Why does 720 24p have less skew than 1080 24p

[Jan Crittenden Livingston]

Originally Posted by Thomas Horton

Jan,

I appreciate the input, but can you tell us why the HPX300 has far less skew in 720 24p than 1080 24p? The full CMOS chips (1920x1080 raster) are being utilized in both, but the only difference is that 720 24p is recorded @ 1280x720 (down rezed from 1920x1080)... Don't get me wrong, I love the camera, but just don't understand why there is such a difference in skew between the two 24p recording modes...

Figure the diff between the two sizes, 1920 X1080 vs 1280 X 720. One is twice as large as the other. The smaller is clocking faster because it is smaller. Watch the video for ideas on how to work with it.

Best,

jan

[Thomas Horton]

But... they are both being read off of the imagers (CMOS chips) at exactly the same full raster size.. 1920x1080. 720 24p mode is simply down rezzing after the fact.. after it has been processed from the imager.. If it was a case of smaller versus larger, then there would be a crop factor negative size value offset. Both 720 24p and 1080 24p clock the same... 24 fps..

I did watch the video.. and yes.. it is very helpful :)

[Jan Crittenden Livingston]

Originally Posted by Thomas Horton

But... they are both being read off of the imagers (CMOS chips) at exactly the same full raster size.. 1920x1080. 720 24p mode is simply down rezzing after the fact.. after it has been processed from the imager
I did watch the video.. and yes.. it is very helpful :)

But when it comes off the dsp, it is going to a smaller format. Its smaller, therefore less. Think about it, this all happens in time frame that I can't even measure, I am not an engineer.

Don't know that that helps, but....

Jan

[David Heath]

Originally Posted by Jan Crittenden Livingston

But when it comes off the dsp, it is going to a smaller format. Its smaller, therefore less. Think about it, ...........

I can't see how that explains it, not if the 720p/24 image is derived from a full-sensor 1080p/24 image - and it must be so, since if it was as simple as using the middle half of the sensor (in area terms) the angle of view would change between 720 and 1080 modes for the same lens focal length. (And it doesn't.)

If we imagine the camera panned across a vertical line, lets assume the panning speed is such that in the 1080 frame, the bottom of the line is offset 60 pixels with respect to the top of frame. The skew then becomes 60/1920=1/32 of the frame width from top to bottom.

Do a downconvert to 720p, and those 60 pixels must scale down to 40 pixels (60x 1280/1920), so yes indeed, smaller, therefore less. BUT those 40 pixels still represent 1/32 (40/1280) of the frame width - and consequently exactly the same skew!!