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Jefe, what is a "plethora"?


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#1 Keith Walters

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Posted 14 September 2006 - 07:29 AM

Somebody else raised this somewhere, and I think it's an excellent question:

What exactly do they mean by: "66dB"?
I know what it means (or is supposed to mean), but I'm beginning to wonder how many other posters here could write an accurate explanation.
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#2 Jim Jannard

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Posted 14 September 2006 - 08:51 AM

[quote name='Keith Walters' date='Sep 14 2006, 05:29 AM' post='127370']
Somebody else raised this somewhere, and I think it's an excellent question:

And Graeme answered it the same place you read the question. Maybe you should have just posted his answer instead of the game?
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#3 Greg Lowry

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Posted 14 September 2006 - 09:45 AM

Somebody else raised this somewhere, and I think it's an excellent question:

What exactly do they mean by: "66dB"?
I know what it means (or is supposed to mean), but I'm beginning to wonder how many other posters here could write an accurate explanation.


I wonder if it's against the rules here to call someone a pretentious, self-important bore? You certainly qualify.
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#4 Nathan Milford

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Posted 14 September 2006 - 01:24 PM

I wonder if it's against the rules here to call someone a pretentious, self-important bore? You certainly qualify.


While ad hominem attacks may be frowned upon, you could just say his behavior resembles that of Jim Murdoch... most members will extrapolate exactly your meaning.

- nathan
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#5 Stephen Williams

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Posted 14 September 2006 - 02:16 PM

While ad hominem attacks may be frowned upon, you could just say his behavior resembles that of Jim Murdoch... most members will extrapolate exactly your meaning.

- nathan


Nathan,

He joined on 16 August...............

Stephen
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#6 David Sweetman

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Posted 14 September 2006 - 02:28 PM

Well I asked the question in a previous thread, and the only real answer I got was "better than Viper," which I had already concluded based on the hype. But what I never got and what I was really looking for is exactly how the figure 66db relates to the image, for example what is the relation of 1db to 0db.

Furthermore what does db stand for?

And by the way, the title of this thread is a quite respectable reference. I love that movie.
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#7 Phil Rhodes

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Posted 14 September 2006 - 05:10 PM

Hi,

The decibel (dB), one tenth of a Bel (after Alexander Graham, telephone guru), indicates the ratio between two quantities, and as such is unitless and rangeless (like a percentage). It's logarithmic, which is why it's often used in audio engineering. The definition is that XdB equals 10log[10](X/Xo), where X is the quantity and Xo is a reference (often one). There are variances for measuring voltage, but we'll overlook that for now.

Example time: if A dB is 3dB higher than B dB (with the same Xo in each case), B is almost exactly twice A, since 10^(3/10)=1.995. This is why most video cameras offer 3dB gain increments, roughly equal to one stop more exposure.

I'm not going to try and prove what 66dB means in this case, because it isn't clear whether it's the weighted or unweighted value, or if they've factored in the RMS of either (which generally adds about 3dB).

Phil
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#8 Keith Walters

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Posted 15 September 2006 - 06:13 AM

And Graeme answered it the same place you read the question. Maybe you should have just posted his answer instead of the game?

Sorry, what I should have asked is: what is meant when a manufacturer quotes a figure of (as in your case) "66dB". I wan't necessarily referring to your sensor in particular, what I was asking was basically: "If a manufacturer quotes his sensor as being 66dB or 55dB or whatever, what exactly does that mean?

I notice that a lot of people seem to like tossing "DeeBee" figures around willy nilly, without, I suspect, realling understanding the terminology, apart from the fact that "65dB is better than 60dB" and so on.
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#9 Keith Walters

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Posted 15 September 2006 - 06:36 AM

Hi,

I'm not going to try and prove what 66dB means in this case, because it isn't clear whether it's the weighted or unweighted value, or if they've factored in the RMS of either (which generally adds about 3dB).

Phil


No that was not my intention.

You have given a textbook definition of what a 66 decibel figure means, but personally I prefer to say that it involves a voltage change of about 2,000 times, which in a linear photosensitive device means a change of about 11 stops of illumination.

What this usually means is that the "2,000" end of the scale is where the camera starts going into white clipping, but it's down toward the "1" end of the scale that the "creative accountancy" starts to come into play. At that end of the scale the photosensor is producing only a very feeble signal, which requires 2,000 times as much amplification to bring it up to a normal contrast level. The problem then is that the various signal "contaminants" (clock phase jitter, thermal noise, quantum noise and so on) also get boosted 2,000 times and start to become visible.

What constitutes a "useable" signal in that situation seems to be somewhat open to "interpretation" by the various manufacturers, who may (or may not) resort to various forms of so-called "noise reduction" (more accurately: "noise disguising"). There is also no iron-clad definition of what constitutes an "acceptable" amount of white clipping or crushing.

Hence, as you say, a straight decibel figure is not really going to tell you all that much.

Unfortunately a lot of people in this industry seem to think that it will.
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