Hector Martin

This actually got me thinking of the DSP implications. I know how sampling and aliasing work in the frequency axis, but clearly phase matters too. Shifting the sampling phase shifts the phase at which the upper spectrum half folds over? I need to think about this.

Like, obviously when the downsampling aligns with the nonzero samples you get constructive aliasing interference, and when it aligns with the zeros you get destructive interference, but why?

Paging @xiphmont. Any pointers as to how to explain this time-domain phenomenon in the frequency domain? How does the phase of aliased downsampling affect the phase of the spectral components that get folded together?

Honestly, never thought about aliasing in detail except how best to avoid it. In the early says of ADC/DAC, the imaging filters purposely allowed some aliasing to get perflectly 'flat' response to nyquist, but that fell out of favor. Possibly due to related question :-)

There's nothing *that* tricky about it I suppose--- it would be a fun excercise when I'm not busy. Can probably just graph it out and that would be enough.

I think the answer will be obvious by simply plotting the phase response of a pure time delay. Going to try that now.

"straight line", and I suppose it just folds at Nyquist like the magnitude.

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.