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AMD's Radeon HD 2400 and 2600 graphics processors

The rest of the family arrives

AMD FINALLY PULLED BACK the curtain on the remainder of its DirectX 10 GPU lineup recently, and we, erm, kinda stumbled on getting the review out at that time. I offered various excuses, including word of a small fire in Damage Labs and limited time with the cards themselves, all of which were true—and dramatic, which really helps sell an excuse. But I was also held back from producing a review by the same borderline obsessive-compulsive impulse that drives us to produce detailed reviews with extensive test results and commentary. We had to get things tested to our satisfaction.

At last, I'm pleased to report, our review is complete. The result isn't perfect by any means (as we are keenly aware), but we do have a number of intriguing things to offer, including a look at the new Radeon HD cards' Avivo HD video acceleration capabilities, with tests of CPU utilization, image quality, and power use during playback. We also have a 3D graphics performance comparison, complete with some thoughts about why ATI's new GPUs tend to fall short of expectations in that department. Keep reading for our take on the new low-end and mid-range Radeons.

RV630 and RV610 burst onto the scene
AMD's family of DirectX 10-class graphics processors is comprised of a trio of GPUs, the R600, RV630, and RV610. We first reviewed the R600 when it launched back in May as the Radeon HD 2900 XT. We covered the basic R600-series technology then, and I'll try to avoid repeating myself here. Go read that review if you want to know more about the core tech. The two new chips with which we're concerned today are derivates of R600, GPUs largely based on the same internal logic but scaled back with less internal parallelism to make smaller, cheaper chips.

The RV610 and RV630 share the R600's unified shader architecture, which dynamically deploys on-chip computational resources to address the most pressing graphics problem at hand, whether it be for pixel shading or vertex processing and manipulation. AMD's new five-wide execution unit is the basic building block of this shader engine. Each of the five arithmetic logic units (ALUs) in this superscalar unit can execute a separate instruction, leading AMD to count them as five "stream processors." In theory, this architecture ought to make RV610 and RV630 more efficient than their immediate predecessors in the Radeon X1300 and X1650 series.

As DirectX 10-compliant GPUs, these R600 derivatives can perform a number of helpful new tricks, including streaming data out of the shader core—a modification of the traditional graphics pipeline needed to enable DX10's geometry shader capability. And for you true propellerheads, these GPUs offer a more complete implementation of floating-point datatypes, with mathematical precision that largely meets the requirements of the IEEE 754 spec.

Beyond the 3D graphics stuff, RV610 and RV630 pack some HD video playback capabilities that they didn't inherit from the R600. AMD has packaged up these video playback features under the marketing name "Avivo HD." The most prominent of them is a facility AMD has dubbed UVD, for universal video decoder. UVD handles key portions of the decoding process for high-def codecs like H.264 and VC-1, lowering the CPU burden during playback of HD DVD and Blu-ray movies. (Despite AMD's initial hints to the contrary, the Radeon HD 2900 XT lacks UVD acceleration logic.) The lower-end Radeon HDs also feature hardware acceleration of deinterlacing, vertical and horizontal scaling, and color correction. With considerably more power at its disposal, the big brother R600 handles these jobs in its shader core.

You may want to display your games or movies on a gloriously mammoth display, and the Radeon HD family has you covered there, as well. All R600-family GPUs have dual-link DVI display outputs with support for HDCP, that wonderful bit of copy protection your video card must support if you want to play HD movies on a digital display (without taking the additional 30 seconds required to install software to bypass it). AMD has embedded the HDCP crypto keys directly in these GPUs, simplifying card design by removing the need for a separate crypto ROM and—we hope—ensuring consistent support for HDCP across all Radeon HD cards.

If your display of choice has an HDMI connection, as many big-screen TVs do these days, the Radeon HD can talk to it via an HDMI adapter that plugs into a DVI port. Uniquely, the R600 and its derivates have an audio controller built in, which they can use to pass 5.1-channel surround sound through an HDMI connection—delivering your digital audiovisual content in a nice, big copy-protected package, just as Hollywood has demanded. This may not be the most exciting of features, but it's more or less necessary for playing back HD movies with digital fidelity.