Radeon HD2900XT review

Decoding
Eh? What’s this entropy decode we’re talking about in regards to UVD? Basically when a video stream is encoded losslessly, information is thrown away. Entropy decode can rebuild this exactly using existing data, providing a video that is identical to the original before encoding.

AVIVO HD completely accelerates the HD video decoding process, while older techniques still rely on the CPU to complete

Three common methods that can be used for entropy encode are Variable Length Coding (VLC is just basic compression – look for repeated datasets and reduce this to a smaller code – this is used in MPEG-2 and VC-1), Context Adaptive Variable Length Coding (CAVLC is where multiple coding schemes are used, with compressed elements bundled with the key used to encode – compatible with H.264) and Context Adaptive Binary Arithmetic Coding (CABAC is also used in H.264 and allows compressed elements to be stored as non-integers).

The second part UVD handles is Frequency Transform. If you read the codec feature, this can often involve Discrete Cosine Transformation (DCT), a lossy form of compression. In layman’s speak, when a video is being encoded, a given set of numbers are divided by a preset number, and that result is then rounded off to a lower level of precision to reduce the amount of data that needs to be stored – for example, a floating point number may be reduced to the nearest integer. During the decoding process Inverse DCT (iDCT) is performed, and a coefficient matching the earlier divisor is applied – the final result is close to the original number, but is not exactly it. The final image is close to the original but not perfect. Post-processing can be performed in an attempt to remove any errors introduced by the iDCT process.

Benchmarks
The ever super-glowy happy people at Australia IT sent us an HD2900XT from the equally super-glowy happy people at PowerColor for testing.



Preliminary results showed the 2900 to be all over the place – just below the GTS in some benchmarks, and almost reaching the GTX in others. To this end, we waited for a driver revision (8.37.4.2) and then widened our usual benchmark set to get a picture of just what this enigma card was.

We'll be testing this card again with the 7.5 drivers, and will post new results at the first opportunity.

Conclusion
As you can see using the existing driver set, the HD2900XT in most circumstances performs close to or under the 8800 GTS 640MB. Taking this into account and considering it has a stonking maximum power draw of 225W – the GTS is around 106W – a slightly louder cooler and significantly higher weight, we’d buy an 8800 GTS over a HD2900XT at this stage in the game.

AMD never intended to release a GTX killer – and it’ll be interesting to see if there’s any fallout as a result. The top-end cards typically don’t pull in a lot of revenue – it’s mainstream where the money is. The perception though is that they achieve a halo effect – resulting in the idea that if technical and performance domination is achieved at the top-end of the scale, surely this must filter down to the lower-end.

Of course this isn’t necessarily the case, but the general public is a slow and ponderous beast, and if a simple statement is made that ‘Brand X is the fastest’, Joe Average typically tends to apply it to a full range regardless of fact. We’ll be fascinated to see if this theory holds water – to our knowledge there has never been a deliberate attempt at not producing a competing card for the high-end since the ATI and NVIDIA war started proper.

Let’s take a perspective check here though. For a start, the performance tests done here are DirectX 9 only – so the picture may change later on in the game with Vista, excluding any enhancements offered by drivers. DirectX 10 demos are now available – such as a Call of Juarez benchmark provided by AMD, or an NVIDIA-supported version of Lost Planet – however both are incomplete, buggy and are biased towards each vendor. As such we’ll be waiting for a release-quality game or benchmark before providing relevant results.

The R600 architecture brings some fascinating ideas to the table (tessellation), and some welcome additions (guaranteed HDCP, sound over HDMI, and in the mainstream cards, UVD) that hint that in the next round R700 could be something special indeed.

For those in the market for a good value performance card, the HD2900XT is a decent offer – however in the performance stakes it is outdone in almost every avenue by the 8800 GTS. If the price is kept low enough, it could be a good solution. Those hunting for ultimate performance though don’t even have to make a choice – the only answer is NVIDIA’s 8800 GTX.

Now AMD, bring on the mainstream cards!