My question is if the transition band in the question below would be from 4Khz to the Nyquist frequency of 22.05Khz or from 4Khz until 44.1Khz-4Khz=40.1Khz?
1
$\begingroup$
$\endgroup$
If you don't want DAC noise near 40-44 kHz to alias down into your audio, you want the narrower/lower transition band reconstruction filter.
-
-
1$\begingroup$ If the signal bandwidth is $B$, the LPF response should be flat up to $B$, then go to 0 as steeply as feasible. $\endgroup$ – MBaz May 1 '16 at 18:32