Product: AMD Radeon HD 6790
Company: AMD
Authour: James Prior
Editor: Charles Oliver
Date: April 6th, 2011
Package & Specifications

AMD's Radeon HD 6790 uses the Northern Islands series Barts GPU first seen in the AMD Radeon HD 6870 (Barts XT)and HD 6850 (Barts Pro) products. The Barts architecture is a refresh of the Evergreen series Cypress design, with fewer stream processors and AMD's updated 7th generation tessellation unit, featuring improved thread management and buffering, and dual rasterizers. Like Barts XT and Pro based products, the AMD Radeon HD 6790 features a VLIW-5 thread processor design with 80 stream cores per SIMD and 256-bit memory interface to 1GB of GDDR5 memory.

The number of stream cores/ALUs is decreased following the same pattern as the 6870 to 6850 scaling by fusing off 160 SPs, or two SIMDs. Clock speed splits the difference between the 900MHz HD 6870 and 775MHz 6850, at a robust 840MHz. The 1GB of GDDR5 is clocked at the same speeds as the 6870, at 1050MHz or 4.2Gbps QDR. Our engineering sample board is based on the same PCB as the HD 6870, although AMD indicates that there will be a lot of creative product variants from add-in board (AIB) partners in both PCB and cooler design as well as clock speed settings.

Barts Architecture
Barts Architecture

The biggest differences from the other Barts based products is the number of render back ends and stencil units, which is more like the HD 5700 series than the HD 6800 series. The four 64-bit memory controllers feature 16 color ROPs (half the usual number of 32 in Barts Pro/XT). Reducing the number of SIMDs scales down the texture unit count, settling at 40 for Barts LE, vs 56 for Barts XT and 48 for Barts Pro. Like the HD 6800 series, a single CrossFire connector is included for dual-GPU configurations.

AMD Eyespeed Support

The AMD Radeon HD 6790 supports AMD Eyespeed. AMD Eyespeed is the marketing umbrella that covers UVD, General Purpose GPU compute, and AMD Catalyst Video technology. GPGPU is supported via DirectX11 DirectCompute 5 and OpenCL 1.1. With the release of AMD Catalyst 11.3, OpenCL 1.1 acceleration on the GPU is enabled by default, and is being leveraged by mainly video processing and editing applications currently. This is because these companies were the first to work with AMD's close-to-metal (CTM) programming methodologies, such as Brook, CAL and ATI STREAM to perform video transcode, upscaling, and image enhancement. AMD's STREAM has now evolved into support for OpenCL, the open standard compute language that will allow developers to simply and easily use the specialized high-performance floating point (1.34 TFLOPS single precision for the HD 6790; dual precision is unsupported) capabilities of compatible hardware.

AMD Radeon HD 6790 Engineering Sample
AMD Radeon HD 6790 Engineering Sample

AMD Catalyst video controls allow quite granular enhancements for selecting post processing effects to improve the quality of video playback, from both local and web streaming sources. These functions are designed to improve the media consumption feedback by providing deblocking and denoise filters, as well as advanced handling of interlaced and compressed video. The split-screen demo mode allows users to simply and quickly see the effects of their settings vs. the original, unprocessed video. The typical AMD settings are designed to provide a more colorful and vibrant picture, while offloading from the CPU at the same time.

Unlike the Juniper based products, Barts ASICs feature AMD's Universal Video Decoder 3 (UVD3), which supports hardware accelerated decoding for several key video formats including Blu-ray, Blu-ray 3D, Flash, DVD, and DivX. The HD 6790 reference design features two DVI outputs (one Dual-Link and one Single-Link), a HDMI 1.4a output, and two mini-DisplayPort v1.2 outputs. The HD 6790 supports AMD Eyefinity in several configurations, requiring at least one DisplayPort connected monitor. Up to two 'legacy' display outputs can be used, which means either both DVI outputs or one DVI and the HDMI output. Three or four displays can be connected, depending on how many DisplayPort outputs you use. Self-powered active DisplayPort to DVI adapters can be found for around $30, and work well for Eyefinity. The HD 6790, like all AMD Radeon HD 6000 cards launched so far, is Eyefinity 6 capable but requires the use of both DisplayPort outputs and either DisplayPort 1.2 daisy chain capable monitors or Multi-Stream Transport hubs, providing three outputs per mini-DP 1.2 output. Neither solutions is yet available on the market, some 6 months after AMD launched their products - a combination of niche usage and quick adoption of the DP 1.2 standard.

All of the feature disabling measures allow Barts LE to hit the performance target for the $150USD price point AMD is aiming for. Although we'd like to see nice clean lines of delineation for branding of products, with all the Barts based products called one series (in this case 6800s), when AMD did that with the HD 5830 the criticism leveled at was it was more of a 5790 than 5830 because of the backend. Apparently AMD marketing took that feedback to heart, and decided to use the '90 designation of the basically empty 6700 series, which coincidentally matches up nicely with the World's Fastest Graphics Card, the AMD Radeon HD 6990.

AMD Radeon HD 6790 Engineering Sample
AMD Radeon HD 6790 Engineering Sample

As previously stated, the drawback to using a binned ASIC is they typically require higher power, and produce more heat, which in this case leads to the 6790 sharing the form factor and power requirements of the 6850. The plus side is you have a card set up for overclocking - you know you won't be power limited with 225W of power headroom on tap. Indications from AMD are that these cards are quite overclockable and there will be a variety of special overclocked custom cooled cards on offer as we progress through Q2 2011. But of course, they would say that, although this is the segment most sensitive to product differentiation, and a nice cooling design plus a healthy overclock can convince gamers to plump an extra $20 or more for the custom editions.

What this product really highlights is how timely AMD's Powertune technology could have been. Both the HD 6870 and 6790 could really, really have benefited from power containment technology being implemented in the Barts ASIC to provide some really smart products in the gamer sweet spot price range. In this respect AMD are a victim of their own technological prowess, as if we had not seen the stability and performance that the 6900 series offers in guaranteed power profiles we wouldn't have much to complain about with the HD 6870 and 6790 cards. As it is, we have seen it, and absolutely wish PowerTune existed in the HD 6700 and HD 6800 products, where lower rated and quality power supplies are more predominant, making systems more susceptible to problems with gamer performance class video cards installed. AMD could probably have released the HD 6700 series to retail as well as OEM channel if it had featured power containment as well as Blu-ray 3D certification.

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