In message ,
Alan Browne wrote:

Yes, I owuld guess that it is so. When below, eg, ISO 1600, the ADC is
stepped, and at 1600 and above the sensor readings are left shifted.
There may or may not be an overlapped zone where some gain is in the ADC
and some gain is in shifting left.

Then, there may be some filtering in the ADC as well as (of course)
making the RGB image from the Bayer image that will disgusise (filter,
badly or otherwise) the quantization effects.

The dynamics of the noise are greater than the dynamics of quantization
at the highest ISOs. The lower ISOs are the ones that show the most
quantization effect.
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John P Sheehy