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what is the"dB" means?


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#1 YongLee

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Posted 10 June 2007 - 07:36 AM

What is "dB", for example ,dynamic range of HD usual is 72dB.
what difference between the stop and dB?
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#2 David Auner aac

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Posted 10 June 2007 - 10:21 AM

What is "dB", for example ,dynamic range of HD usual is 72dB. what difference between the stop and dB?


DB stands for deci Bel. Bel a is measure of the magnitude of a physical quantity (e.g. power) in comparison to a reference level. For example it's used to measure sound pressure in relation to the human hearing threshold. For more info see wikipedia.
In the video world, the dB is used as a measure of signal amplification. When shooting in low light you can use a gain setting to boost your signal to make the image brighter, but that introduces grain because the signal is amplified by a DSP. Six dB equal about one stop. So 72dB would equal 12 stops, if I am not mistaken and it's the same here as it is with gain.

Cheers, Dave
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#3 Chris Keth

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Posted 10 June 2007 - 11:11 AM

Six dB equal about one stop. So 72dB would equal 12 stops,



That sounds like way too much for HD dynamic range, no? Where did the 72 come from, YongLee?
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#4 David Mullen ASC

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Posted 10 June 2007 - 06:55 PM

That sounds like way too much for HD dynamic range, no? Where did the 72 come from, YongLee?


Due to noise, not all the stops may be usuable, plus some cameras may lop off some of that info before processing/compressing.
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#5 Matt Sandstrom

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Posted 18 June 2007 - 07:33 PM

you can't translate it into stops like that because it doesn't take clipping into account, nor gamma.

/matt
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#6 Michael Collier

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Posted 18 June 2007 - 08:19 PM

The 72db rating might be a signal to noise ratio. That would be the measurement of noise (referance level) to signal (dB). The higher the STNR the better, since the higher it goes, the less noise there is compared to signal strength. if that were the case it would be just about right. Most cameras fall between 65-80dB signal to noise range.
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