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More important? 8bit vs 10bit or 35 vs 50Mbps?

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jono kardno
More important? 8bit vs 10bit or 35 vs 50Mbps?
on Feb 16, 2011 at 12:32:53 am

We are about to do a series with alot of greenscreen keying in it. We are looking at 2 cameras for the filming - the Sony EX3 and the Canon XF305. We are recording direct to computer (so we can use the HDSDI for higher quality capture). In this scenario the EX3 will do 10bit 35Mbps and the XF305 will do 8bit 50Mbps as far as I am aware.

My question is what would be the best choice for Keying? Is it better to have the 10 color depth at only 35Mbps or only 8 bit color depth but 50Mbps?

Any assistance gratefully received,

Jono


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Tim Ward
Re: More important? 8bit vs 10bit or 35 vs 50Mbps?
on Feb 16, 2011 at 3:19:42 pm

Both the EX3 and XF305 record 8-bit 4:2:0 Long-GOP video. HD-SDI is uncompressed (1.5 Gb/s). When you record from it, you bypass the camcorder's internal codec. I don't know (and can't find it in the owner's manuals) if they output 8-bit or 10-bit or what color space the HD-SDI output uses but I'd assume that it's 4:2:2 (and not 4:2:0), which is more important for chroma-keying than bit depth or bit rate - within reason. I would use the EX3 because of its 1/2" imager (compared to the XF305's 1/3" imager).


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jono kardno
Re: More important? 8bit vs 10bit or 35 vs 50Mbps?
on Feb 16, 2011 at 6:20:25 pm

Thanks for the great reply. I have found the output specs for the xf305 and it says:

The HD-SDI output provides an uncompressed, 1.485 Gbps signal at 1920x1080 resolution with 4:2:2 color sampling and embedded audio and time code. http://www.usa.canon.com/cusa/professional/products/camcorders/hd_video_cam...

and the HD-SDI on the EX is 10bit 4:2:2.

We will be capturing to ProRes 4:2:2 HQ 1080p (Target data rate of approximately 220 Mbps (1920 x 1080 at 60i)).
If this is the case... do you think the XF's uncompressed signal would win over the EX?


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Dave LaRonde
Re: More important? 8bit vs 10bit or 35 vs 50Mbps?
on Feb 16, 2011 at 6:50:25 pm

Unless you have a device that will interface between the camera and a desktop (not laptop) computer, the selection is pretty much moot, y'know? Get that settled first.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Tim Ward
Re: More important? 8bit vs 10bit or 35 vs 50Mbps?
on Feb 16, 2011 at 7:28:36 pm

What Dave said.

The REAL difference that comes into play is which camera looks better. You can tweak the looks of the cameras, but you should get a cleaner picture out of the EX3 because that larger sensor will generate less noise in the image, which would also mean a cleaner key. The larger sensor also means you can achieve shallower depth-of-field (focus) which will result in a blurrier (read: smoother) colored background which will make keying easier.


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David Foster
Re: More important? 8bit vs 10bit or 35 vs 50Mbps?
on Feb 17, 2011 at 5:28:01 am

The Canon xf300 is 4:2:2 long GOP 50Mbps FYI


Dave


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Tim Ward
Re: More important? 8bit vs 10bit or 35 vs 50Mbps?
on Feb 17, 2011 at 2:23:19 pm

Thanks for that - I thought I'd verified it. I stand corrected.

Canon XF-305 IS 4:2:2.

That doesn't change my answer to the original question. However, for on-camera recording, I would have to do some tests between them to see which one would gives cleaner keys. The Canon ON PAPER should, but I'd want to compare the two.


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Dave LaRonde
Re: More important? 8bit vs 10bit or 35 vs 50Mbps?
on Feb 16, 2011 at 3:46:24 pm

To expand on Tim's comments, that 4-2-0 color sampling (aka color resolution) is particularly important when chroma keying. When either of those two cameras record video, they record four times LESS color information than luminance information. It's enough to fool the human eye, but computers aren't fooled so easily.

Here's a very simplified example -- think of the picture as two separate layers of pixels: one black & white, and one color. The color layer has half the vertical & horizontal resolution of the B&W layer. So to compensate, 4-2-0 color resolution makes those color pixels twice as large in both dimensions when playing back. Needless to say, those color pixels look pretty bad at the edges of the subject and the green screen.

What do you need for a good COLOR key? That's right, accurate color information. So getting clean edges when you record the image in-camera will be a definite challenge.

That's why Tim's advice about recording the camera's HD-SDI out (or HDMI out) is spot-on: you retain all the color information.

An AJA I/O box would allow you to connect the camera to a computer, letting you capture video directly. I expect it's too late in the game to rent one of those, so follow Tim's camera recommendation.

Just understand that getting a good, clean edge on your keys won't be a day at the beach.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Rafael Amador
Re: More important? 8bit vs 10bit or 35 vs 50Mbps?
on Feb 20, 2011 at 4:35:50 am

[jono kardno] "In this scenario the EX3 will do 10bit 35Mbps"
Forget about the "35Mbps". That is what gets recorded on camera.
The SDI puts out a full HD Uncompressed 10b/422 stream.

[jono kardno] "XF305 will do 8bit 50Mbps "
Again, forget about those 50Mbps. You are not recording on camera.
The camera put out full HD Uncompressed 8b/422.

So you have to HD Uncompressed YUV/422 streams.
The one that is 10b (SONY), obviously allows you a 10b Uncompressed workflow, while with the Canon, you are limited to an 8b Uncompressed one.
If, as I guess, your idea is to record through a video card to your computer (Prores): The SONY is a better option.
rafael

http://www.nagavideo.com


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jono kardno
Re: More important? 8bit vs 10bit or 35 vs 50Mbps? Thanks!
on Feb 20, 2011 at 6:10:53 pm

Thank you all so much for your advise. It is very much appreciated. The EX is a go!

Thanks again,

jono


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Praveen Kumar
Re: More important? 8bit vs 10bit or 35 vs 50Mbps?
on Oct 25, 2014 at 6:14:24 pm

Hi,
I need to know if canon eos 600d is capable of recording at 50 Mbps and at 4:2:2 colour depth?
Thanks
Praveen


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