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Power
The power consumption of an add-on video card can be estimated by comparing
the total system power draw with and without the card installed. Our results
were derived thus:
1. Power consumption of the graphics card at idle - When CPUBurn is
run on a system, the video card is not stressed at all, and stays in idle mode.
This is true whether the video card is integrated or an add-on PCIe 16X device.
Hence, when the power consumption of the base system under CPUBurn is subtracted
from the power consumption of the same test with the graphics card installed,
we obtain the increase in idle power of the add-on card over the
integrated graphics chip (Intel GMA950). (The actual idle power
of the add-on card cannot be derived, because the integrated graphics does draw
some power — we'd guess no more than a watt or two.)
2. Power consumption of the graphics card under load - The power draw
of the system is measured with the add-on video card, with CPUBurn and FurMark
running simultaneously. Then the power of the baseline system (with integrated
graphics) running just CPUBurn is subtracted. The difference is the load power
of the add-on card. (If you want to nitpick, the 1~2W power of the integrated
graphics at idle should be added to this number.) Any load on the CPU from FurMark
should not skew the results, since the CPU was running at full load in both
systems.
Both results are scaled by the efficiency of the power supply (tested
here) to obtain a final estimate of the DC power consumption.
|
Power Consumption Comparison (DC) |
|
Card |
Est. Power (Idle) |
Est. Power (Load) |
|
PowerColor SCS3
HD 4650 |
15W |
28W |
|
Asus EAH3650 |
18W |
39W |
|
ATI Radeon HD 4670 |
3W |
40W |
Our PowerColor HD 4650 exhibited slightly lower power consumption than Asus'
passively-cooled HD 3650 when idle (3W), and significant savings at full
load (11W). It's a nice improvement considering the 4650 is the 3650's direct
successor. The HD 4670 remains the
king of idle power consumption however, even though it is faster and uses much
of the same software and hardware under the hood.
It's difficult to pinpoint exactly why the idle power is so high. It may be
the result of differences in chipset implementation by the manufacturer, or
the use of slower, yet more power hungry memory, or a combination of both. It's
hard to believe either factor alone could result in an extra 12W. We doubt drivers
are the issue — ATI has had plenty of time of to sort out any serious power
issues for their HD 4000 series, especially since they release a new set of
drivers every month. Some form of power management was in effect — the
core/memory speeds lowered to 300/299MHz when the system was idle and during
video playback it alternated between idle and reference speeds.
Video Playback
|
Video Playback Comparison
|
|
Video Clip
|
HD 4650
|
HD 3650
|
HD 4670
|
|
Mean
CPU
|
AC
Power
|
Mean
CPU
|
AC
Power
|
Mean CPU
|
AC
Power
|
|
Rush Hour
(H.264)
|
4%
|
~102W
|
2%
|
~102W
|
3%
|
~94W
|
|
Coral Reef
(WMV-HD)
|
30%
|
~115W
|
28%
|
~117W
|
28%
|
~105W
|
|
Drag Race
(VC-1)
|
64%
|
~137W
|
72%
|
~141W
|
63%
|
~129W
|
As the video decoding hardware is the same in both the HD 3000 and 4000 series,
CPU usage measurements were unsurprisingly similar, passing every test with
ease. The HD 4650's power consumption during video playback was within a few
watts of its predecessor, the HD 3650, but far behind the ultra-efficient HD
4670. This is due to the higher idle power consumption of the HD 3650 which
carries over to light-load situations like video playback.
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