Video Gain at DVinfo.net

Greg Walsh · April 7th, 2009, 10:19 AM

Hi all,
I see in the posts that people often restrict the maximum gain level while videoing. Can someone tell me why. I have a FX7. Should one turn off AGC while recording? Any advice here will be appreciated.

Adam Gold · April 7th, 2009, 10:57 AM

No, just set the AGC limit in your menus to 12dB. More than that and you will get "noise" or "grain." Up to 9dB it's beautiful; at 12dB it just looks a little like film grain. At least that's what I tell myself. I wish the FX7 had a setting for 9dB, which would be perfect, but you have to choose between 6dB and 12dB.

Page 61 of US manual, 58 of FX7E manual.

Seth Bloombaum · April 7th, 2009, 11:09 AM

Important to distinguish between two types of gain:

a) video gain is a circuit that applies current to the cmos sensor (that's where the optical image is converted to an electrical signal). The result is an increased low-light capability.

b) AGC usually refers to "automatic gain control" of an audio input. We can say "automatic volume control" and be a little more descriptive of what's taking place.

The primary disadvantage of video gain is increased video noise. In a static scene with too much gain the individual picture elements, pixels, are changing color. This will first be seen in the blacks, but is really happening throughout the picture to some extent.

My experience is with the V1 - the FX7 may be exactly the same. Up to 6db of gain, no perceptable noise. 9db - maybe. 12db and above - definitely. So, users who like automatic exposure may use the menus the V1 offers and limit automatic gain setting to 6 or 9db.

Why do we care? Well, the aesthetics of the HD image are kind of ruined when there is visible noise, and, if compressing for internet distribution, can have very poor results.

This is one of the reasons that professional shooters almost never use automatic exposure settings - they consider themselves smarter than the camera in understanding how iris, shutter speed, and gain will affect the image they're after.

***********************
AGC (audio) is a different beast. In short, its primary deficiency is in not knowing which sound is important. If, for example, you're interviewing someone in an area where there is some background noise, an AGC will tend to boost the noise to a "good" recording level when the subject isn't talking. That's bad.

So, again, pro shooters and sound engineers rarely use AGC - they want control.

Adam Gold · April 7th, 2009, 11:17 AM

Note that on this cam AGC refers to video gain only. There are no adjustments for audio gain other than switching from AUTO to MANUAL. The AGC LIMIT control is in the Camera Set menu, not Audio Set.

Seth Bloombaum · April 7th, 2009, 04:18 PM

Ah, thanks for that clarification Adam.

AGC-Audio on the V1 is set via a hardware switch next to each XLR input, and is referred to as Auto/Man.

And quite right, automatic video gain is referred to as AGC Limit in the V1 menus as well, also with only 0, 6, and 12db.

Mind those details... because manufacturers seem to have no concern about changing terminology.

As to turning AGC (video) off when recording, that relates back to the question of whether you like auto exposure at all. If you set AGC limit to 0db, auto will only use iris and shutter speed, which has it's own problems.

It can be very helpful to do some testing - see what a shutter speed of 1/4, 1/8, 1/15, 1/30 of a second looks like. See what 6db, 9db, 12db, 15, 18 of gain looks like. Then you'll know what your camera is choosing in low-light situations.

Oh, and see what auto gain for your microphone sounds like!

Greg Walsh · April 8th, 2009, 11:03 AM

Thanks guys, busy experimenting. I find that my camera hunts for exposure a bit radically and at the wrong times.

Adam Gold · April 8th, 2009, 11:43 AM

Hunts for exposure? Odd. Focus, yes; exposure, not that I've ever seen.

Set AE RESPONSE to SLOW. Same page as the AGC limiter.

Stelios Christofides · April 11th, 2009, 02:17 PM

Quote:

Originally Posted by Seth Bloombaum

Ah, thanks for that clarification Adam.

AGC-Audio on the V1 is set via a hardware switch next to each XLR input, and is referred to as Auto/Man.

And quite right, automatic video gain is referred to as AGC Limit in the V1 menus as well, also with only 0, 6, and 12db.

Mind those details... because manufacturers seem to have no concern about changing terminology.

As to turning AGC (video) off when recording, that relates back to the question of whether you like auto exposure at all. If you set AGC limit to 0db, auto will only use iris and shutter speed, which has it's own problems.

It can be very helpful to do some testing - see what a shutter speed of 1/4, 1/8, 1/15, 1/30 of a second looks like. See what 6db, 9db, 12db, 15, 18 of gain looks like. Then you'll know what your camera is choosing in low-light situations.

Oh, and see what auto gain for your microphone sounds like!

Seth
All of the above might be a bit complicated for someone with less knowledge than you (like me for example) , so don't you think then it's better to leave the setting in auto gain?

Stelios

Stephan Stryhanyn · April 11th, 2009, 02:46 PM

It's all very simple actually - auto or manual exposure regardless - because in auto mode the camcorder will only increase gain as a last resort. Turn on data display, so that you get camera settings continuously in the VF (gain, shutter speed, iris), and do monitor them. If you see gain creeping beyond 9dB, you're in trouble... so just turn on the lights then. AGC limit is helpful, but only there for the occasionally absent-minded.

Seth Bloombaum · April 11th, 2009, 04:10 PM

Quote:

Originally Posted by Stelios Christofides

Seth
All of the above might be a bit complicated for someone with less knowledge than you (like me for example) , so don't you think then it's better to leave the setting in auto gain?

Stelios

You're right, it is complicated, especially to someone new to manual controls.

However, the principles of exposure are not that difficult. Sometimes, the camera manufacturers make them more difficult to understand than they need to be.

Here's the issue, as I see it: If one is shooting on automatic and motion looks bad; or if the shot is too grainy/noisy; or the subject is too dark or too light; or several other possibilities, then understanding the basic principles of exposure will help you to make sense of what the camera is doing when using automatic settings, even if you don't normally shoot manually.

If the image always looks great, or at least good enough, then I agree with you - why fix something that isn't broken?

On the other hand, the FX7 and V1 are both very capable cameras, and a knowlege of exposure and the advantages of manual or automatic settings allows the user to take full advantage of what the camera can do. Understanding this stuff is essential to intentional creation of the image.

April 7th, 2009, 10:19 AM	#1
Greg Walsh New Boot Join Date: Dec 2008 Location: Port Elizabeth, South Africa Posts: 12	Video Gain Hi all, I see in the posts that people often restrict the maximum gain level while videoing. Can someone tell me why. I have a FX7. Should one turn off AGC while recording? Any advice here will be appreciated.

April 7th, 2009, 04:18 PM	#5
Seth Bloombaum Inner Circle Join Date: Sep 2003 Location: Portland, Oregon Posts: 3,420	Ah, thanks for that clarification Adam. AGC-Audio on the V1 is set via a hardware switch next to each XLR input, and is referred to as Auto/Man. And quite right, automatic video gain is referred to as AGC Limit in the V1 menus as well, also with only 0, 6, and 12db. Mind those details... because manufacturers seem to have no concern about changing terminology. As to turning AGC (video) off when recording, that relates back to the question of whether you like auto exposure at all. If you set AGC limit to 0db, auto will only use iris and shutter speed, which has it's own problems. It can be very helpful to do some testing - see what a shutter speed of 1/4, 1/8, 1/15, 1/30 of a second looks like. See what 6db, 9db, 12db, 15, 18 of gain looks like. Then you'll know what your camera is choosing in low-light situations. Oh, and see what auto gain for your microphone sounds like! Last edited by Seth Bloombaum; April 7th, 2009 at 08:38 PM. Reason: typo

April 7th, 2009, 10:57 AM	#2
Adam Gold Inner Circle Join Date: Jan 2007 Location: Woodinville, WA USA Posts: 3,467	No, just set the AGC limit in your menus to 12dB. More than that and you will get "noise" or "grain." Up to 9dB it's beautiful; at 12dB it just looks a little like film grain. At least that's what I tell myself. I wish the FX7 had a setting for 9dB, which would be perfect, but you have to choose between 6dB and 12dB. Page 61 of US manual, 58 of FX7E manual.

April 7th, 2009, 11:09 AM	#3
Seth Bloombaum Inner Circle Join Date: Sep 2003 Location: Portland, Oregon Posts: 3,420	Important to distinguish between two types of gain: a) video gain is a circuit that applies current to the cmos sensor (that's where the optical image is converted to an electrical signal). The result is an increased low-light capability. b) AGC usually refers to "automatic gain control" of an audio input. We can say "automatic volume control" and be a little more descriptive of what's taking place. The primary disadvantage of video gain is increased video noise. In a static scene with too much gain the individual picture elements, pixels, are changing color. This will first be seen in the blacks, but is really happening throughout the picture to some extent. My experience is with the V1 - the FX7 may be exactly the same. Up to 6db of gain, no perceptable noise. 9db - maybe. 12db and above - definitely. So, users who like automatic exposure may use the menus the V1 offers and limit automatic gain setting to 6 or 9db. Why do we care? Well, the aesthetics of the HD image are kind of ruined when there is visible noise, and, if compressing for internet distribution, can have very poor results. This is one of the reasons that professional shooters almost never use automatic exposure settings - they consider themselves smarter than the camera in understanding how iris, shutter speed, and gain will affect the image they're after. *********************** AGC (audio) is a different beast. In short, its primary deficiency is in not knowing which sound is important. If, for example, you're interviewing someone in an area where there is some background noise, an AGC will tend to boost the noise to a "good" recording level when the subject isn't talking. That's bad. So, again, pro shooters and sound engineers rarely use AGC - they want control.

April 7th, 2009, 11:17 AM	#4
Adam Gold Inner Circle Join Date: Jan 2007 Location: Woodinville, WA USA Posts: 3,467	Note that on this cam AGC refers to video gain only. There are no adjustments for audio gain other than switching from AUTO to MANUAL. The AGC LIMIT control is in the Camera Set menu, not Audio Set.

April 8th, 2009, 11:03 AM	#6
Greg Walsh New Boot Join Date: Dec 2008 Location: Port Elizabeth, South Africa Posts: 12	Thanks guys, busy experimenting. I find that my camera hunts for exposure a bit radically and at the wrong times.

April 8th, 2009, 11:43 AM	#7
Adam Gold Inner Circle Join Date: Jan 2007 Location: Woodinville, WA USA Posts: 3,467	Hunts for exposure? Odd. Focus, yes; exposure, not that I've ever seen. Set AE RESPONSE to SLOW. Same page as the AGC limiter.

April 11th, 2009, 02:46 PM	#9
Stephan Stryhanyn Regular Crew Join Date: Dec 2006 Location: France Posts: 48	It's all very simple actually - auto or manual exposure regardless - because in auto mode the camcorder will only increase gain as a last resort. Turn on data display, so that you get camera settings continuously in the VF (gain, shutter speed, iris), and do monitor them. If you see gain creeping beyond 9dB, you're in trouble... so just turn on the lights then. AGC limit is helpful, but only there for the occasionally absent-minded.