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In this test, graphics cards without hardware-based H.264 acceleration must use the CPU to decode video. This explains why the Radeon 2900 XT reports much higher power consumption. All of the Radeon HD 5000-series cards run at higher clocks than idle. But, for the most part, the graphics core isn’t doing much work.
With support for hardware-based video acceleration, modern graphics cards can lower overall power consumption levels significantly. The system consumes around 80 W with the Radeon HD 5670 and 90 W with the HD 5770. As for efficiency, look at the Radeon HD 5670’s results. It has roughly the same power consumption as the integrated Radeon HD 3300, likely due to the similar video decoding block inside the graphics processor.
You can see the difference hardware decoding offers by comparing the two discrete 5000-series cards and the Radeon 2900 XT. With higher idle power consumption and the processor running full speed, the system consumes almost 70 W more power, or almost twice as much.
A mere 20 watts separate the Radeon HD 3300, HD 5670, HD 5770, and HD 5870 1 GB. So, in certain cases, the Radeon HD 5870 1 GB can still save enough power to close in on its more mainstream derivatives. Again, this is the case because the cards use a fixed-function video engine to assist in decoding acceleration, which is the same from one board to the next. Thus, even a high-end card behaves like a lower-end product in such a workload. This is very important, as you will see later on.
The Radeon HD 5870 2 GB draws much more power than the 1 GB version. Too much, in fact. It's about 20 W higher than the 1 GB card. Checking the GPU voltage and measuring idle power draw did not shed any light on this behavior. GPU voltage is the same for both cards, and the HD 5870's idle consumption is only about 5 W higher.