ATI Radeon HD 2900 XT: Calling a Spade a Spadeby Derek Wilson on May 14, 2007 12:04 PM EST
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The New Video Decode Pipeline: UVD
UPDATE: Since the launch of the HD 2900 XT, we've learned that all HD 2000 series parts except the high end R600 based parts will have UVD. This means that the HD 2900 XT will feature the same video decode acceleration offered on the R5xx hardware. As higher end cards are usually paired with faster CPUs, AMD feels that UVD on R600 is unnecessary.
This comes as a bit of a surprise to us and to certain board partners who's box art claims UVD as a feature of the HD 2900 XT. We do apologize for any confusion we may have caused at launch.
NVIDIA beat AMD to the punch with their full H.264 decode acceleration on G86/G84. Of course, AMD is one upping NVIDIA this time around, as their UVD (Unified Video Decode) architecture is also capable of decoding VLC bitstreams on all three HD media codecs. This means that there are no cases where AMD hardware will not handle 100% of the video decode process (after the CPU has dealt with pulling the encrypted content off the disk and preparing it to send to the GPU that is).
Here's a diagram of the landscape as it stands now. Notice that G80 is not capable of the bitstream decode or the inverse transformation (either iDCT or otherwise), but G84/G86 come very close to matching AMD's capabilities.
At the same time, we should remember that bitstream decode is only really heavy under H.264. Certainly VC-1 and MPEG-2 will see some benefit, but they are already fairly manageable. NVIDIA stated that building the hardware to handle VLC bitstreams wouldn't have a high enough return on investment. AMD, however, indicated that their bitstream processors are at least a little bit adaptable and it wasn't that difficult to include VLC decode.
Either way, the best way to figure out what's going on is to take a look at performance and see if there really is any advantage to R600 over G86. Unfortunately, try as we might, we could not get UVD to work with the current drivers provided by AMD and the PowerDVD release that is supposed to enable the hardware acceleration on HD 2000 series parts. We will have to take a second look at hardware decode when AMD and CyberLink or Intervideo get their software in order.
For now, our information leads us to believe that performance won't be hugely improved over G84/G86 in MPEG-2 and VC-1 CPU offloading. Where we might start to see a difference is in AMD's 65nm HD 2000 and mobility series parts. These have the potential to decrease power consumption by large amounts and provide quiet running systems for HTPCs, or longer battery life for notebooks. We will have to wait to get our hands on the higher volume R6xx based parts though. Also worth nothing is that AMD's high-end hardware does something that NVIDIA's 8800 series cards currently don't, so NVIDIA users that want fast H.264 decoding support are stuck with slower 3D performance.