results
