Dynamic range and RAW file "bit depth"

Started 11 months ago | Questions thread
bobn2
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Re: Ramping up the ADC - Does Nikon Accelerate its Ramps?
In reply to Jack Hogan, 11 months ago

Jack Hogan wrote:

DSPographer wrote: 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?).

Yes, according to Chipworks it's a Sony IMX128. Compare its part number to these .

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.

It would seem rather wastful to do that before writing data to the raw file - as opposed to doing it after having pocketed the space/time saving... In other words, if the data rolling off the ADC is 'accelerated' to start with, follow Sony's lead no? Unless it's a marketing gimmick (we've got an 'uncompressed' mode and you don't).

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.

I see (said the blind man). The allegedly ramp-accelerating A7 is actually a bit slower in peak FPS than the D610 (5 vs 6). Maybe the A7 simply has a slower DSP, though. I know that whether the ramp is accelerated or not would make virtually no difference in practice to a photographer, although the potential astrophotographer in me would be slightly disappointed.

But now I am really curious. Does Nikon accelerate its ramps or not? Does Canon? Anybody know?

I'd be interested to know more about the evidence that Sony is using this accelerated (non-linear) ramp technique. I haven't seen any evidence in the D800 files, but perhaps I haven't been looking in the right place.

Knowing how the Exmor ADCs work, where one DAC feeds the comparison signal to a whole line of comparators, and the ADCs are simply latches which latch the value of the counter feeding the DAC when the comparator strobes, what you'd expect is that at the top end of the scale there would be a lot of missing codes, since each ADC would skip exactly the same codes. I've never observed that.

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Bob

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