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Old January 26th, 2011, 08:40 PM   #1
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Green Screen Bit Rate Recommendation

Does anyone have a recommendation?

We're out with a JVC gy-hm700 shooting on green screen--mostly talking heads. Ultimately being played on large screens. Right now the plan is to shoot in 30p.

The edit is slated for FCP.

IFrame or Long GOP? And at what bit rate?

I'm looking for the best of both worlds (within reason) when it comes to keyable visuals and ease of working with assets in Final Cut.

Mike
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Old January 26th, 2011, 09:44 PM   #2
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Not familiar with that JVC camera. Does it have 1920x1080 sensors?
I normally recommend 220 Mbps (or 280 if your cards can handle it) I-frame.
But I've also had excellent results with 100Mbps Long-GOP when data storage and transfer times were an issue.
That said, don't plan on using the built in Keyer in FCP. Most of my clients get their best results with Boris FX, and tell me it produces a far superior matte to what they can get from FCP, even under the best circumstances. I do have another client who swears by AfterEffects, but he's using a different NLE.
Of course most importantly, do a good job with lighting and makeup, and make sure that you're not dealing with shiny clothing (or jewelry) that reflects the green. I also tend to go with a higher shutterspeed to minimize blurred edges that might produce keying problems.
Best of luck with it.
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Old January 26th, 2011, 09:55 PM   #3
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How are you recording data? If you're doing MOV files go for the maximum (35Mbps?). However, I think it uses the MPEG-2 XDCAM EX codec, so if you're using that then does it allow you to record I-frame?

Anyway, I-frame and highest bitrate and if possible 4:2:2 (or more) sampling - this is what you need to pull off a good key that will hold on a big screen. Also, FCP keyers might not be good enough for that screen size.
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Old January 27th, 2011, 03:59 AM   #4
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Dear Mike,

All of our bit-rates above 35 Mbps are 4:2:2, HD in 18 Mbps or 35 Mbps are 4:2:0, so please do not use those low settings.

100 Mbps Long-GOP can produce a very nice key.

If you want more, you can record 220 Mbps or 280 I-Frame Only.
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Old January 27th, 2011, 02:53 PM   #5
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Quote:
Originally Posted by Dave Sperling View Post
Not familiar with that JVC camera. Does it have 1920x1080 sensors?
I normally recommend 220 Mbps (or 280 if your cards can handle it) I-frame.
But I've also had excellent results with 100Mbps Long-GOP when data storage and transfer times were an issue.
That said, don't plan on using the built in Keyer in FCP. Most of my clients get their best results with Boris FX, and tell me it produces a far superior matte to what they can get from FCP, even under the best circumstances. I do have another client who swears by AfterEffects, but he's using a different NLE.
Of course most importantly, do a good job with lighting and makeup, and make sure that you're not dealing with shiny clothing (or jewelry) that reflects the green. I also tend to go with a higher shutterspeed to minimize blurred edges that might produce keying problems.
Best of luck with it.
Dave:
Thanks for the reply. Yes the JVC has a 1920 x1080 sensor. At present we have not been out on a job with the nanoflash, so deciding if we have enough room to store and time to transfer are all up in the air. I have experimented with the nano to see that the short records are of course large in size--we're interviewing 23 people over 5 days on greenscreen. I might stick with 100Mpbs Long GOP just to be prudent.
As for FCP, I'll be producing at a mid-level post house that does Smoke and FCP, my guess is that they're well past the FC keyer.
Thank you,
Mike
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Old January 27th, 2011, 02:56 PM   #6
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Quote:
Originally Posted by Dan Keaton View Post
Dear Mike,

All of our bit-rates above 35 Mbps are 4:2:2, HD in 18 Mbps or 35 Mbps are 4:2:0, so please do not use those low settings.

100 Mbps Long-GOP can produce a very nice key.

If you want more, you can record 220 Mbps or 280 I-Frame Only.
Thanks Dan,

Because I'm a producer more than a tech guy--why would you suggest 100 mbps rather than a higher rate? Are there any benefits to going higher in Long-GOP? Or are there "sweet spots" in both GOP and I-Frame?
Thanks,
Mike
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Old January 27th, 2011, 03:03 PM   #7
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Quote:
Originally Posted by Sareesh Sudhakaran View Post
How are you recording data? If you're doing MOV files go for the maximum (35Mbps?). However, I think it uses the MPEG-2 XDCAM EX codec, so if you're using that then does it allow you to record I-frame?

Anyway, I-frame and highest bitrate and if possible 4:2:2 (or more) sampling - this is what you need to pull off a good key that will hold on a big screen. Also, FCP keyers might not be good enough for that screen size.
Thank you Sareesh,

I will be recording .mov to both the camera sdhc cards and the nanoflash. The limit on the sdhc cards will be 35 mbps. In discussion now is do I have the room or transfer time to deal with I-Frame at 220 or 280. I have over 23 interviews which may be in excess of 20 minutes apiece on green screen. I fear that I will need several terabytes to transfer when we're finished with the shoot.

I think we'll save the I-Frame 220 and 280mbps something a little more interesting and controllable.

Thank you
Mike
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Old January 27th, 2011, 03:24 PM   #8
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Hi Mike,
From my limited testing and playing with various LongGOP bit rates, I've grown to feel that 100 is a noticeable improvement over 50, but that the quality increase when going above 100 wasn't incrementally as great or noticeable as the differences when going from a lower bit rate up to 100. Hence for clients who are 'data size and transfer time conscious' I've settled on 100 LongGOP as my 'sweet spot'.
Also -- You should have a conversation with your post house about all this. First, find out if they'd prefer .MOV files; but second, they may also have input as to whether their keying system works significantly better with I-Frame than with Long-GOP. That could be the deciding factor. (If they're not used to the NanoFlash files, it might even be worth giving them a few samples ahead of time for them to test.)
Whatever bit rate you pick, I'd recommend having (more than) enough CF cards on hand to record a full day of interviews. I find that even though I have 4 64GB cards and a bunch of 32's as well, I like to get the transfers going as quickly and often as possible, so I'm usually not waiting to fill a full 64 before taking it out and starting the transfers.
Best,
Dave S.
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Old January 27th, 2011, 07:53 PM   #9
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Dear Mike,

Dave answered your question very nicely! Thank you Dave!

I never recommend 100 Mbps I-Frame Only.

And as Dave said, 100 Mbps Long-GOP is a very nice sweet spot, very good quality and much smaller file sizes than 220 or 280 Mbps.

So, for many users, 100 Mbps is fine, just run you own tests.
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