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although the fact that it can record two and play a third is pretty amazing.

Hi Def streams are likely to put less load on the TiVo when recording, as all it needs to do is record the digital stream, instead of encoding an analog one into digital. Similar to how a DirecTiVo has an easier time since it just records what comes over the dish.

Playback on the other hand will put a higher load since it is a higher quality signal, and the TiVo has to do sone digital -> analog conversions for the component ports.