Frame rates and bit rates

[Filmbuff28]

I have been looking at the technical info from some of my 1080 clips and found something odd, at least to me. 1080i24pa has a bit rate of 11.3 and 1080i30p has a bit rate of 14.1. I am confused because they both use 60 fields which should mean equal file sizes. anyone know what is up?

Also as a side note, why isn't there a 1080i24p capture setting in FCP? There seems to only be 1080i24pa. is there any benifit that I am missing by not being able to use 24p instead of 24pa?

I ask this because I have noticed something strange in all of my 24pa clips. In the top left of the image there is a wobble of what seems to be the top horizontal line of the image. I have also noticed this in 480i24pa shot in the hvx. This is easily fixed by increasing the scale by 1% but I feel that I should have to do that in every 1080i24pa project I do. Has anyone else run into this problem? Any solutions?

Barry, would you be so kind as to share your expertise on the explanation of the 1080 bit rates?

Any thoughts, comments, or suggestions are appreciated.

 

[ullanta]

When you capture 24pA into FCP it removes the redundant fields to give you true 24p video (23.98, really). Without any redundancy, 24 frames per second has only 80% of the information/data rate of 30 frames per second - that is, 24/30 = 11.3/14.1 = .8

-Barry

I'm not sure about the wobble you mention... but if that is really the case, you'd be better off cropping the top line than scaling the whole image.

 

[Filmbuff28]

ullanta,
Thanks, that explains difference in bit rates very well. One question about your "wobble" solution. When you say crop do you mean take of the first couple lines of the top? Could that end up looking wierd if the top has a black line and the bottom doesn't if it were on a widscreen tv? Basically what I'm asking is what are the advantages of cropping over scaling?
Thanks again for the help

 

[ullanta]

In general, TV's don't show the whole image - depending on the technology and model, you lose up to 10% from each side of the image (this is what "safe zones" are all about. A computer will typically display the whole image, and if you look carefully you'll notice that footage from many cams (like the DVX) often has a black line on one or more edges. For us humans, a little black around the edge isn't really noticeable (especially on TV's which always have a little bit of black mask around the edges).

However, scaling the whole image requires it be recompressed, the whole thing loses a little resolution, and you're losing information from all edges of the image - that is, the quality of the entire frame degrades a bit. Blech!

So, if it's really just one line and you're not already scaling - cropping rocks!

Now you could also find a way to make the next-to-top line replace the top line (that is, copy it) to fill the exact frame without a black area. But that's a little more work for little more benefit.