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-   -   -3db gain (https://www.dvinfo.net/forum/canon-xl-gl-series-dv-camcorders/112752-3db-gain.html)

Guy Godwin January 19th, 2008 11:05 PM

-3db gain
 
I am very new to this type work and since getting my XL2 I have learned a whole lot of stuff. However, I have not learned as much about gain as I would like.

Since I have found out that the gain adds alot of noise I have set my gain to "0" and left it at that. However, tonight I was browsing some of my footage that took just tinkering with the camera and I noticed 1 shot that I thought was good with "0" gain. However, just after that shot I noticed that I changed my gain to -3. (This was my footage with in the 1st hour of ever using the camera)

Anyway, the -3db footage appeared better so I captured it and am now burning a DVD to see how this footage looks in the final mode.

So based on this I would like to know what exactly does the -3db gain do? I know that adding gain adds light/noise and "0" gain adds nothing. I undertand I can removed light, but what does this do to the noise factor? and can it really remove light?

Please explain what takes place when I use the -3db gain option.

P.S. Exposure lock and AE shift are other areas I have ignored (Anything I need to know just yet?)

Bill Ravens January 20th, 2008 08:56 AM

CCD's are Charge Coupled Devices. That means they take a voltage across the element. When a light photon hits the element, the photon is converted to a signal out of the element. The amplitude of the output signal(gain) is proportional to the voltage impressed on it. CCD's are designed and optimized for a particular voltage, the voltage at zero gain. More voltage increases the sensitivity but also the spurious noise signal. Less voltage reduces the noise signal, but, there is a baseline spurious noise that is native to the CCD design. So reducing the gain voltage will eventually bump into the background noise of the CCD block. The optimum S/N ratio(signal/noise) is at 0 gain. negative gain is useful for situations where you have too much light, rather than use an ND filter. I wish I had a problem with too much light.

Jack Barker January 20th, 2008 09:11 AM

Nice one, Bill!

Jurgen Geevels January 20th, 2008 12:17 PM

I thought that (with the xl1 at least) the camera is set at 0 db but in theory its not 0 but +3 db, but since there is just a small difference in the noise you get at that point, but much better senstivity.

So basicly 0 is not 0, i'd keep it on -3 if I were you if you have the choice for it.

Guy Godwin January 20th, 2008 10:02 PM

Bill,
That was a very good explanation. Considering my engineering background it made simple and logical sense. So is it safe to assume that that "zero" gain puts the least possible amount of "stress" into the CCD's and therefore should always be the optimum value for eliminating noise?

Bill Ravens January 20th, 2008 10:16 PM

Generally speaking, I would say yes. However, one enever knows what lurks in the minds of the camera makers. Jurgen could very well be correct, I, simply, don't know. I always shot my own XL2 at -3dB gain The images appeared no more noisy than 0 dB.

Jurgen Geevels January 21st, 2008 01:07 PM

I am almost sure that -3 = less noise :)

John DeLuca January 21st, 2008 04:45 PM

No, -3db just makes the image darker. 0db is ideal.

John DeLuca January 21st, 2008 04:54 PM

Hmm...... Maybe the XL2 does have a higher base sensitivity at 0db. A higher base sensitivity doesn’t necessarily mean higher noise at 0db vs -3db. Anyone care to clear this up?


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