https://curiouser.cheshireeng.com/2015/01/16/pdm-in-a-tiny-cpu/
https://tomverbeure.github.io/2020/12/20/Design-of-a-Multi-Stage-PDM-to-PCM-Decimation-Pipeline.html
https://www.dsprelated.com/showarticle/1337.php
10log(1/((.998-4cos(8*2pi*x)+6cos(8*4pi*x)-4cos(8*6pi*x)+cos(8*8pi*x))^2+(-4sin(-8*2pi*x)+6sin(-8*4pi*x)-4sin(-8*6pi*x)+sin(-8*8pi*x))^2))-100+3*10log((sin(128pi*x)/sin(pi*x))^2)
https://www.micromodeler.com/dsp/
https://www.analog.com/media/en/technical-documentation/dsp-book/dsp_book_Ch16.pdf
https://fiiir.com/
http://t-filter.engineerjs.com/