To Reiterate that point: visualize sampling the upper spectrum this kind of the new sampling level is at 50 % of $f_ s1 $, suitable in the center with the "significant frequency" spectrum parts we depicted. Which means then that the spectrum centered relating to this new sampling rate must https://share.evernote.com/note/3862b16f-f83a-cbec-2fb6-07ed32465225