was I right that the recording had made the same error
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Yeah, I can't quite picture it in my head but I think aliasing affects phase (negates it or something like that), so then depending on input phase it can make the resulting signal either completely in-phase or out of phase with the folded alias. Going to try some jupyter-fu.
Yeah, so the phase of an alias folds around Nyquist *and* gets negated (in the φ=-φ sense, not the 180° shift sense). That's the trick. Notice how 90...180 gets folded into -180...-90. Then when you add both halves together, they cancel out. Remove the 1s delay, and they don't.pic.twitter.com/JpGkBme5Xo
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So maybe it's more correct to say that aliasing folds magnitude around Nyquist but *rotates* phase around the Nyquist,0° point.
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And if I haven't completely forgotten my complex math, negating the phase is equivalent to negating the imaginary component of the complex result at a given frequency. This is interesting, it means there's a discontinuity at Nyquist; aliasing isn't "smooth" I guess.
End of conversation
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