CPU Usage Benchmarks
While HD post processing tested in the HQV benchmark only works on the 2600/8600 cards, the new low-end 2400/8400 cards are supposed to be able to take almost the entire load off of the CPU when HD video is being decoded, just like their big brothers.
The two main codecs used in HD disks are the VC-1 codec, and the newer H.264 (AKA MPEG-4) codec. VC-1 is the simpler of the two and is therefore easier for the PC to decode. Newer titles use the H.264 codec, which requires much more work to decode, and will bring many modern CPUs to their knees. The 2600/2400 cards will accelerate the entire video pipeline when decoding both VC-1 and H.264 titles. Nvidia's 8600/8400 cards will accelerate the entire H.264 process, but do not assist the entire pipeline when decoding VC-1, so it will be interesting when comparing the results when decoding King Kong (a VC-1 title).
Let's see how hard the CPU will work when we play HD video with both the VC-1 and H.264 codecs on Windows XP and Windows Vista, in conjunction with these cards.
The VC-1 Codec In Windows XP And Windows Vista
As we expected, the Radeon cards have an advantage with the VC-1 codec, due to the fact that the GeForce cards do not assist in accelerating the entire VC-1 decode pipeline. However, even though the Radeons have a 20% CPU advantage, the GeForces are still taking a good sized load off of the CPU. This is a testament to the newer Geforce drivers, as the ones we previously tested only showed decode acceleration in Windows Vista, and not in XP. Let's see if these results will remain consistent when we move from Windows XP to Windows Vista.
Indeed, the results do look similar, with the Radeons requiring fewer CPU cycles than the Geforces. It also appears that the entire playing field has been bumped up closer to the level of unaccelerated CPU usage, probably due to Vista's increased overhead over XP. It also looks like there are more spikes of CPU usage in Vista compared to XP.
Now that we've examined the VC-1 title on both operating systems, let's see what H.264 looks like. In theory, the results should be even closer, as both Radeons and GeForces accelerate the entire H.264 decoding pipeline.