At 19:26 29/08/02, David wrote:
>But what do you mean when you say that "dynamic range is a range"?
>
>Dynamic range is _always_ a ratio. If it's not, it's something different.
Yes, the number is a ratio. As I have said many times, this is because
this is the ONLY way you can quantify a range in a single number so that
the number is independent of irrelevant things. But the thing the ratio is
measuring is a RANGE. Just look at the equation, it is the biggest level
divided by a littlest level. It measures the ratio of these two levels,
that is it quantifies the range they represent, the range of which they are
the endpoints, the range between the littlest and the biggest. Look at the
diagram which is from one of the texts that Austin apparently likes.
(I'll try and attach it. If it doesn't get through Tony's ISP's
filters, go to
. The diagram
is Fig 5, about a quarter down the page).
You can't get much clearer than this. The dynamic range is the range from
the peak level (25dB) down to the noise floor (-65dB). It shows
unambiguously the max level at the top of the arrow. It shows
unambiguously the min discernable signal level at the bottom of the
arrow. The arrow illustrates the range. Not a difficult concept. The
dynamic range is the ratio of these two levels, or in logs since they have
already done the work for us, 25-(-65)dB = 90dB. That's all there is to
it, it is VERY straightforward.
> >>>>>>>>>>>>>>>
>These values can be obtained from testing, and the bit-depth/resolution
>within that range is immaterial to the DEFINITION of DyR. It may be material
>to the values you will measure in testing, but it is immaterial to the
>definition/formula.
><<<<<<<<<<<<<<<
>
>Yes, I think, sort of, maybe. If you are looking at the analog signal prior
>to the A/D converter.
The bit depth is one of the two things that might determine the minimum
discernable signal of the scanner. If the smallest (one-bit) step is
bigger than the noise level, then it will be the determinant of MDS. If
not, the noise will be the determinant.
As Todd says, bit depth may be material to the values you will
measure. But it is not part of the definition. It MIGHT be in the formula
if you know that it dominates the noise level in terms of determining
MDS. In this case, your dynamic range formula would be DR = max signal /
smallest step size.
Otherwise the formula would be DR = max signal / noise level (*ASSUMING*
you are using the ISO definition of how to measure DR. You don't have to
do this.)
>Austin's explained this: in any dynamic range calculation, the maximum
>signal level can be seen as corresponding to the range of levels handled,
>assuming the minimum level is defined. The noise (or minimum recognizable
>signal level) (and the maximum signal level) defines how many meaningful
>steps the maximum signal level is from the minimum signal level. That's all
>dynamic range is: the number of meaningful steps from min to max. That's
>normally expressed as a ratio...
David, can you tell me why you have to include all these words like "Can be
seen as corresponding to ..."? You don't need to be so convoluted, the
reality is much simpler. Try this in place of your complex paragraph - "In
any dynamic range calculation, the maximum signal is itself. The noise
defines the minimum recognizable signal level. The ratio of these is the
dynamic range".
Much simpler, more accurate, and agrees with the definition formulae. You
do not need to tie yourself up with complex equivalences and second order
ideas, just keep in mind that dynamic range is what is says it is and what
the equation says it is, a range. Very easily measured, very easily
visualised, very easily calculated. There is another related concept
(usually related but not always) that is the resolution. Most often you
calculate these two different but related things using the same formula,
because that is what the normal assumptions are. But not necessarily.
> >>>>>>>
> If
>a 1-bit scanner can assign any range a value that is 50% of its density
>range, what bit depth scanner is it that will assign a signal the entire
>scanners density range?
><<<<<<<<<<<<<<<<
A 1-bit example is pretty pointless and gets people confused. If you want
to do it, it is quite simple.
The max signal is 1. The min signal DISCERNABLE SIGNAL is 1. The dynamic
range = 1/1 = 1 or 0dB.
>My point is that a value reported by a scanner corresponds to a range of
>possible values in the film, and that the size of that range is given by the
>worse of the noise in the electronics or the bit resolution of the scanner.
Absolutely. Has there been any argument against this concept?
Julian
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