No that is not my understanding... it's what you think is my understanding of it.
It's the consequences of the waveform in the amplitude domain over time.
Have a look at the amplitude of the 10 and 16kHz signal in post #310 and look at the amplitude which varies and it should not do this.
the amplitude thus drops and rises (varies) constantly depending on synchronization of the frequency in question with the sample frequency.
That dropping of amplitude is the 'roll-off cutting in the audible band'
The 12kHz square and sinewave comparison is not what this is all about.
That has to do with related harmonics.
What if the amplitude of that 12kHz, that should be constant, varies in amplitude between nominal and way below nominal constantly and the average of it is lower than what it should be ?
Waveform shows various frequencies at the same time.
You see a fluctuation of the amplitude but this fluctuation doesn't exist at the frequency of the fundamental tone. It graphically shows when you add oranges and apples, different frequencies. But different frequencies don't add in nature, they simply coexist.
Also it's irrelevant whether if it's harmonics or some other frequencies which show in the NOS waveform. All frequencies which show in a waveform are higher than fundamental frequency. And the only thing that matters is whether any of them (aside of fundamental frequency) belong to the audible range. If not...you won't hear them, you will only hear a fundamental frequency sine tone.
This is why you can't hear a difference between a 12kHz tones. First harmonic is 24kHz, you can't hear it.
If you look carefully graphs of NOS waveforms, you'll understand that those frequencies that change the shape of the fundamental tone sine all belong to >20kHz range. If you filter them out above Nyquist, of course you'll get a (almost) perfect sine, because this garbage frequencies are all above Nyquist.
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