Yes the ADC anti aliasing filters have a tap length requirement, and the requirements are "fairly" simple - linear phase, aliasing set to a level that is no longer audible. But if you set that to say -144 dB (24 bit accuracy), you need about 2K taps or more - which is much greater than normally employed. ADC chips do have significant aliasing, and normally the chip designer says its OK as the errors are at say 21 kHz. What they fail to realize is that that is audible, as it will change the time domain linearity, which is audible.
Now the mathematics is pretty clear - a DAC interpolation filter needs an infinite tap length, infinite oversampling FIR filter to perfectly reconstruct the original bandwidth limited signal. So I will keep pushing the tap length, and oversampling rate until one can no longer hear the difference. For the ADC filter, the maths simply states that at >= FS/2 the level is zero, i.e. there is no signal at 22.05 kHz and greater for 44.1 kHz. The fun will start when I actually listen to these filters, and find out what stop band attenuation is really needed. But even if 200 dB was good enough, then that would not need enormous tap lengths.
The pulse array ADC project has been running for 5 years so far, so its quite well advanced.
Rob
