The dust has scarcely settled since AMD released the first couple of graphic cores built on its ‘Northern Islands’ 40nm architecture, the mainstream Bart XT and Bart Pro – better known as the HD6870 and HD6850 to you and me. Now the next pair of cores have made an appearance, the eagerly awaited higher-end Cayman XT and Pro, better known by their marketing names of the Radeon HD6970 and HD6950.
As with the HD6870 and HD6850, confusion reigns over AMD’s new numbering system. Like a great many people, you might have expected the HD6970 to replace the HD5970 and whip Nvidia’s GTX580 into a quivering wreck.
In fact, AMD has pitched the new card against the GTX580′s slower sibling, the GTX570, so it sits below the HD5970 in AMD’s hierarchy. A tad surprising, if not downright confusing, to say the least.
While the Bart’s core architecture was seen as a tweak of the earlier Cypress architecture, which was used by AMD’s HD5000 series of cores, the Cayman architecture is much more of an evolution of Cypress.
The most important change that has been made is installation of two graphics engines to feed the card’s arithmetic logic units (ALUs). Compared to the Bart core, the Cayman feeds twice the throughput into the processing cores and the eighth-generation tessellation engine.
Both AMD and its chief rival Nvidia have focused on increasing the tessellation and geometry performance of their newest cores, the reason being that newer games – and titles released in the future- will make heavy use of tessellation and geometry to make them appear even more lifelike.
A couple of new things for the HD6900 series are EQAA (Enhanced Quality AntiAliasing), an improved form of edge filtering to provide better image quality, and Power Tune. Power Tune is AMD’s attempt to fundamentally alter the way graphics cores are designed by controlling their performance in terms of the core’s thermal design power or TDP – the effective maximum power limit before a chipset overheats. This is a complete reversal of the situation that existed previously, where performance dictated power usage. To this end Cayman uses sensors to detect exactly how much power the card is drawing and compares it against the TDP rating, hiking up or throttling back on clock speed to maximise performance within that given power limit.
Sapphire has taken the bull by the horns and released three versions of the HD6970; a standard version; a Gaming Edition (the version we reviewed), which comes with a full version of the Battlefield game and a code to download Battlefield: Bad Company 2: Vietnam; and a Special Edition, which comes with the Gaming Edition extras but arrives in a fancy aluminum flight case. All three cards run at standard clock speeds, and apart from the game the only difference between the three are the stickers on the coolers of the gaming versions.
As previously mentioned, Sapphire’s HD6970 uses the reference clock speeds. Core and shader (1,536 shaders) engines are clocked at 880MHz while the 2GB of GDDR5 memory runs though a 256-bit memory bus at 1,375MHz (5.5GHz effective), which gives the HD6970 a very useful bandwidth of 176GB/s.
Performance-wise, Sapphire’s HD6970 sits between Nividia’s GTX580 and GTX470. When benchmarked with Crysis Warhead at 1,920×1,200 resolution with 4x full-screen anti-aliasing and 4x anisotropic filtering and the in-game detail set to Enthusiast, we got an average frame rate of 33fps compared with 36fps for the GTX580 and 31fps for the GTX470, while the older Radeon HD6870 gave a result of 28fps.
Tessellation performance demonstrates the improved throughput of the Cayman architecture over the Bart GPU quite nicely when benchmarked with Unigine’s Heaven benchmark. With a resolution of 1,920×1,200 resolution, 4xFSAA and 4xAF filtering at the normal tessellation setting, we achieved an average frame rates for the HD6970 and HD6870 of 43.8fps and 29.6fps, respectively.