Video Decoding CPU Utilisation

NVIDIA have now been using their "Pure Video" programmable video decoder initially introduced on NV40. The Pure Video decoder was enhanced on the rest of the NV4x line for better high definition WMV9 support and the decoder for G70 is largely derived from this later implementation with some further tweaks and enhancements to improve performance.

The video decoding capabilities of the graphics processor should offload more video operations from the CPU to be processed directly on the video processor. Here we'll take a look at the CPU utilisation under three decoding operations, DVD MPEG-2, High Definition (1080i) WMV9 and DiVX in order to see which board provides the lowest CPU utilisations under each of the formats. In each case we used the same Terminator 2 trailer clip found at Microsoft's High Definition Video site, converted it into the other two formats and played each one back through Windows Media Player 10, monitoring the CPU utilisation via the Performance Monitor in the Microsoft Management Console.

DVD MPEG2 (% CPU Usage) Min Max Average
7800 GTX 1.56% 10.94% 6.15%
6800 Ultra 4.69% 12.50% 8.36%

DVD MPEG2 files are not the highest of resolutions, nor is MPEG2 a particularly complicated codec these days hence the SPU utilisation in the first place is fairly low on these boards. However, here we can see that the GeForce 7800 GTX has a lower CPU overhead than the 6800 Ultra has, shaving a few percent off the overall CPU utilisation.

DiVX (% CPU Usage) Min Max Average
7800 GTX 18.75% 32.81% 25.22%
6800 Ultra 17.19% 26.56% 21.54%

With DiVX, which utilises high compression ratios than MPEG2, has a higher CPU utilisation in general, however in this case the 7800 GTX actually has a slightly higher CPU utilisation than 6800 Ultra.

WMV8 - 1080i (% CPU Usage) Min Max Average
7800 GTX 31.25% 54.69% 42.91%
6800 Ultra 34.38% 56.25% 46.02%

Finally, the WMV9 format here has the highest CPU utilisation due to the higher resolution of the video output. In this instance the GeForce 7800 GTX is performing with a slightly lower CPU utilisation than the (PCI Express) 6800 Ultra is.

Overall it looks like the performance of the video engine in G70 has been upgraded a little amount, but not a great deal, and the higher CPU utilisation under DiVX is probably due to drivers not being fully optimised for the 7800 GTX at this point in time.