Learned something new today and it isn't good.

Please don't ask me to explain this in any more detail as I will already be pushing the limits of my knowledge. Use Google if you want to learn more.

Nyquist rate DACs have a zero-hold structure. They are not flat to 20KHz and roll off towards 20KHz. A zero-hold means it holds the same value between sample updates. I understand ADCs don't normally have this issue because they take a snapshot in time. To compensate for this roll-off, the CD could have pre-emphasis to compensate and get a flat response. The roll off is not very big. When up sampling DACs became standard, this was not needed as the roll-off at 20KHz was very small.

The redbook CD standard has a flag on the CD that says pre-emphasis was applied. The CD player should read this and tell the player what it needs to do if anything.

Pre-emphasis is the first part of a noise reduction technique in which a signal's weaker, higher frequencies are boosted before they are transmitted or recorded onto a storage medium. Upon playback, a de-emphasis filter is applied to reverse the process. The result is a higher signal-to-noise ratio (SNR); the original frequencies are restored, but noise that was introduced by the storage medium, transmission equipment, or analog/digital conversions is quieter than it would have been if no filtering had been done. Pre- and de-emphasis can be collectively referred to as just emphasis.

Emphasis was sometimes used in digital storage media in the late 1970s through early 1980s, including on a small percentage of audio CDs. Emphasis is akin to Dolby noise reduction for tapes, or the RIAA equalization curve for vinyl records.

From what I gather emphasis was implemented to try to improve the sound of the CD players, not the cd's themselves. This is a case where the media was better than the technology to play it.

Pre-emphasis is very likely dead IMHO, because ripped CD's data doesn't carry pre-emhpasis bit. A lot of people use servers and music with pre-emphasis would sound bad (+9.5dB@20kHz) without de-emphasis.  Some ripping programs might contain plug-in to de-emphasis if needed. There are programs (like SoX) to de-emphasis digital data, but it is additional hassle (to find out if it is needed and convert every song).

(In common CDP to DAC setup  S/Pdif carries pre-emphasis bit to DAC)

I have not played a CD disk that I would have suspected Not having such de-emphasis.  Like I hear playing a record without RIAA de-emphasis.  Or Dolby encoded tapes.  It's not broken.  You don't have to deal with it.

This has nothing to do with noise or noise reduction. Why does that keep getting repeated??

It was a necessary step to deal with the natural operation of early DACs.

One of my colleagues sent this. It is not light reading but the concept seems easy to understand.

https://www.analog.com/en/technical-articles/equalizing-techniques-flatten-dac-frequency-response.html