One more note (not a criticism at all, just more information);
It's common for reconstruction anti-imaging filters to allow some
aliasing in the reproduced transition band... that is, they don't slam
to zero between 20050 and 22050, they actually use 4kHz, from 20 to
24kHz or so. Since the aliasing happens in the folded transition
band, it's still all up above 20kHz.
Embedded hardware is normally more constrained than software on full
sized CPUs, and every extra gate is money (so I'm told)...
OTOH, I'm also used to seeing modern Sigma-Delta designs do ~ no
output filtering at all. You can't use them in demos with
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