As for filters - look at typical response of 2and 8 pole 20kHz Bessel filter in dB:Kijanki, you provided us w/ the freq resp of ANALOG Bessel filters. I agree with you & I did write this in my prev post - analog filters cannot creat a sharp cutoff like what the author has shown - large attenuation between 20K-24K.
2pole 8pole
20kHz -3 -3
22kHz -3.63 -3.67
40kHz -9.82 -13.68
80kHz -20.32 -51.81
But, how about digital FIR filters? Can they create such a sharp roll-off?
Yeah, sure they can! Did you bother to read any of the links I referenced in my post? The paper from Dan Lavry shows 1 example & then there is that AES paper by Julian Dunn that also shows 4 filters that have 100dB atten & only a modest # of taps. All FIRs have flat group delay in-band.