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Re: er...

I assumed the same for many years.

In reality just about any commercially available ADC chips*, including top models like the PCM1804, do their anti-aliasing filtering with linear-phase half-band FIR filters with relatively short length:
-this is cheap in silicon
-only 6dB attenuation at fs/2
-often only about 80dB of stop-band rejection above fs/2 (i.e. full-band aliasing occurs at levels coincident with the source's ambience and fine detail)
-filter impulse response with pre and post echo, about 1 ms wide (upper 60dB of impulse). You rang, mylord?

All of this is very poor. Luckily indeed that many have reverted to 88.2kHz or 176.4kHz recording followed with downsampling in the DAW, hopefully using better filters.

(* There exist pro ADC units that do better, notably dCS and a few others. These do not rely on commercial ADC chips from TI/BB/Crystal/AKM but rather on in-house developed discrete solutions, often with much more rigorous AA filtering)

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One filter I have made is:
.flat to 15kHz
.-3dB at 18kHz
.-130dB at 20kHz
.600us impulse width (almost twice as fast as a regular CD-audio AA filter)
This is a 'safe' filter in that it allows even for cheap DACs with primitive half-band reconstruction filters to play without imaging.

Another filter I have is more like (don't have the details here):
.flat to 16kHz
.-3dB at 24kHz (!)
.-130dB at 28kHz
.400us impulse width
This filter is remarkable in that it deliberately allows aliasing to occur, folding back in the 16-22kHz band. The reasoning is two-fold: many adults (including me) don't hear beyond 15kHz, and above ~12kHz the human ear cannot distinguish pitch, so aliasing occuring above 12kHz is perceived as a brightening, and not as a mixing-in of dissonant tones. dCS follow the same strategy.
I use this filter mainly for recording analogue-master LPs, often with purely acoustic contents and innately smoothly rolling off above 12kHz or so. In short: when the source programme is not rich in treble contents.

All work in progress. Present filters are long and computationally intensive, and FIR/linear phase. I want to arrive at much shorter filters, asymmetrical FIR, and somewhere between linear phase and minimum phase.




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