Graphics Cards Guide

ATI Radeon HD 2900 XT 512MB (Reference Card) review

ATI Radeon HD 2900 XT 512MB (R600)

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Architectural Enhancements Part 2

True 512-bit Memory Interface

You will recognize the Ring Bus memory architecture on the R600 as similar to that found on the Radeon X1000 series. The main difference is that the memory bandwidth has been expanded once again. ATI had dubbed its original Ring Bus memory interface as 512 bits; in truth it was just a pair of 256-bit wide rings going in opposite directions, combined to give 512 bits. It is also not a true ring, since there are direct bus connections passing through the memory controller at the center of the ring for memory read/write requests, making it more of a hybrid ring design.

Thanks to a doubling of the I/O pads that are the bottleneck between memory module and the ring bus, the R600 now has a true 512 bit bus. These pads have been increased in density to allow for 512 bits, leading to greater bandwidth. The Radeon HD 2900 XT for example has one of the highest bandwidth we have seen in a graphics cards at 105.6GB/s, which even surpasses the 103.6GB/s of the GeForce 8800 Ultra. Also, the ring is fully distributed now and hence it has become more scalable than ever since it is now possible to remove any component within the ring without needing to drastically alter the design, which would have been the case for the older ring bus architecture.

Doing HD the Right Way

From the dedicated video processing unit on the new mainstream GeForce 8600/8500 series that actually surpass the one found on the older G80 graphics cards, and now to new Avivo HD video processing logic found on the Radeon HD 2000 series, it won't be an understatement to say that both graphics chipmakers are staking everything on High Definition (HD) taking off in a big way. The brisk sales of HDTVs in the West are probably contributing to their optimism, though there is still an element of uncertainty surrounding the competing Blu-ray and HD DVD.

Fortunately for graphics chipmakers like ATI, the outcome of the HD format battle is immaterial, as the goal is to reduce CPU usage by having dedicated video processing hardware onboard the GPU. In this case, ATI is fully prepared for the HD revolution (after all, the HD inserted into its name is not done on a whim.) All the new Radeon HD graphics cards will come with the Avivo HD package, consisting of a Universal Video Decoder (UVD) and an Advanced Video Post Processor (AVP). These dedicated hardware will take over the video processing, especially the entropy decode segment which taxes the CPU most, and ensure that even a budget PC with a modest CPU will be capable of HD playback without stuttering, provided that they purchase at least a Radeon HD 2400 card. Previously, on the older ATI cards, the bitstream processing and entropy decode portion of HD video processing was not done on the GPU. Similar to NVIDIA, ATI has now remedied this weakness in its Avivo technology. We will have an article in the near future that will delve specifically into the technology behind PureVideo HD and Avivo HD and how they reduces CPU usage so interested readers should look out for that.

Next on the HD agenda includes a common headache faced by PC enthusiasts when it came to assembling a HD home theatre PC. These HTPC system integrators found themselves stepping into a DRM minefield, as prospective buyers had to hunt down which graphics cards had HDMI support for their HDTVs and which had proper HDCP support so that they could playback their HD discs without a hitch. Coupled with the usual FUD and misinformation released by manufacturers, it could actually be more worth it for enthusiasts to wait till the situation has settled down before stepping in.

ATI aims to change this with the Radeon HD 2000 series by having the last say in HDCP compliance graphics cards. Besides having the HDCP encryption keys integrated onboard within the GPU, ATI has gone further by integrating a HD audio controller as well. Partly, this was done to ease the full HDMI implementation with a proper Windows Vista Premium Logo standard HD audio controller that conforms to the protected content output path. This essentially allows HD Video output streaming to your HD television in your living room with audio and video via HDMI, while the PC is still capable of outputting digital audio for gaming or other purposes through the motherboard's audio CODEC. Thus this also ensures the ensured that the entire Radeon HD 2000 series is immediately compliant with Windows Vista Premium Logo requirements, (which states that the audio output cannot be separated from the video for HDMI).

The Radeon HD 2000 series GPUs and their built-in HD audio controller is complemented by a special, Radeon active DVI-to-HDMI converter that allows full video and audio support through the conventional DVI outputs of the graphics card. It is no shabby audio controller either, supporting formats of 32kHz, 44.1kHz and 48kHz 16-bit PCM stereo. It also supports AC3 (5.1) compressed multi-channel audio streams like Dolby Digital and DTS. For the HTPC enthusiast, this is excellent news as any Radeon HD 2000 series card will be able to connect to any HDTV without any fear of conflict since it is guaranteed HDCP compliance and the DVI-to-HDMI converter is all it needs to connect. It works too for ATI since vendors can easily package the same hardware for a variety of market segments; i.e. adding a converter for a HTPC oriented bundle and then removing it for the vanilla model. This reduces both confusion for consumers and adds more flexibility for ATI's retail partners.

** Updated on 1st July 2007 **

The Radeon HD 2900 XT UVD Fiasco

At the time of publishing the article, the above mentioned information was accurate. However, over the course of time, ATI has revealed that the Radeon HD 2900 XT part doesn't have the Advanced Video Post Processor (AVP) and more importantly it doesn't have the Universal Video Deocode (UVD) engine to drastically reduce both the GPU resources and CPU utilization for HD video decoding. The Radeon HD 2400 and 2600 series however have all of these features present to truly support ATI's new Avivo HD initiative that encompass all of these mentioned features.

However, the Radeon HD 2900 XT is still marketed as supporting ATI Avivo HD technology. This is true to a certain extent because the Radeon HD 2900 XT still boasts of an onboard 5.1-channel audio processor, has built-in HDCP and uses its massive GPU processing prowess to still fulfill full HD video acceleration as well as with the assistance of the CPU. So unlike the Radeon HD 2400 and 2600 series where the entire HD video acceleration process is handled by the dedicated UVD engine, the Radeon HD 2900 XT utilizes CPU power for bitstream processing and entropy decode while the rest of the processing blocks are handled by the GPU's traditional processing pipelines (sort of like the original Avivo architecture). Functions executed by the AVP are also handled by the Radeon HD 2900 XT GPU.

As such, expect the Radeon HD 2900 XT to still handle all HD video acceleration needs, just that you'll need a powerful CPU to go along with it - which AMD expects this user demographics to have. In addition to the powerful processor requirement, the other downsides are high CPU and GPU utilization as well as high power consumption. These characteristics are inline with the competing GeForce 8800 series from NVIDIA, but the difference is that ATI didn't make its offerings clear from the very beginning and causing an uproar in the tech community.