A reconstruction filter rolling off 19-22kHz sounds better to me at the top end - for some recordings anyway - and I don't feel I'm missing anything. In fact the opposite. I don't know whether this is:
1. Simply the amps/speakers being asked to produce less energy >19kHz.
2. A gradual filter roll off. Does linear phase mean this should not matter?
I assume brick wall filters are done because they are easy/cheap/low processing power though.
The ideal reconstruction filter, sinc(x), is infinitely steep. It stands to reason that the industry attempts to approach this for digital replay. (But doing so, it ignores the ADC side of things, but that is a different story). So it is to be expected that most commercial CD filters, at least before the often entirely misunderstood apodising craze started, are quite steep and cut at 22kHz. Making them steep adds cost. Making them cut at exactly Fs/2 halves the number of coefficients, reducing cost again. Thus symmetrical linear phase half-band FIR. Since steepness is
not infinite, imaging occurs.
This would be perfectly innocuous if the recording side was done properly, meaning with zero content at Fs/2. In such case the DAC side would not ring (since zero content at Fs/2), and would generate a minimum of images, since the ADC side already ensured that there is not much near Fs/2.
But that is not how industry works. Seeing those nice and economic linear phase half-band FIR filters in DACs, they decided to adopt the same filter style for in-ADC downsampling (and, broadly, also in downsampling software, which, until recently, was abysmally bad, see ProTools, Pyramix, Merging, ... at src.infinitewave.ca). So the ADCs also got steep half-band filters. These ensure a couple of bad things:
-aliasing happens, infecting the 20-22kHz part of the recording
-strong ADC filter ringing is inserted at 22kHz
-DAC-side ringing is triggered
-DAC-side imaging is triggered
In short: industry practice guarantees that the 20-24kHz range of CD is buggered during replay. Can't hear it directly, but metal domes will ring, and systems with IMD issues may show this up.
And yet, the recipe is simple, provided we drop that old fetish of needing flatness to beyond 20kHz.
Just somewhere in the chain, ideally at the ADC side, start rolling off at 18kHz, and reach zero, or at least a suitably low level (most music hasn't got that much of high treble anyway), at Fs/2. That gives the filter a 4kHz-wide transition band. 4kHz is also, give or take, the width of the highest critical band in the ears of healthy young people. This then means that the filter's ringing is of the same order as the innate temporal acuity of that highest critical band, i.e. good enough. So in one step you ensure that:
-DAC-side ringing is not triggered
-DAC-side imaging is not triggered
-any ringing is inaudible, even for those with fresh ears
-the downstream system is only fed with the music, nothing else.
they are easy/cheap/low processing power. But filter shape aside, fair to say a lot digital filtering falls significantly short of current SOTA (which I believe is Saracon), and
iZotope is the reference (*). Has been for a long time.
(* When configured properly. Last year it was very fashionable at CA to dream up iZotope settings and validate them by ear. A lot of garbage was generated, and people loved it. Of course, the tiniest settings differences were always day-and-night audible.)