[filmscanners] Re: Density vs Dynamic range

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

Hi again Austin,

Well I tried!  You think I am not reading and comprehending your posts, I
get the feeling you are not reading what *I* say, then we get tangled up
with you telling me the easy bits that we both should be taking for granted
and I wonder if you give me any credit at all for my training!


Julian: > You say:
> > "Simply put, dynamic range is the (max value - min value) divided
> > by noise."
>
>Austin: Correct...when expressed in simple values...
>
>J: > But NO - dynamic range is max value - min value (when expressed as a
> > difference) ***OR*** max value divided by min value (as a ratio).
>
>A: Hum.  I don't recall saying that...BUT...it IS max - min, when min is NOISE
>AND they both are in log values!

No, you didn't say that, I did.  And when I say max - min I mean max - min,
not logs.  I am using the fundamental english language definition of a
range as the difference between two values in a sequence.  You can express
such a range as a difference or as a ratio.  Forget the logs, I am talking
values here.

Anyway, please look at the following:

> > DR = (max - min) / noise
>
>Correct, and I, short hand, don't include the log etc. since it's obvious
>(at least to me) when log applies and when it doesn't...and log is simply
>
> >          = (max - noise) / noise   ...(1)
>
>If...min value is equal to noise, then yes...


( and yes I agree, that's why there is no logs in my discussion)

Now elsewhere you  say, quoting a reference:



> > >DR (dB) = 10log10(largest signal/smallest discernable signal) or:  ...(2)

One of my points is that these two ideas/equations, both from you, are NOT
consistent.

Making the assumption that min value = noise, and translating (2) into the
same simple but meaningful jargon as (1) and forgetting the logs, and
putting (1) and (2) side by side, we have:

DR = (max - noise) / noise ...(1)

DR = max / noise ...(2)

These are NOT the same!  I agree as I said last time that the actual
difference in the figures will usually be small, but it is the concepts and
where it has lead some of the discussion that is a problem.


But as for "dynamic" and what that means, we have totally different ideas
and I absolutely do not agree with your interpretation.

>No, it is not.  It is a DYNAMIC range.  Range has a meaning all in and of it
>self.  If it was a simple range, as you suggest, it would simply be called a
>range...but it's called "dynamic range".

I went to some trouble to explain what is dynamic range where I come from,
but you haven't responded to that.  Dynamic range is a range, like a red
house is a house. It is not a number of steps.  For the rest of it please
have a look at my previous simple examples of what is the difference
between dynamic range and "static" or "maximum" or "total" range.

Austin:
>Look up the word dynamic:
>
>"Characterized by continuous change, activity"
>
>In other words, the changes over a given range...and change must be
>perceived for it to be change.  You must specify the range for dynamic range
>to be meaningful.  Obviously, if you double the range you are resolving over
>(and keep the resolution the same), you get a higher dynamic range (and a
>higher range)...but no better resolution.

Even better, here is a definition of dynamic range, the first I found...

dynamic range: 1. In a system or device, the ratio of (a) a specified
maximum level of a parameter, such as power, current, voltage, or frequency
to (b) the minimum detectable value of that parameter. Note: The dynamic
range is usually expressed in dB. 2. In a transmission system, the ratio of
(a) the overload level, i.e., the maximum signal power that the system can
tolerate without distortion of the signal, to (b) the noise level of the
system. Note: The dynamic range of transmission systems is usually
expressed in dB. 3. In digital systems or devices, the ratio of maximum and
minimum signal levels required to maintain a specified bit error ratio.

There it is, a ratio.  Nothing to do with resolution at all, although it is
true that for those situations where the minimum level is determined by
self-sourced noise, then that lower level = noise = the resolution.

Later you said:

>And this I completely disagree with.  Dynamic range defines the resolution
>within a range, and a "range" simply specifies the endpoints, and says
>nothing about the resolution within that range.
>
>Here's an easy example.  You measure one foot in 1" increments.  That gives
>you a min of 1 and a max of 12.  Dynamic range = max/min, or 12 in this
>case.  Range is still 12.  Now change your increment to 1/4".  Dynamic range
>= min/max, or 12/.25 and the result is 48.  Hum.  Over the EXACT same range,
>we now have two different DYNAMIC ranges.

But no!  It is NOT the exact same range.  In the first case, the range is 1
to 12 i.e delta = 11 or ratio = 12.  In the second case the range is 0.25
to 12 i.e. delta = 11.75 or ratio = 48.  That is ----  two different
ranges, or dynamic ranges, same thing in this example.  My interpretation
follows if you mean that the minimum you could *measure* was 1" and 1/4"
because there is some kind of measurement noise operating. If not, then you
can measure 0" perfectly well in both cases, and the range in this case is
in fact 12" in both cases.  There is no meaningful ratio to be made, so any
attempt to do so is invalid.  The resolution is increased in the second
example, but this has NOTHING to do with the range. The ruler example is
dangerous because it is not clear what the limitation or "minimum signal"
is, in turn because it is resolution limiting you here, not noise.

I won't carry on any more about this, so this is forewarning that I won't
write again (on this topic anyway!).  I don't mean this rudely and I hope
you *do* respond to this, but I really don't think we are going to converge
and my efforts are not likely to help anyone else.  I now think I
understand what you mean by dynamic range, but I don't agree and I thought
(maybe I was wrong) that  your definition was causing confusion.  As I am
absolutely sure that I am not improving comprehension for anyone else,
there is no  usefulness in continuation except for our intellectual
satisfaction and I am happy to agree to differ.

Cheers,

Julian


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