I-Frame or Long-GOP?

[Ronald Jackson]

Apologies if this question has been asked/answered before but I'd appreciation any recommendations relevant to my simple needs.

I video birds in the main via my XLH1 and nanoFlash, editing with FCP. I am currently using I-Frame 180mbps. Birds of course often move quickly and I have noticed some motion "blur" or distortion on clips when they are moving quickly e.g. feeding or walking.

I recently changed from Long-GOP 100, and notice that earlier clips shot at that setting seem to be less prone to such distortion.

I am unsure as to the criteria that would make someone "in the know" opt for I-frame over Long-GOP.

Maybe I would be better switching to Long-GOP 140 or 180. File size should not be a problem, and my editing involves very little rendering.

Ron

[Simon Wood]

What shutter speed are you using? You could try increasing the shutter speed as this would have more of an impact of blurring than I-Frame/LongGOP I would imagine.

[Ronald Jackson]

A 50th and at 25p. Never adjust shutter speed, following recommendations for 25p shooting.

Still interested in finding out the difference between I-Frame and Long-GOP and why some prefer one to the other,

Ron

[Dan Keaton]

Dear Ron,

Intraframe codecs compress each frame individually.

Long-GOP codecs take into consideration that the current frame of video may be very similar to the preceeding frame, or may be very similar to the following frame.

Since Long-GOP codecs can, in some circumstances, encode just the differences between two frames and can undstand that the image may have been panned or tilted or both, they can be more efficient than Intraframe codecs.

Here are my general guidelines.

50 Mbps Long-GOP using our Sony XDCam 4:2:2 codec is broadcast quality (and works well in Avid).

100 Mbps Long-GOP is widely accepted as being better. But will not work in all versions of Avid.

I-Frame Only is an Intraframe codec, so it is less efficient than Long-GOP

Our minimum bit-rate is 100 Mbps for I-Frame Only.
I recommend using 50 Mbps Long-GOP instead.

140 Mbps to 280 Mbps I-Frame Only flavors are recommended.

Thus, the flavors that I typically recommend are:

50, 80, 100 Mbps Long-GOP
(50 Mbps Long-GOP only for Avid except for the very latest version of Avid)

140, 180, 220, 280 Mbps I-Frame Only

[Ronald Jackson]

Thanks Dan. Will I gain much then by using 140 Long-GOP as opposed to 100 ?

As I mentioned I do not feel constrained either by storage capacity or any NLE issues, little rendering and I'm a FCP man.

Ron

[Dan Keaton]

Dear Ron,

If you are filming a movie, then you may want to consider an option higher than 100 Mbps Long-GOP.

If so, you may want to consider 220 Mbps I-Frame Only or 280 Mbps I-Frame Only.

[Ronald Jackson]

Thank you again Dan. I'm no movie maker so Long-GOP 100 will suffice I think. Just a final point about I-frame, excellent for video grabs for those who might need them,


Ron

[Dan Keaton]

Dear Ron,

There is a common misconception that the image quality of a single frame of video is better if I-Frame Only is used instead of Long-GOP.

And some think that fast action will not be properly handled if Long-GOP is uesd.

Both of these concepts are wrong.

Please note that since we support both equally well, we can be neutral in our recommendations.