In the previous post we mentioned Apple's listed hardware requirements for QuickTime 7 HD H.264 playback and suggested that most G4 Macs will have problems with this material. While true, it seems that Apple's latest dual G4 Power Macs can handle at least 720p24 material (but not 1080p material) at full frame rates.
Thanks to those people with dual G4s who posted their results. Going by these reports and by reports at Ars, it seems that Apple has taken a conservative approach and may be overestimating the hardware requirements slightly.
So far the lowest end hardware reported that can sustain 24 fps for the 720p material is a dual 1.0 GHz G4 Power Mac or 1.6 GHz G5 iMac. (The few reports of dual 867 Power Macs so far state that they cannot maintain the full frame rates.) Apple lists support for neither the 1.6 GHz iMac nor any G4 Macs. Apple recommends at least a 1.8 GHz G5. However, one report of a 1.67 GHz PowerBook states that it comes very close, at least for 1280x544 material at 24 fps, so even if the 1.8 GHz G5 970GX doesn't make its way into the next PowerBook, a 1.8 GHz G4 7448 with its doubled L2 cache and 200 MHz bus might just be enough.
Both my Cube G4 1.7 GHz and my PowerBook G4 1.0 GHz get about 12 fps (half the native frame rate) with the 720p Batman Begins trailer. At 12 fps, the video is watchable, but stutters somewhat. It interesting that despite the 700 MHz clock speed advantage, if anything the Cube sometimes does slightly worse than the PowerBook. While the PowerBook has a better GPU, it's possible the difference lies in the bus speed, which is 33% faster on the PowerBook. Fortunately, there are no audio dropouts whatsover on either machine, at least when using 10.4 Tiger, despite the reduced video frame rates. This is even true for 1080p. The audio is fine, even if the video is unwatchable.
The lowest end hardware reported so far that is able to play back 1080p material is a dual 1.8 GHz G5 Power Mac, which is lower than Apple's recommendation of a dual 2.0 GHz Power Mac.
Note, however, that Apple hardware recommendations are based on 30 fps material. Most of the clips on Apple's website are 24 fps. Furthermore, Apple's "720p" material often isn't full 1280x720. The Batman Begins 720p trailer is 1280x544 for instance. Similarly, Apple's "1080p" material often isn't full 1920x1080. The Batman Begins 1080p trailer is 1920x816. It is possible that Apple is being conservative in its hardware recommendations to ensure playback is smooth even in the worst case scenarios, ie. 1280x720 @ 30 fps, and 1920x1080 @ 30 fps.
It will be interesting to see how Windows x86 hardware fares with this material, but Apple has not yet released QuickTime 7 for Windows yet. Apple says it's coming soon though.