Digital BW, The Print

An A/D takes a fixed voltage range say 0 to 1 volt. The signal needs to 
be in that range in order for the A/D to provide the a digital 
conversion that maintains the SNR.  For example if you clip you will not 
only have artifacts you will lose some SNR. Same thing if the input 
voltage out of the sensor is to small.

If the A/D is designed with with the same SNR expected out of the sensor 
electronics then as long as everything is correct, i.e. the exposure is 
perfect. On the other hand if it is under or over exposed the resulting 
SNR out of the A/D will be less than the input SNR. An 8 bit A/D will 
produce a 36 dB SNR signal for an infinite input SNR signal 
(quantization noise). A 12 bit has 60 dB and a 10 bit 48 dB. If you have 
48 dB coming out of the camera electronics the output of the A/D is 
limited by the A/D quantization noise so 8 bits are not sufficient. If 
the exposure is perfectly set so the input voltage to the A/D is 0 to 1 
volt 10 bits will be sufficient. However, if it is one stop 
underexposed, the top bit won't be used hence the resulting output SNR 
of the A/D is 45 dB - hence there is a loss of SNR. If a 12 bit A/D is 
uesd the output signal will sitllb e 48 dB because of the extra bits (11 
bits would be used).

So have enough bits for some margin is not a bad idea. If the most 
significant bits are not used - they can be thrown away. Most likely the 
image will be scaled to max range in processing and with the extra bits 
that will not be a problem. I suspect that in practice 12 bits is 
sufficient for digital photography - and it gives you a comfortable margin.

Truman



Paul D. DeRocco wrote:

> > From: Truman Prevatt [mailto:tprevatt@...]
> >
> > One reason to have more bits in the A/D is to give yourself a margin for
> > error in an image for under or over exposure or in RF systems for bad
> > gain settings. So there are two issues, the S/N required and the dynamic
> > range that insures that S/N is still captured even in the case of a
> > screw up.
>
> That's a good reason to want more S/N in the system, not just more bits in
> the A/D. There's never any reason to have more bits in the A/D than 
> the S/N
> justifies, because any extra bits will be garbage.
>
> --
>
> Ciao,               Paul D. DeRocco
> Paul                mailto:pderocco@...
>
>


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