I fail to understand the purpose of the (-3) Gain setting? at DVinfo.net

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Canon XL H Series HDV Camcorders
Canon XL H1S (with SDI), Canon XL H1A (without SDI). Also XL H1.


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Old December 19th, 2005, 06:56 PM   #1
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I fail to understand the purpose of the (-3) Gain setting?

I'm over here seting up various camera settings so I can save them to the SD card and email them to Chris s he can upload them for others to use as a base starting point for visual looks like "GLADIATOR" battle scenes and "ANY GIVE SUNDAY" game day scenes and "GOSFORD PARK" dramatic period scenes and "GANGS OF NEW YORK" fight scenes and a couple others like Requiem For A Dream, Schindler's List and City Of God. Also creating a few settings for those who like HIGH COLOR & CONTRASTY Music Video looks as well. (even though I'm not a DP...I know a lil' sumn sumn) I wish I knew how to re-create the Amelie look.

Anyhow, my question is.....is -3 Gain better for cinematic shooting? Does it remove more noise then "0"? All I ever seen is the picture darkening, which indicates to me it's a supplement to the ND filter, but I'm not sure. Anybody have any insight on this? I never used it on my XL1s because I only used my xl1s for event videography.

Also, has any XL owners read this article posted by Chris Hurd....pretty clever way of using Time Value mode and Exposure Lock. I wish I would have thought of that long ago:
http://www.dvinfo.net/conf/showthread.php?t=45694

Wassup with -3 Gain?

- ShannonRawls.com
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Last edited by Shannon Rawls; December 20th, 2005 at 12:59 AM.
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Old December 19th, 2005, 07:03 PM   #2
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You could do a search in the old XL1/XL1s boards.
I don't know anymore, but I thought I read once that it gives an even more clean signal which should/could be better if you are putting postproduction effects on it.

But I'm not sure, but I am sure you can find the explanation in the old XL1/XL1s boards. MAYBE also the XL2 boards, but I doubt it.
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Old December 19th, 2005, 07:07 PM   #3
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Quote:
Originally Posted by Shannon Rawls
Wassup with -3 Gain?
On my XL2, before I sold it, I used -3 Gain anytime I had enough light to use it. It results in a cleaner image. I'm not sure why they had to call it -3. Perhaps they figured most people in low light settings find that they need 0 Gain, and -3 is too dark.
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Old December 19th, 2005, 07:10 PM   #4
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Shannon,

-3db gain reduces noise in the image, reduces the sensivity of the chip and flattens out the characteristic curve of the range of all the refectivities in the image...helping the camera capture images that have a bit more latitude...a bit more like a film emulsion. It's a great thing, if you have enough light. I use it all the time.

Thanks, by the way, for all your posts about the XLH1. It's an exciting camera and I'm looking forward to shooting with it in 2006.

Thanks also to Chris Hurd for providing a place to collect so much information.

Steve
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Old December 19th, 2005, 10:06 PM   #5
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Hello :-)

-3db also allows you to open the iris to shallow the DOF were applicable.
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Old December 19th, 2005, 10:31 PM   #6
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"The numbers all go to eleven. Look, right across the board, eleven, eleven, eleven." -- Spinal Tap.

Best,
Christopher
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Old December 19th, 2005, 10:40 PM   #7
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Quote:
Originally Posted by Christopher Glaeser
"The numbers all go to eleven. Look, right across the board, eleven, eleven, eleven." -- Spinal Tap.

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But why not just make it ten?
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Old December 21st, 2005, 09:29 AM   #8
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Most CCD frontends, the part that applies the gain to the signal, double samples it, and converts it to digital, has the lowest gain of 0 db.

So what i'm suspecting that when the camera is set to -3db the ccd frontend is actually set to 0db. So when the camera is set to 0db the frontend could actually set to +3.

Or it could mean that when the camera is set to -3db, the frontend can't go that low so its just 0db, but usually digitize higher resolution, sometimes as high as 14 bit. So the DSP just compresses the range down more so it sees more of the top bits that are normally cut off at higher gain.

Really would like to know where they are appling the gain in the camera. Is it in the frontend or in a DSP. Really curious now. But from a math standpoint it doesn't matter because it should achieve a near similar effect of reducing the overall gain.
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Old December 23rd, 2005, 10:31 AM   #9
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Just a mee-too post: I use -3 db gain on my XL 1s whenever the lighting permits it. It results in a cleaner, less contrasty image.

I'd be curious, too, where exactly the benefit of -3 db gain comes from from an electrical / math perspective. If the CCD block was already at gain 0 when the cam is set to gain 0, then there would be no benefit to reducing the gain, IMO. It would just reduce the signal without visible benefit, right?
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Old December 23rd, 2005, 12:49 PM   #10
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The sensor has "dark current" which means it produces a noiselike signal even when no light is present. This noise is present even when light is present and gets amplified and recorded along with the response to light. It is what causes that "crawling" appearing business you see in shadows. Now if there is lots and lots of light you don't need much amplification. Under these conditions you can open up a stop and reduce gain by 3dB (this ignores gamma correction but I hope you get the idea). When this happens the noise is amplified less and so is less noticeable in the recorded images. Remember that gain is a relative thing. If they wanted to they could lable the +3 position 0 in which case the 0 position would become -3 and the -3 position -6dB and we'd have no basis for complaint. We don't know what the gain is realative to and we don't need to. What we do need to know is that reducing gain by 3 dB suppresses noise by 3 dB (again ignoring gamma correction). Whether that makes a visible difference in the picture or not depends on the scene and the lighting.
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Old December 23rd, 2005, 01:10 PM   #11
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Thanx, A.J. - that puts things in perspective. I guess the gain settings are all relative and clearly not comparable between different models/brands.

Happy Holidays!
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Old December 23rd, 2005, 09:45 PM   #12
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ssettting

Shannon how do I get these settings from Chris
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