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Old December 23rd, 2002, 01:16 PM   #1
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Why -3db gain?

I've seen a number of post where folks say they set the gain to -3db for almost everything. But I'm not sure why. I've played with it and I don't notice any particular difference between doing that and leaving the gain control on auto.

Maybe you can s'plain it to me, Lucy.
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Old December 23rd, 2002, 01:34 PM   #2
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Cleaner footage. If you have enough light to expose the shot, lowering the gain is a way to increase the SNR.
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Old December 23rd, 2002, 01:40 PM   #3
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Good question Charles. This matter first came up (to my knowledge) regarding the XL1. Many folks (including moi) remarked that the XL1's standard image was a bit "hot" and that black areas were prone to grayness and grain. Turning down the camera's gain to -3db produced better contrast by producing a better quality of black, so to speak. Some even say it produces a more film-like image with the XL1 on many scenes.

Personally, I use it sparingly with the XL1s which has a slightly different default image than the XL1. If the scene is dim I do find it can seem to smooth-out the image. But in bright scenes I'll generall leave the gain on auto. Use your own judgement. Your eyes are all that count on this subject.
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Old December 23rd, 2002, 02:18 PM   #4
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-3db

Well, I'm shhoting with an XL-1s so I guess that's why I didn't notice any difference... except in low light, of course.

Thanks
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Old December 23rd, 2002, 11:54 PM   #5
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When you turn up the gain, it gives this artifical brightening that makes the image look video-ish. If it is the desired effect, that's fine, but I would explorer keeping the gain down and lighting or unlighting a scene as needed.

This isnt an insult, but you may not have closely observed film and video stock shot at different settings long enough to know the difference right away.

crafting an image and understanding the results for a medium it is delivered to is something that comes with time. If it looks wrong to you, I say crank up that gain dial till it does.

Im sure there is some great stuff whose story justifies it being shot a +12 gain.
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Old December 24th, 2002, 07:42 AM   #6
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The problem with auto gain, is the same as any auto device on video cameras. They can induce more problems than they cure. Auto gain on the audio, auto white balance, auto focus, auto exposure etc. all have their place and it isn't being left on mindlessly. As JeepBastard suggests take some time and learn your camera and it's many settings. But leaving all the settings in auto will yield far less than optimum results.

I'm not anti automatic settings. I use AF and AWB a lot. Each one of the items is a tool to be used. But what separates a pro from an amateur is how well they use their tools. I own a hammer, but believe me, I'm no carpenter.

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Old December 24th, 2002, 09:04 AM   #7
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I think this has been a pretty useful post about auto settings. To refresh this point the advice is if lighting conditions permit, using -3dB gain will reduce the noise in the picture.

I guess as long as we are here, what about posterization? Could having -3dB increase this problem when shooting sky shots?
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Old December 29th, 2002, 08:04 PM   #8
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I'm no fan of auto gain as well. Better to choose a gain level
manually (-3, 0, +3 etc.). Just pick one that you like best. Keep
in mind that grain is a lot harder to see on the small LCD viewfinder
on the camera than on a big TV or Projector!! I either use -3
db or 0.
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Old December 29th, 2002, 08:45 PM   #9
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The -3db setting will not increase or decrease the banding (posterization) commonly seen in shots of the sky. Banding is more the result of 8 bit video. There are not enough colors or luminance levels to produce smooth gradations of subtle level changes.

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Old December 30th, 2002, 05:28 AM   #10
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Recall that gain amplifies both signal and noise read from the CCD. If the gain is set at -3 dB and the light reching the CCD is increased by +3 dB by virture of a larger aperture (or slower shutter) you have a ~3 dB improvement in signal to noise in the image. That can be valuable in some situations, although most viewers on ther home TV will never notice.
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Old December 30th, 2002, 07:36 AM   #11
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Although gain reduction has its advantages (noise , DOF) when there is sufficient ambient light, there is also a drawback due to the limited "full well" capacity of CCD structures. The reference gain is normally set at allmost full well for peak white. Reducing gain, and allowing more light on the CCD, potentially generates well overflow which induces extra blooming and streaks. In practical terms this means that high contrast scenes tend to show more blooming and streaking in the -3db(or more) gain shift mode.
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Old December 30th, 2002, 10:46 AM   #12
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Does anyone know whether the gain on the XL1 is an analog or digital gain? That is, does the gain knob amplify the analog video signal coming off the CCD prior to AD conversion, or does it simply multiply the brightnesses of the digital pixels?

If it's an analog amplification, turning up the gain will aid in reduction of posterization. Increased noise enhances the overall perception of a quantized signal. This is known as dithering.

Andre, good point about the well overflow. For those that don't know what he's talking about, take your video camera outside and shoot some street lamps at night. You'll see vertical streaks emanating from the flare of the lamps. The CCDs in Sony's CineAlta cameras don't seem to have this issue.
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Old December 30th, 2002, 01:48 PM   #13
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Analog CCD pixel voltages are, after a fixed amount of amplification, digitized a first time (in 10-bit and more resolution) . on this level all kinds of processing are applied (gain, gamma, shading correction...). In a second step, the (DV) 8-bit 720x480 (576) sampling is being applied. So gain-up adds noise before the final 8-bit quantisation and thus reduces quantisation banding visibility by intensity dithering.
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Old December 30th, 2002, 02:15 PM   #14
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In the XL1, pedestal adjustment, AGC, AWB, gamma correction, are applied in the "analog proces IC". The output from the analog process IC is fed to a 9-bit A/D converter for further processing in the DSP chip (e.g., pixel shift, digital zoom, noise reduction, Y/C processing). The analog process IC is fed from the CCD through a sample/hold IC.
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Old December 30th, 2002, 02:41 PM   #15
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Strange...to my knowledge all modern camcorders only have precompressing analog amps (pre knee) and some direct to the CCD related functions like dark current compensation(pedestal). All other processing is done in the digital domain. Analog gain control and gamma compression of video signals is known as unstable and unreliable.
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