Again, thank you for all the discussion. It continues to twist and turn in many directions, but I think we are contributing useful information.
I would like to respond to discussion about music player apps and how they relate to gapless playback.
Some people experience gapless playback with their normal ripping and music playing apps. That’s great.
However, not all music players have this “gapless playback” behaviour.
I did a quick test of a few music player apps on one Linux system (Lubuntu) and one macOS system which I had handy, using the Apple Music m4a format music files of the Release The Ship, where the original LP and CD play continuously with no break between tracks.
On Linux, some apps played back gaplessly, some played back with gaps, and some just didn’t work very well. (I didn’t take note of the app names, sorry.)
On macOS, VLC 3.0 left short but definite breaks open in playback as it moved from one track to the next. I would say the breaks were 0.1-0.5sec long. This is not surprising, because gapless playback has been a missing but requested feature for 15 years, which may finally appear in VLC 4. (If you are getting gapless playback from VLC 3, you are doing well.)
Quicktime Player only opens one file at a time, and won’t open a sequence of files to play together, so it has no way of attempting gapless playback.
And, I must report that my earlier report about iTunes was incorrect. I reported that iTunes inserted gaps, until I added a tag pgap (which Picard calls gapless) to the music files; then iTunes played the files without gaps. I was passing on an observation by someone else, and they were using streaming services rather than iTunes in their initial test. iTunes (12.8, macOS 10.13.6 High Sierra) plays these music files gaplessly, by default, even without the pgap tag.
Thus, I now know that macOS, Windows, and Linux each have some music player apps which will play back an album’s worth of per-track music files gaplessly by default. And, I no longer have examples of major music playing apps which require special gap-related tagging to enable gapless playback.
Understanding this greatly reduces the value of adding metadata about inter-track gaps to MusicBrainz, because one can already get the desired gapless playback experience even without it. The value proposition is now only that the metadata describes true facts about Release layout on CD or LP or cassette media, and it advances MB’s mission of being the “ultimate source of music information”. That is not enough. There are many other MB improvements which also have these qualifications, plus deliver more user value.
That said, I have found out some interesting things about the CD-DA format, which responds to other parts of this thread. Those follow in a separate message.