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Decklink Studio, Canon HG20, HDMI capture for green screen

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Graham Jones
Decklink Studio, Canon HG20, HDMI capture for green screen
on Jul 2, 2010 at 7:16:06 pm

Hello, (posting this on Canon and After Effects forums too)

I am trying to set up a home green screen studio on a very tight budget, to enhance my podcast work. I've already ordered a number of soft lights and a chroma key backdrop (from cowboystudio.com -- great site by the way). As for camera, I have a Canon HG20, which records 1920x1080 AVCHD, at a maximum of 25 Mbit.

I am a TV editor, and recently have had to do a pile of green screen keys for Animal Planet, which were shot at 1080i60 with Sony 35 Mbit EX cameras. They were lit well enough, but very difficult to key, because of the very compressed format and color sampling. I know the 25Mbit AVCHD from my HG20 will be much worse.

Canon confirmed that their cameras' HDMI ports properly output the uncompressed signal directly off the CCD, before applying the lossy AVCHD compression. Since I have a Decklink Studio card, I am interested in capturing this uncompressed HDMI output directly to the hard drive.

Since the HG20 can shoot 1080p30, and the Decklink Studio can capture 1080p30 to Apple Prores 422 HQ, I thought this would be a pretty nice setup for green screen, with higher 4:2:2 color sampling and much higher bit rate, and not quite as much of a hit on my hard drive as full uncompressed HD.

I will use After Effects for the keying.

My question is -- does anyone have any real experience with this? Anything I should know about? Gotchas?

I should mention that I plan to buy a second camera, but am limited to $1,000. I am thinking of staying with Canon so the cameras are easier to match. Any recommendations?

Thanks,
Gray.


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Joshua Helling
Re: Decklink Studio, Canon HG20, HDMI capture for green screen
on Jul 2, 2010 at 9:53:00 pm

While we haven't done this with that particular camera, we have done this exact thing with other cameras. So I can't see why it wouldn't work.

I would probably confirm with the camera manufacturer that it can output for extended periods of time in this mode without enabling the demo mode. The demo mode (which is typically used in stores) will change the output of the camera to a screen saver of sorts when the record function is not being used. So if you let it sit long enough it will start outputting a screen save. This wouldn't be good. But typically this can be disabled in the menu. Just might be worth confirming.

Sincerely,

Joshua
Director of Support
Blackmagic Design Inc.


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Graham Jones
Re: Decklink Studio, Canon HG20, HDMI capture for green screen
on Jul 2, 2010 at 9:57:42 pm

Hi Joshua,

thanks for your response. I was thinking I would record to the camera's hard drive as a backup anyway, so probably wouldn't have to worry about the demo mode.

Since I have the Decklink Studio and camera already, I'll run some tests to see if there are any issues.

Thanks,
Graham.


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Graham Jones
Re: Decklink Studio, Canon HG20, HDMI capture for green screen
on Jul 2, 2010 at 11:35:45 pm

Hmmm...

I decided to do a very quick down and dirty test -- low lighting in my office to exaggerate the grain, tried capturing 1080p30, 1080p24, 1080i60 by HDMI (ProRes HQ), and recording at the same time to the camera at 24 Mbit AVCHD to import with Log and Transfer.

Notes:
- The camera appears to Final Cut as 1080i60 regardless of which internal mode the camera is set at, whether using HDMI or Log and Transfer. I notice the 24p-style motion when recorded in that mode, but not true progressive frames. Bummer. This may not be a FCP issue -- when I check the HDMI Status in the camera menus, it says 1080i60 and has no other options.

- Though the HDMI-captured image does have significantly more colour and more detail in moving areas, it also has significantly more noise. This doesn't make sense to me -- unless it is an issue of the HDMI more accurately capturing the color noise of the grain.

- Because of this, in low light the non-HDMI actually looks better.

- That said, in my broadcast color grading experience I know that even very high end pro cameras look terrible when there is gain noise in low light. I am eager to see how these tests might differ in full studio light. If I don't have to deal with the grain, I think the HDMI improvements will be worth it.

My light kit will ship in another week or two -- I'll report back when I have more tests to share.

Thanks,
G.


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Graham Jones
Re: Decklink Studio, Canon HG20, HDMI capture for green screen
on Jul 3, 2010 at 1:42:40 am

Hi,

Silly me... I just read further in the forums and realized that though the HG20 captures 30p as 1080i60, it really is sending progressive frames. So apparently you just set FCP's sequence settings to fields "none" and it is progressive.

To my eye there's very little difference between the 1080i60 and 1080p30, but we'll see once I'm dealing with higher light levels,

Thanks,
G.


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