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Re: A lot of people don't understand dither

What Todd correctly points out is that if the original signal contains wideband noise, with a suitable distribution, and at a level at or above the target word length's equivalent noise floor, then the signal is self-dithering and truncation to the target length can be done without additional dither.

In real-world music productions the total amount of noise originating as thermal noise prior to the ADCs, in the final digital-domain high-resolution master more often than not exceeds the 20bit -120dB noise floor and truncations to word lengths of 20 bit or more don't need dither.

Your 24bit/16bit picture analogy is not quite valid as truncation to 16 bit (i.e. 5/6/5 bits for R/G/B respectively) makes the target noise floor exceed that part of the noise floor in the original image that could have worked as self-dither. In fact, as image sensors offer 60-70dB (10-12bit) of original SNR (10-12bit) in the dark and are shotnoise-limited in the light one can safely say that in almost all applications the original non-correlated image noise lies way below the quantisation noise floor of the 8/8/8 '24' bit format and thus will never serve as self-dither.
If the original raw image does contain visible noise (and trust me, it will ;-), then keep in mind that this high-level noise originates as shot noise or as image processing truncation noise and thus is correlated and non-dithering.




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