[filmscanners] Re: Density vs Dynamic range

["Julian Robinson" <jrobinso@pcug.org.au>]

Morning Austin,

>JULIAN....oh, JULIAN... (R that is...)

>Julian said: > The term "dynamic range" is simply defined for ANY system
>as the ratio of
> > the largest possible signal to the smallest possible signal which can be
> > handled without changing system parameters (usually smallest is determined
> > by "noise" or some equivalent).

>Austin said:
>Above, you CLEARLY say dynamic range is based on noise...

No!!!!!   I don't!!!!!  Please read.  I say it is "usually" determined by
noise, because noise is what USUALLY determines the smallest possible
signal.  WHat  I actually say is dynamic range is based on largest possible
signal and smallest possible signal.  I thought that was pretty
straightforward.  Sheesh.

> >Julian:  This example system for some reason has a noise of 1V, a smallest
> >discernable signal of 2V and a largest signal of 10V.
>
> > I'll do it here:
> > DR = largest signal/smallest discernable signal
> >          = 10/2
> >          = 5
> >
> > That is, the dynamic range of our example system is 5.
>
> > 7) If you agree they are different, then this explains why you say the
> > example dynamic range is 8 and the definition that is used by everybody
> > else uses gives a dynamic range of 5.

Austin:
>OK, your point 7) here makes me believe YOU believe the DR of the above
>example is 5, YET YOUR use of the terms for that equation clearly shows that
>equation is NOT based on "noise", but on the smallest signal LEVEL, and in
>the example you CLEARLY show that noise is different than smallest signal
>level (which you are mistakenly calling "smallest discernable signal", which
>is not the correct term).

i) Yes, the DR here IS 5.  Correct, so if you believe that I believe that,
then we have a small success!

ii) You are correct - my use of the terms DOES show that in this example
the equation is not based on noise.  That is because the noise IN THIS
EXAMPLE is not the determinant of smallest discernable signal.  The
smallest discernable signal here does NOT equal noise.  And dynamic range
is measured based on smallest discernable signal.  There it is in your
agreed formula.  Have a look, there it is on the bottom -  "smallest
discernable signal".  It doesn't say "noise", it says "smallest discernable
signal" .  Heck, what more do you want?  I *define* the smallest
discernable signal is 2V, the equation calls for smallest discernable
signal, so I put 2V in there.  This means that in THIS EXAMPLE, the dynamic
range is not determined by noise,  that was the point of this example.  The
dynamic range is NOT what you think it is.   Someone else described your
version of DR as ..."(the range) / (min possible increment within the
range)" and as far as I understand you this is a fair statement of your
view.  But dynamic range is NOT that - rather, it is the ratio of the
largest signal to the smallest signal.  It is not the number of meaningful
levels inside the range even though that is a nice concept, it is just the
ratio of largest signal to smallest signal.

iii) How can you tell me that  "smallest discernable signal" is not the
correct term!?  I DEFINED it as being the value it was, and selected the
phrase because that is the term used in the equation.  You may not like my
use of it, but you can't tell me it is incorrect.  I DEFINED it for this
imaginary example and gave it the value I selected.

>OK, so why do you now say that DR IS based on noise,

  I don't say it IS determined by noise, I say that most of the time it
is.  Because MOST of the time, the "smallest discernable signal" is
determined by noise, so MOST of the time dynamic range is determined by
noise.  The importance of this semantic juggling is twofold, first, it is
important to understand the DEFINITION of dynamic range, and the fact that
it is NOT defined in terms of noise, it IS defined in terms of "smallest
discernable signal".  Second, on those odd occasions when "smallest
discernable signal" is NOT determined by noise, then you need to make sure
that noise is NOT in the equation!  (which is one reason why your equation
has a problem).

Once again, if you want me to describe such a box where the "smallest
discernable signal" is NOT determined by noise, just say so .   I don't
want to spend the time on this unless there is some point.

Julian

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