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Birate versus Image Size

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Jimmy Stephans
Birate versus Image Size
on Feb 4, 2012 at 5:03:34 pm

I have a group of about 57 video clips, 3-4 minutes each, that had been exported as 8-bit uncompressed from FCP (6) / Compressor a year ago. 1920x1280.

We now want to export them as h.264 / MOV files for customer downloads in both 1080P and 720P sizes.

I was playing with a few and using "automatic" for bitrate just doesn't work. Some of the files are bikini models moving around on beaches with many trees and other objects blowing in the wind behind them. One three minute clip, compressed on auto, came out as 1.24GB. Too large for our needs.

I've been testing and seem to have found a sweetspot for limiting the bit rate at about 7500. All my testing was done from the 8-bit file to 1280x720 and I'm happy.

My question is: When I go back through them all to make 1920x1080 MOV clips should I assume that I need to raise the bitrate?

I'm not fully understanding the bitrate versus size equation.

Thanks in advance,

Jimmy


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Rafael Amador
Re: Birate versus Image Size
on Feb 4, 2012 at 5:41:02 pm

Hi Jimmy,
In the end you are dealing with "pixels" as the smaller piece of information that you have to codify.
The bigger the picture, the bigger the number of pixels.
A 1920x1080 contains 2.073.600 while 1280x720 has 921.600.
That is a bit more than the double number of pixels, so to get similar quality on the 1920x1080 clips you should at least duplicate the data rate of the 1280x720.
That accounting that all the clip have the same number of frame per second.
7/8Mbps for 1280x720 and 15mbps for 1920x1080 are quite reasonable data rates to get a decent picture without making too big files.
BTW, set "Key frames: automatic", Multipass and "Frame reordering" to get the best quality.
rafael

http://www.nagavideo.com


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Jimmy Stephans
Re: Birate versus Image Size
on Feb 4, 2012 at 6:19:24 pm

OK, thanks -- that help a bunch.

As a small sidebar question or two: I expect some customers to want to eventually play / stream the clips through a media player, or even an XBOX, to a TV.

1- Would .mp4 (h.264) be better (not sure XBOX does .mov)?

2- If I go with .mp4, does anybody know if bit rates above 10,000 are OK with an XBOX. I stumbled on a forum that said said NO, and another that said yes.

Thanks


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Michael Gissing
Re: Birate versus Image Size
on Feb 4, 2012 at 9:05:34 pm

Online video sites like Vimeo have published specs. They usually have data rates for HD content around 8000. I think that is both a sweet spot for H264 and the ability to play smoothly on a wide range of computers.

As Rafael says 1920 x1080 should require a higher data rate but if you want maximum compatibility on players then perhaps exceeding 10000 might start to cause playback issues for some players or systems. DVDs are all throttled to less than 10,000 and that seems to have set a benchmark.


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Jimmy Stephans
Re: Birate versus Image Size
on Feb 4, 2012 at 9:21:00 pm

Thanks.

I did find a 2007 Microsoft / X-Box page with 10,000 listed - but not as a MAX, just a common spec for 1080P.

I'll add more here if I find it, and hope others will too.


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Phil Balsdon
Re: Birate versus Image Size
on Feb 5, 2012 at 8:13:12 am

Might be of interest

http://library.creativecow.net/lawrence_david/Video-Purifier_and_x264/1

Cinematographer, Steadicam Operator, Final Cut Pro Post Production.
http://philming.com.au
http://www.steadi-onfilms.com.au/


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Rafael Amador
Re: Birate versus Image Size
on Feb 5, 2012 at 10:47:07 am

Hi Jimmy,
XBox plays Blu-ray, so should support that data rate.
The default data-rate for BR in Compressor are:
- Average bit-rate = 30Mbps
- Max bit-rate = 35Mbps.
That should work for 1080i25/30 and for 720p50/60.
With that data-rate you get almost two hours movie in a 25GBs disk.
So that's more or less the data rate in commercial BR disks and should play in domestic BR players too.
rafael

http://www.nagavideo.com


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Jimmy Stephans
Re: Birate versus Image Size
on Feb 5, 2012 at 6:07:20 pm

Thanks guys. I did create a file, send to a pal with XBox360, and it played at 12500 just fine for him.

Also sending from his iMac to Apple TV it worked fine.

Jimmy


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