So, you have the specifications for the hardware ? In that case, feel free to share. If not, your assumption is unjustified. Just because settings like this would not require more performance does not mean they can actually be set. Readout of the CCD depends on a lot more than just the CPU.
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Ok, I have a senzor in my camera that is capable to make movies at max. 640x480/30 fps. I can understand that due to the senzor limitation, it is (maybe - because, nobody cannot prove it, yet) impossible to use this senzor to make HD movies like 1280x720, but, how I can belive that it is impossible to use same senzor to capture smaller resolution ?... It is like you say for cannon sx20is, that is capable to make movies in HD 720p format (1280x720) but cannot make VGA movies...
If chdk have not support to enable 16:9 format for low resolution, this is it. But, this, do not proove that is impossible... To have 640x360 instead 640x480 it is a matter of software. Hardware (senzor + cpu) that is capable to compress 640x480 can also do 640x360. Maybe I'm wrong, but nobody try to explain why...