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Old April 21st, 2002, 09:47 AM   #1
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Signal to Noise Ratio - please explain

Would greatly appreciate it if one of you highly experienced boffins out there would explain "Signal to Noise Ratio" in a nice way for a newbie to the world of DV to understand.

What is it, how does it affect image quality, what changes it etc etc...

Thanks

Ron
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Old April 21st, 2002, 04:11 PM   #2
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Signal-to-noise (S/N) ratio typically is the ratio of the amplitude of the desired signal to the amplitude of the noise in system or signal you are evaluating. It is expressed in dB (decibels ). 6 dB is an amplitude ratio of 2:1, 12 dB is 4:1, 20 dB is 10:1 and so on.

For an analog video signal it is typically probably referenced to the maximum video signal (100 IRE). VHS recorders have a S/N on the order of 40 dB or so. The analog ouput of a MiniDV camcorder with good lighting should be around 50 dB or higher.

However when speaking of digital signals it is often expressed as carrier to noise ratio.
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Old April 21st, 2002, 05:26 PM   #3
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Hi,

In real simple terms signal is the good, clean portion of the signal or image. Noise is the grain or fuzzy portion of the signal. I usually explain by, imagine your looking through a screen door. The signal is your view through the screen. The noise is the screen itself. One layer of screen only slightly degrades the signal (your view outside). If you shoot in lower levels and boost the gain it's the same as looking through another layer of screen. As noise increases the image gets worse and worse.

Lower light levels and the subsequant increase in gain, increase noise. Heat and cold can also cause fixed pattern noise. In analog conversion noise is added by the various circuts and components. Hence, its best to keep the signal digital all the way through. Noise can be external, like certains types of lights or devices ( flourescent lights and microwaves). i think you get the idea. I hope this helps.

Jeff
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Old April 21st, 2002, 07:38 PM   #4
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Thanks folks.. I think I get the picture.

If I relate it to still photography (with which I am more familiar), it's kind of like exposing a picture in very low light conditions, and then overdeveloping the film to get something on the neg - the result is a very grainy (noisy) picture.

Regards

Ron
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Old April 22nd, 2002, 02:14 PM   #5
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I think you 've got it.

The gain setting is a bit like push processing film. As when you push film, the grain gets larger and the color balance may shift a bit. When you add gain, the noise is amplified along with the weaker desired signal, and may become apparent in the image, especially at the high gain settings.

Noise appears as the random specks that are like snow, and also it may have a fixed grain pattern component that results from the variations in sensitivity of the individual CCD pixels.
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