Digital BW, The Print

Hi Roy,

<snip>

> You
> quoted one sentence that contained "dynamic range describes the actual
> limits of how many tones the scanner can really differentiate
> from light to
> dark".  You seem to be associating:  dynamic range <--> how many tones,
> when I think the real association is:  dynamic range <--> actual limits.

READ IT.  "actual limits of HOW MANY tones".

It is unquestionable that the number of bits IS the LIMIT of the dynamic
range of a digital system.  Even the Higgins book CLEARLY says that, and I
quote:

"The relationship between number of bits, M, recorded digitally and the
dynamic range DR in dB is

DR(dB) = 20log10(2**M)"

and it qualifies the 20 at the bottom saying it is typically 10log10...

> Reading just this one sentence might be a little ambiguous,

That statement is perfectly clear in and of it self...and he should have
left it at that, but unfortunately...

> but
> read the whole
> paragraph.  Immediate following this sentence is the staircase example:
> dynamic range <--> height of staircase,  bit depth <--> number of steps.

He states he is paraphrasing from Bruce Frasier, who is a PhotoShop guru,
not a hardware engineer (or a signal processing expert).  Look, this is just
not debatable.  Number of bits IS dynamic range.  I thought you said you
were an engineer?  I take it not an electrical or computer engineer.  That
is just so "basic" to anyone who has any experience in digital signal
processing.

Again, supporting exactly what I've said, the term dynamic range is very
misunderstood, and is very misapplied by people who don't have the
background to understand that they are misapplying it...as no one ever
really explained it to them, and the proliferation of misinformation on it
that is out there.

Go read this:

http://www.prosig.com/signal-processing/adcbits.html

BTW, none of the authors of the book "Real World..." have ever designed any
scanners, much less any digital imaging systems.  The main author is an
expert on QuarkExpress and Photoshop.  He is NOT an electrical engineer, and
I am reasonably sure, doesn't completely understand what he is writing about
with respect to dynamic range.  He certainly understands scanning enough to
write a decent book on scanning, but that doesn't mean he gets every
technical detail "right".

> Please consider your position as it relates to these references.  These
> are both very explicit references to dynamic range as it relates
> to imaging.

I have "considered" my position and I am unquestionably right, not only in
what I've said before, but unquestionably so in this case.  This FACT that
dynamic range is limited by bit depth is so basic to the discussion of
digital signal processing, which is what we are talking about when we talk
about bits and dynamic range.

Here are some additional references of OTHER professionals who explain that
dynamic range of a digital system is the same thing as bit depth:

http://www.styrex.se/kunskapsbanken/STYREX_HTML/Tutorial/realtime.html

Where the go on to simply say "The digitisers vary in conversion speed,
dynamic range (number of bits in the result), stability and linearity."

http://www.ndt.net/article/0598/linas_eq/linas_eq.htm

Go down half way, and there is a table that shows number of bits required
for a different dynamic ranges...and it conforms to the equation I listed
out of Higgins above.

And a very interesting application of dynamic range:

http://www2.erdas.com/supportsite/Glossary.htm

"radiometric resolution - the dynamic range, or number of possible data file
values, in each band. This is referred to by the number of bits into which
the recorded energy is divided. See pixel depth."

And from a manual for some medical imaging software, but relating dynamic
range to monitors (as they apparently do understand digital imaging, since
that is what they do ;-), showing they believe number of bits represents
dynamic range, as well as dynamic range is number of tones:

http://www.expasy.ch/UIN/html1/projects/osiris/osirismanual.html

"Image dynamic range versus displayed dynamic range
The dynamic range of some images may often exceed the dynamic range of
intensity levels that can be displayed at one time on a screen. The software
performs an optimal mapping of the image intensity values into the display
range. Typically medical images have an intrinsic dynamic depth range of up
to 12 or 16 bits while most of the common display systems support 8 bit
displays that allow only 256 levels to be displayed simultaneously on the
screen. Interactive adjustment is made in real time on the available dynamic
range of the display (typically 256 gray levels) and when done, the image in
its full dynamic range is remapped on the screen."

And...a discussion on digital signal processing:

http://www.qsl.net/on7yd/136narro.htm

"The number of bits of the A-D convertor determines the dynamic range of the
spectrum."

And since I'm getting tired, and this is time consuming, here is a final
resource (requires registration):

http://www.the-scientist.com/yr1999/august/profile2_990830.html

"The dynamic range of a pixel (vertical resolution) is a function of the
linear well size and noise. A pixel that can hold 500,000 electrons with a
read noise of five electrons will offer a signal-to-noise ratio of 100,000:1
or approximately 17 bits of information (dynamic range = 2n, where n = the
number of bits). To take full advantage of this, the on-board processor of
the chip and the downstream components need to have 16- to 18-bit
capability."

Austin