Bit Rate, Frame Dropping, Frame Bending, Mathematics

L

Lyndrick Collins

Member
Wanting to mix GH2 24p footage with Sony Action camera Footage.

Let's say you have an Action camera with the following specs:

1080p at 50fps Bit Rate = 28mbps
1080p at 25fps Bit Rate = 16mbps

You intercut the 50p (25p) (Close range and POV) with Multicam DSLR footage of 24p (Medium and Wide).
You Drop the Footage into a 24p timeline with a 24mbps Bit Rate and have the option of either dropping frames or bending frames.

1- How much information is lost conforming 50p to 24p?
2- Will the amount of information lost from the 50p footage lead to the 25p (16mbps) footage maintaining a higher Bit Rate in the 24p timeline?
3- Is it better to drop the 50p into a 25p time line, render at 25p, and then drop the new file into the 24p timeline?
4- Is it better to just forgo the 50p and use the 25p (16 Mbps)?
 
The bitrate doesn't have much, if anything, to do with frame rate mixing or conversion. A file "retains" its bitrate in any timeline.

You should pick your frame rate because it gives you the motion (live-looking (50p), or fictional-looking (25p, 24p)) you want the footage to have.

If you want to deliver as 24p or 25p, or if you're planning to convert 50p to 25p, then there's no point in shooting 50p at all unless you plan to use it for slow motion, because the conversion to 25p or 24p will cause the footage to lose its 50p motion.

If you want to preserve the live-looking motion of 50p, then you should edit and deliver as 50p. The 25p and 24p footage will retain its "fictional" look.

As for what makes a better conversion to 24p -- converting 50p to 25p first and then converting to 24p, or converting 50p directly to 24p -- either could work; it depends on your software. This is where you'll have to experiment and test. Shoot 50p and test it. But as I said, there's really no point in doing this; just shoot 25p. (Using 50p for slo-mo is an entirely different process.)
 
David, Thanks for the reply. I was looking at bitrate more because I will be doing a lot of post work, and I need something that will hold up better. I would rather shoot those sections in 24p, but the camera I have only records in in 60, 50, 30, and 25. 25 is the closest to the rest of the workflow, but also has the least amount of picture information.
 
If those are your frame rate options, and you want 24p delivery, then shoot in 60p. 60p will convert to 24p better than any of the other listed frame rates, and if you decide you want to retain the live look of 60p, you can edit both 60p and 24p on a 60p timeline with no worries. 24p renders out in a 60p stream with no problems at all. If you're going to convert it all to 24p, just be careful when you shoot -- you need to move the camera exactly the same as you would if you were shooting 24p, else when you convert, you'll see all the same strobing problems you'd get by moving the camera too quickly shooting 24p.

You COULD shoot 30p, but 30p and 24p do not convert to each other very well. You could edit them both on a 60p timeline and then export as 60p, and everything about both will be preserved (no need to convert one to the other), but you'll find that 30p and 24p don't look exactly the same. But 60p converted to 24p will look 99% identical as long as you handle the camera well.

Yes, higher bitrates are better for color correction and other post manipulation, but really don't factor into frame rate conversions. Higher never hurts, but it doesn't really matter.
 
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