Dynamic range and RAW file "bit depth"

Started 10 months ago | Questions thread
DSPographer
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Re: Ramping up the ADC - step by step or skipping?
In reply to Jack Hogan, 10 months ago

Jack Hogan wrote:

All Exmors (even those co-'designed' for Nikon DSLRs)?

I am asking because if I understand the article correctly the referenced ADC starts skipping levels as signal (therefore shot noise) increases - which makes sense from an information theory/physical standpoint and would explain why many current Sony ILCs (including a7s, RX1s etc.) encode all linear data from the sensor non-linearly before writing it to the Raw file without giving the user the option to record the full linear data. The benefit to the designers in terms of faster operation and lower power consumption (and possibly even less 1/f noise?) seem intuitive.

Sony's 'most faithful' raw data mode appears therefore to be similar to Nikon's Lossy Compression mode and its data encoding step could potentially closely correspond to the levels actually sampled by Sony's ADC.

Sample Sony look-up table embedded in Raw file. Ignore captions: they pertain to a different topic

On the other hand Nikon DOES give the user the option to save data in the Raw file linearly and 'uncompressed'. Below are the histograms of the entire raw files resulting from DPR's Studio Scene captures for the A7 and D610 at ISO 100. In the D610's most significant bit there is sparse but apparently full data covering virtually every linear level - while the A7's is clearly skipping levels per the non-linear table above. Tellingly, in the green channel the D610 uses 12074 unique values to encode the image versus the A7's 1774 values.

So unless Nikon is injecting noise after the ADC (are they? It would not look that way by the shape of the histograms) my guess would be that Nikon ADCs are programmed to ramp up value by value and produce a 'faithful' digital representation of the analog signal+noise, no matter what the source or level of noise. What do you think?

Jack

If I recall correctly, the D800 was using an accelerated ramp much like you show for the A7. I haven't seen an analysis for the D600 or D610 yet, so I don't know what they do (do we know for sure it is a Sony Exmor?). But, just because the histogram doesn't show skipped levels doesn't mean the A-D is using a single slope ramp. I don't think Nikon would be injecting noise, but they are known to process raw data before the file is written. So, there could be something like either an analog or digital column amplifier pattern noise (PRNU) compensation being performed: which could fill in missing levels much like a raw vignetting compensation would. Nikon may also have invented some way to get a full precision number latched even while the ramp slope is accelerated. In that case the digitization error might increase for the higher levels- but without missing codes being produced. Of course, Nikon might have just had Sony put in a mode where the slow ramp is used for the entire conversion range, but that would normally make the digitization so slow that the frame rate would need to be dramatically reduced.

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