why two settings for 640 x 480 and 352 x 240?

There is a question I have with the DVR and probably true for Tivo and
Media center PCs too.

How come we don't set the video size to 640 x 480 and then just use
whatever bitrate to control the recording time?

For example, use 640 x 480, but if we want to record 4 hours, use a
lower bit rate.

Instead, it is first "forced" into 352 x 240, and then use a lower bit
rate. If the compression algorithm could , wouldn't it take care of
compression automatically?


I always wonder if Mpeg1 at 352 x 240 is quite clear with 600MB and is
for 1 hour of video, then for Mpeg2, the size quadruple, and it is 2GB
for an hour... so the improvement from Mpeg1 to Mpeg2 isn't so big as
from Mpeg2 to Mpeg4? It seems that for Mpeg4, only 500MB is needed
for 1 hour of 640 x 480 video.