mystran wrote:Nowhk wrote:Than: if calculating the FFT of the signal in that diagram will return a "signal x + error", why moving sample's magnitude casually around Y axis won't return the same "signal x + error"? Because signal x seems to change, really. Not only the noise...

The "error" is signal dependent unless the signal has sufficient broadband component (read: is noisy enough). Such "sufficient broadband component" is guaranteed when you add dither (although it's also possible for a signal to already be noisy enough to begin with), at which point the quantisation error will be broadband noise. From the engineering point of view, failure to dither properly is essentially a bug and that's why your result is (again from the engineering point of view) garbage. Throw in the dither and all the theory will suddenly make sense.

This doesn't entirely answer to my question.

Let me make an easier example.

Take a collection of N samples, at a sample rate that can express the signal's frequency I'm sampling.

Let say this samples collection express a SINEX (where X is the freq).

Once I change (a bit) the magnitude of each sample, I'll get the same SINEX + NOISE.

Change a little bit each sample of the collection again, and I'm still getting SINEX + more/plus noise (a different one).

And so on.

WHEN it will happens that the magnitude's deviation of some samples is so huge that the SINEX will disappair and substituted by another SINEY (i.e. another frequency) + noise?

It can't be that there is ALWAYS that SINEX + some noise, whatever sample's magnitudes I place within my collection of samples at the same sample rate.

Because this will means that whatever collection of samples I use will ALWAYS express SINEX + noise.

But I know that such collection can simply express SINEZ, maybe without noise (which has no relationship with SINEX and its noise).

So, what's the barrier where a SINEX + noise become another SINEY + noise, simply moving samples? There should be a relationship between samples, else the difference between two random collections of samples would only be "noise" (keeping the same SINEX "under the wood").