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[filmscanners] RE: Density vs Dynamic range

Hi Julian,

What you say below matches my understanding, and, I believe, what I've said
throughout this discussion.  Though, I haven't seen RMS used in a DR
equation.  None of the reference material I have, for audio, signal
processing, and engineering in general use RMS for DR...but that doesn't
mean some vendor can't make their own equation ;-)



> -----Original Message-----
> From: filmscanners_owner@halftone.co.uk
> [mailto:filmscanners_owner@halftone.co.uk]On Behalf Of Julian
> Vrieslander
> Sent: Tuesday, June 11, 2002 3:44 PM
> To: darkroom@ix.netcom.com
> Subject: [filmscanners] Re: Density vs Dynamic range
> On 6/11/02 2:15 PM, "Austin Franklin" <darkroom@ix.netcom.com> wrote:
> >> In engineering terms you are confusing dynamic range with
> signal-to-noise
> >> ratio.
> >
> > Absolutely not the case.  Being that I've also designed a LOT of audio
> > equipment, I know the difference.  They are SIMILAR, but not the same,
> > except at one point, when the SNR is at it's highest, it is the same as
> > dynamic range.
> For audio and video circuits, SNR usually assumes a standard (or claimed)
> reference level, which might be well below the maximum possible signal
> level.  Say you have a an amplifier stage with a noise level of
> 0.0001 volt
> RMS.  The SNR, relative to a 1 volt RMS reference, is 2 * 10 *
> log10(1/0.0001) = 80 dB.  The factor of 2 enters, because AC circuit specs
> are defined in terms of power levels, and power is proportional to the
> voltage squared.
> But that particular circuit might not clip or reach unacceptable
> distortion
> levels until the signal is increased in amplitude by a factor of 10.  The
> difference between noise and the maximum possible undistorted
> output is 2 *
> 10 * log10(10/0.0001) = 100 dB.  This is sometimes quoted as the "dynamic
> range," and it is consistent with the usage that we have been
> discussing in
> the scanner context.  The difference between the SNR number and
> the dynamic
> range number is sometimes called the headroom (20 dB in our example).  For
> an audio component, the headroom spec usually indicates the ability of the
> product to cope with signals that exceed the standard or typical levels.
> > SNR also is an RMS based measurements, and RMS doesn't apply
> > to dynamic range.
> Why not?  I've seen quite a few designers and vendors use the
> above-described convention for specifying dynamic range.  Consumer HiFi
> manufacturers have used other schemes, measuring the limits of their
> products to handle impulses or "instantaneous" signals.  But usually these
> schemes are designed to generate more impressive numbers for
> advertisementss.
> This is getting pretty far afield from scanners.
> --
> Julian Vrieslander <julianv@mindspring.com>

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