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Old July 9th, 2006, 06:45 PM   #1
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What's up with -3 Gain?

If footage supposedly looks best with gain set to -3, why is it labled -3. My understanding is that gain boosts the signal electronically, so my interpretation of -3 gain is that it actually limits the signal. If -3 is the best, than it must offer no signal manipulation, so why not call it zero gain. Can someone exlain this?
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Old July 9th, 2006, 08:54 PM   #2
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The idea of video gain is a relative concept. What is called 0db of gain is not an absolute measure of anything, it is a reference point. Even at "0db", there is gain being applied to the signal coming off the CCDs.

What the manufacturer calls "0db gain" is just an arbitrary place where the tradeoffs of CCD noise and signal amplification are, in the designers opinion, best set.

So +6db of gain truly does add 6db of gain to the signal, but be aware that 0db was an arbitrary reference point.

So all -3 means is that 3db less of gain is applied to the signal than at 0. It's not "limiting" anything.
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Old July 10th, 2006, 04:55 AM   #3
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I used it a lot on this last feature i did and it looks great.
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Old July 10th, 2006, 05:01 AM   #4
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Actually, I believe it's because of the way that the gain is applied.

A gain of 3dB increases the signal level by a factor of 1.414, whilst a gain of -3dB attenuates the signal by a factor of .707.

A gain of 6dB increases the signal level by a factor of two; this can be simply achieved after analog to digital conversion by using a shift register.

If you combine an analog -3dB attenuation circuit with a digital shift register, you can get any desired amount of gain in 3dB steps -

Attenuation on + Shift no bits = -3dB
Attenuation off + Shift no bits = 0dB
Attenuation on + Shift one bit = 3dB
Attenuation off + Shift one bit = 6dB
Attenuation on + Shift two bits = 9dB
Attenuation off + Shift two bits = 12dB

and so on...
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Old July 10th, 2006, 08:29 AM   #5
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So what to shoot?

So should I try to shoot -3 or 0 gain?
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Old July 10th, 2006, 09:45 AM   #6
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I would guess it would be a matter of preference and what particular camera you are using. Shoot with both and see what you like better. If you have a critical shoot coming up soon, go out run several tests in what you expect to be similar lighting conditions and see which one you like better a couple of days before the actual shoot.
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Old July 10th, 2006, 10:33 AM   #7
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I always try to shoot at -3 and it looks very good. Try it. Bob
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Old July 10th, 2006, 12:48 PM   #8
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I think you mean "what's down with -3 gain?"

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Old July 10th, 2006, 03:11 PM   #9
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-3 db

Looks better to me. Less noise. Sharper Image. But you gotta be well lit.. or at least thats my experience.

David
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