Number of iframes different in 24p? at DVinfo.net

Jim McQuaid · November 19th, 2007, 11:48 AM

I cannot find my original source (late night surfing, I'm afraid) but I believe I read somewhere that the EX generates more iframes (or was it only iframes) when shooting at the 24fps rate.

If so it would end any reservations about the MPEG long GOP issue. Does anyone have any information about this or I am just projecting?

Steven Thomas · November 19th, 2007, 11:55 AM

It's long GOP and it looks great, especially at 35 mb!

Piotr Wozniacki · November 19th, 2007, 12:47 PM

Quote:

Originally Posted by Jim McQuaid

I cannot find my original source (late night surfing, I'm afraid) but I believe I read somewhere that the EX generates more iframes (or was it only iframes) when shooting at the 24fps rate.

If so it would end any reservations about the MPEG long GOP issue. Does anyone have any information about this or I am just projecting?

In 25p, the GOP is 12-frames long; I guess 24p is not different.

Thomas Smet · November 19th, 2007, 01:16 PM

Are you talking 1080p or 720p?

with 720p 24p the bitrate is virtually already as good as I frame only because there are 2.5 less frames to encode then 60p.

If you are talking 1080p then I don't know. Be carefull however the more I frames you add in the harder it gets to compress at lower bitrates. If you add too many I frames the compression quality could actually look worse.

The other thing to think about is that even with a GOP of 12 that still only equals 2 I frames per second of video. The same as if you used a 15 GOP with 30p. It's all about the ratio of I frames per the rate of the video. If you could have video at 6p you could have a GOP length of only 3 frames but that would still result in only 2 I frames per second.

November 19th, 2007, 11:48 AM	#1
Jim McQuaid Regular Crew Join Date: Nov 2003 Location: Durham, North Carolina Posts: 61	Number of iframes different in 24p? I cannot find my original source (late night surfing, I'm afraid) but I believe I read somewhere that the EX generates more iframes (or was it only iframes) when shooting at the 24fps rate. If so it would end any reservations about the MPEG long GOP issue. Does anyone have any information about this or I am just projecting?

November 19th, 2007, 11:55 AM	#2
Steven Thomas Trustee Join Date: Sep 2005 Location: Gilbert, AZ Posts: 1,896	It's long GOP and it looks great, especially at 35 mb!

November 19th, 2007, 01:16 PM	#4
Thomas Smet Trustee Join Date: Mar 2004 Location: Milwaukee, WI Posts: 1,719	Are you talking 1080p or 720p? with 720p 24p the bitrate is virtually already as good as I frame only because there are 2.5 less frames to encode then 60p. If you are talking 1080p then I don't know. Be carefull however the more I frames you add in the harder it gets to compress at lower bitrates. If you add too many I frames the compression quality could actually look worse. The other thing to think about is that even with a GOP of 12 that still only equals 2 I frames per second of video. The same as if you used a 15 GOP with 30p. It's all about the ratio of I frames per the rate of the video. If you could have video at 6p you could have a GOP length of only 3 frames but that would still result in only 2 I frames per second.