[filmscanners] RE: scanner dmax discussion

["Chuck Cole" <cncole@earthlink.net>]

> -----Original Message-----
> From: filmscanners_owner@halftone.co.uk
> [mailto:filmscanners_owner@halftone.co.uk]On Behalf Of
> austin@darkroom.com
> Sent: Thursday, July 17, 2003 5:34 AM
> To: cncole@earthlink.net
> Subject: [filmscanners] RE: scanner dmax discussion
>
>
>
> Chuck,
>
> > > > The "steps" mentioned above have nothing to do with intrinsic
> > > > properties of sensor chips and I doubt that anyone tries to
> > > > size an A/D's lsb to some rms noise level
> >
> > > But that is EXACTLY how the A/D is sized in analog to digital
> systems.
> It
> > > isn't sized less, then you aren't getting all the data that is
> available,
> > > and if you size it more, then you simply throw the bits away
> as they are
> > > noise.
>
> > au contraire:  the Shannon waveform sampling theorem would be that
> > criterion, and that says the steps must be smaller by a factor
> of about 3
> or
> > more.
>
> Actually, it's Nyquist, and it's 2 to 1.

Check your Engineer's Handbook: it's Shannon for sampling and Nyquist for
stability.  The concepts overlap at times, but not this time.

The math under Shannon's work says that a real number "greater than two" is
required.  The next integer is usually selected and the fidelity of the
sampled signal improves with oversampling, as is typical in most of today's
top quality digital audio.  The 2x sampling frequency requires lots of
smoothing to remove the sampling signal and harmonic distortion from the
data, while a much higher sampling frequency is easier to smooth out.

 That is simply part of basis
> sampling theory, and is inherent in any discussion of sampling, scanner
> design included.  Your comment said that the "steps" (of the A/D) had
> nothing to do with the intrinsic properties of the CCD and that is simply
> NOT true, which was my point, and the A/D IS in fact sized to the noise,
> and that sizing is 2:1 typically.

I think you are wrong on this technical point.  The film "noise" is
adequately and wholly captured by HP's 12 bit A/D, according to their tech
notes.  I believe that the CCD noise in scanners is substantially smaller
than film variabilities (ie, "noise"), though it is significant.  CCD
offsets (eg, fixed cell-to-cell zero light values) are likely to be much
larger than what is called "noise" like Johnson noise on CCD spec sheets.
That would mean that the models with 14 and 16 bit A/Ds are using a finer
step that is not directly sized to film noise or intrinsic CCD noise.

>
> > The A/D is scaled to keep the min to max signal
> > range between just a few bits and below full house
>
> I'm not sure what you mean by "The A/D is scaled..." because it is not.
> The analog front end simply "matches" the output voltage range of the CCD
> with the input voltage range of the A/D, which is not scaling the A/D at
> all.  It IS "scaling" the voltage TO the A/D.

Difference of viewpoint only I think: A/D designs come in ranges for 0-5
volts, -5 to +5, etc.  Similarly, once a suitable bipolar or unipolar A/D is
selected or scaled for the signal range to be captured, the actual reference
voltages for offset and gain (ie, 0 volts = 0 bits and max volts = full
house bits like 4096 for 12 bit A/Ds) usually can be adjusted for the
signal's characteristics.  I'm used to doing basic design where the A/D
subsystem characteristics can be scaled but the physics of light detection
or film variabilities cannot.


> > The size of the lsb is just not relevant nor is it significant (as
> > for frame-to-frame waveform analysis/synthesis) in these
> scanner systems.
>
> The size of the LSB IS in fact relevant, and IS a property of the noise,

The lsb selection has no necessary or direct control of noise from a sensor
but sizing the lsb (ie, oversampling on both axes - time and voltage)
affects sampling noise and signal representation.  NOT a property of
intrinsic CCD or film noise, but an electronic artifact that bad design can
introduce and make significant.  Maybe we differ in terminology reference
alone since we're not debating existence of the phenomena.


> and again, the choice of number of bits used in the A/D matched to the
> noise (by a factor of two, but it's still matched to the noise).  If you
> have more bits, fine, but you can't have less, or you lose good data.

Specific matching by a factor of two would be bad design IMHO, and "not the
best" according to handbook criteria for quality signal recovery with low
distortion.
According to HP's and other tech lit, that matching for film noise would
occur at about 9 bits of A/D.  We're not connecting on this point somehow.

Some of this techie stuff is relevant to scanners, but that may show when we
emerge from this corner of techie space.  I apologize to all for pursuing
this minutia a while, but I have a reason.

I need to wallow in this mud a bit to prepare for some assessment of "color
space" issues (ie: my asking about them).  I want to keep physics and
electronic technology concerns separate from judgemental or other issues of
colors and gamma-like stuff in film scanners.

Regards,

Chuck




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