GPUs continue to become more like CPUs not just in terms of computational capabilities, but also in the way they manage power consumption and heat production. AMD took a nice step forward with Cayman by introducing a power-limiting feature called PowerTune, which is almost the inverse of the Turbo Core capability built into AMD microprocessors. By measuring chip activity, PowerTune estimates likely power consumption and, if needed in specific cases of very high utilization, reduces the GPU's clock speed and voltage to keep power in check. The cases where PowerTune steps in are relatively rare and are usually cased by synthetic benchmarks or the like, not typical games. Knowing PowerTune is watching, though, allows AMD to set the default clock speeds and voltages for its GPUs higher than it otherwise could. That's one reason Tahiti is able to operate at a very healthy 925MHz aboard the Radeon HD 7970.
This new chip takes things a step further by introducing a new GPU state somewhat similar to the "C6" or "deep sleep" feature added to CPUs some years ago. AMD has experience with deploying such tech on the graphics front from the development of its "Llano" CPU-GPU hybrid. Now, with Tahiti, AMD calls the feature ZeroCore, in a sort of play on the whole Turbo Core thing, I suppose. The concept is simple. The Tahiti chip has multiple voltage planes. When the host system sits idle long enough to turn off its display (and invoke power-save mode on the monitor), voltage to the majority of the chip is turned off. Power consumption for the whole video card drops precipitously, down to about three watts, and its cooling fan spins to a halt, no longer needed. A small portion of the GPU remains active, ready to wake up the rest of the chip on demand. AMD says waking a Radeon from its ZeroCore state ought to happen in "milliseconds" and be essentially imperceptible. In our experience, that's correct. As someone who tends to leave his desktop computer turned on at all times, ready to be accessed via a remote connection or the flick of a mouse, I'm a big fan of this feature.
ZeroCore has even more potential to please users of systems with CrossFire multi-GPU configs. Even during active desktop use where the primary video card is busy, the second (and third, and fourth, if present) video card will drop into ZeroCore mode if not needed. Although we haven't had a chance to try it yet, we expect this capability will make CrossFire-equipped systems into much better citizens of the average home.
Even when the display isn't turned off, sitting at a static screen, the 7970 should use less power than the 6970—about 15W versus about 20W, respectively—thanks to several provisions, including putting its DRAM into an idle state. We'll test all of these power draw improvements shortly, so hang tight.
Finally, true video encode acceleration in a desktop GPU
Desktop graphics chips have had video decoding engines embedded in them for ages, growing in functionality over time, and Tahiti participates in that trend. Its Universal Video Decoder block adds hardware decode acceleration for two standards: the MPEG-4 format (used by DivX and the like) and the MVC extension to H.264 for stereoscopic content. Also, the UVD block has the related ability to decode dual HD video streams simultaneously.
More exciting is a first for discrete desktop GPUs: a hardware video encoder. Since UVD refers explicitly to decoding, AMD has cooked up a new acronym for the encoder, VCE or Video Codec Engine. Like the QuickSync feature of Intel's Sandy Bridge processors (and probably the SoC driving the smart phone in your pocket), VCE can encode videos using H.264 compression with full, custom hardware acceleration. We're talking about hardware purpose-built to encode H.264, not just an encoder that does its calculations on the chip's shader array. As usual, the main advantages of custom logic are higher performance and lower power consumption. Tahiti's encode logic looks to be quite nice, with the ability to encode 1080p videos at 60 frames per second, at least twice the rate of the most widely used formats. The VCE hardware supports multiple compression and quality levels, and it can multiplex inputs from various sources for the audio and video tracks to be encoded. Interestingly, the video card's frame buffer can act as an input source, allowing for a hardware-accelerated HD video capture of a gaming session.
AMD plans to enable a hybrid mode for situations where raw encoding speed is of the essence. In this mode, the VCE block will take care of entropy encoding and the GPU's shader array will handle the other computational work. On a high-end chip like Tahiti, this mode should be even faster than the fixed encoding mode, with the penalty of higher power draw.
Unfortunately, software applications that support Tahiti's VCE block aren't available yet, so we haven't been able to test its performance. We fully expect support to be forthcoming, though. AMD had reps on hand from both ArcSoft and Sony Creative Software at its press event for the 7970, in a show of support. We'll have to revisit VCE once we can get our hands on software that uses it properly.
..and even more stuff
Tahiti is the first GPU to support PCI Express 3.0, which uses a combination of higher signaling rates and more efficient encoding to achieve essentially twice the throughput of second-generation PCIe. Right now, the only host systems capable of PCIe 3.0 transfer rates are based on Intel's Sandy Bridge-E processors and the X79 Express chipset. We don't expect many tangible graphics performance benefits from higher PCIe throughput, since current systems don't appear to be particularly bandwidth-limited, even in dual eight-lane multi-GPU configs. In his presentation about Tahiti, Demers downplayed the possibility of graphics performance gains from PCIe 3.0, but did suggest there may be benefits for GPU computing applications.
AMD claims Tahiti is capable of supporting the upcoming DirectX 11.1 standard, a fairly minor incremental bump whose feature list is fairly esoteric but includes provisions for native support of stereoscopic 3D rendering. A future beta driver for Windows 8 will add hooks for DX11.1 support, according to AMD.
As if all of that weren't enough, the Radeon HD 7970 is hitting the market alongside a gaggle of software upgrades to AMD's Eyefinity multi-display graphics technology. Collectively, these modifications have been labled Eyefinity 2.0. Some of the changes are available for older Radeons in current drivers, including tweaks to enable several new display layouts and multi-monitor stereoscopic gaming. Upcoming releases in the first couple months of 2012 will do even more, including the display-geek-nirvana unification: Eyefinity multi-displays, HD3D stereoscopy, and CrossFire multi-GPU should all work together starting with the Catalyst 12.1 driver rev. You'll either have a truly mind-blowing gaming experience or get an unprecedentedly massive headache from such a setup, no doubt.