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Sony VX2100 / PD170 / PDX10 Companion
Topics also include Sony's TRV950, VX2000, PD150 & DSR250 family.


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Old April 2nd, 2004, 02:39 PM   #1
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AGC Limit/dB/Auto Gain Limit

Hi guys...

I am familar with Fstops, aperatures, etc. But I don't exactly understand what these things are (my subject).

I have set my camera up with my auto lock in the middle, so I can adjust the aperature according to my zebra pattern reading.

My manual isn't quite descript on this topic, and the search on this board didn't really help (I checked under dB, Auto Gain Limit).

1. What is AGC limit?
2. What is dB?
3. What is Auto Gain limit?

Thanks!
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Old April 2nd, 2004, 03:47 PM   #2
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I'll let users of that cam chime in on the gain stuff.

A dB is a decibel, it's a dimensionless ratio of signal powers calculated as dB=10*log(P1/P2), where P1 and P2 are two power levels. The logarithm is base 10, BTW.

The nice thing about dB is you can simply add them together. A 10 dB gain plus a 15 dB gain equals 25 dB. Can't do that with watts, a 5 watt amp feeding a 10 watt amp does not yield 15 watts of power gain. I'll leave it as an exercise for the ready to calculate the total gain of a 5 watt and a 10 watt amps in series.

Sometimes you will see dBv, which are dimensionless voltage ratios calculated as dBv=20*log(V1/V2). The only reason its "20" is because the relationship of power to voltage is always squared, p~v^2, so the exponent is brought out of the logarithm (basic algebra stuff there).

Cheers.
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Old April 2nd, 2004, 04:24 PM   #3
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Whoa!

Thanks Dan, but my mathmatical skills suck! I have bad memories in high school of my pre-algebra class, and did not venture beyond that! Logarithim, ratios make me shudder! Do I need to take remiedal math at a community college to use these features? :)

Maybe dB/Gain have to do with using lighting, no?
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Old April 2nd, 2004, 04:33 PM   #4
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Linda,
I'm not clear as to whether you are referring to audio or video settings as db is audio, gain can be used in both audio and video. It looks like you might be questioning both.

Go to www.urbanfox.com there is an excellant tutorial about the various settings for the 150. Even though I use 2 of them on a weekly basis, I'm not really great (or even good) at explaning things sometimes and urbanfox would be a good place to start.

Also set up on a tripod and record things trying all the different settings, all it cost you is a tape or 2. You'll find the 150 has some outstanding qualities and a few things you might wish were different but it's a great camera. Play and enjoy.

Don
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Old April 2nd, 2004, 04:35 PM   #5
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I am referring to visual (the dB, gain, etc.) show up along with my fstop reading).
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Old April 2nd, 2004, 04:47 PM   #6
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The Auto Gain Limit sets the maximum amount of electronic gain the camera will add to the video image. The AGL is very useful for controlling image 'grain'. For instance during a wedding reception you can limit the grain in the picture when shooting in darker areas but you'll need to rely more on other sources of light. The higher the gain the more grain you'll have in the video. The only gripe I have is that the two useful settings are 6 and 12db. I would have preferred to have a 9db setting in there as well since at 12db you can already see image grain and 6db isn't anywhere near enough. 9db is a good middle ground. You can always override the AGL with a manual gain setting.

John
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Old April 4th, 2004, 02:14 PM   #7
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Here's another way of thinking of it. On a digital camera (still or video) there are three ways of controlling exposure. Two are the ones you are familiar with from film photography: aperture (size of the opening of the lens) and shutter speed (how long the film or sensing device is exposed to the light). These two can be coordinated in various ways. If you have the shutter open half as long (go one stop faster, say from 1/125 to 1/250), and open the aperture one stop (from f8 to f4, for instance), you let in the same amount of light, and the exposure is the same. If you change just one, you let in more or less light.

Digital cameras add gain control. Gain control does not change the amount of light hitting the sensor, as the other two do, so it doesn't affect the signal you begin with. What it does is amplify the signal, boosting its "gain." That allows you to work with a weaker signal (less light) which is useful in low light situations, but it comes at the price of introducing "noise" or "grain," since it takes an insufficient signal and boosts it. The film photography analogue is "pushing" film by giving it a longer time in the developer.

In film photography, people sometimes push film not just for speed, but for the textural look of the grain, which can be very tactile and cool in the right hands. In digital photography some people may do that too, but in video it moves you more towards the surveillance camera look, with active fizzy stuff in dark areas.

Setting the gain control manually, or limiting it, allows you to avoid that boosted look, but the price you pay is generally slower shutter speed and motion blur (assuming the lens is already wide open). The db steps, like 6db, 12db, and 18db are like "stops" in shutter speed or f stop. In my experience, 6db of gain isn't very noticable, but 18db certainly is.

It's just a trade-off.

Linc Kesler
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Old April 5th, 2004, 05:44 PM   #8
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Thanks guys...

All the info really helpted. Also, I checked out the Urbanfox website. Very informative. I printed out alot of stuff. It's also nice to hear other peoples take on the subject.

I set my custom preset to 6 gain.
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Old April 6th, 2004, 09:27 AM   #9
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I guess it was just a typo Linc, but just in case Linda's confused, you open the aperture one stop from f8 to f5.6, not f4.
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