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[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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