I read this whole thing twice and I now know what pregaps are and the history but still have no idea why people would put them on a CD or why they’re useful for hidden tracks.
If a CD is properly following the Red Book standard, index 0 will point to a 2 second pre-gap of silentce and index 1 will point to the actual start of audio of the track (additional indices are allowed, but not common). The purpose of the pregap is to make life easier for less sophisticated players that aren't able to seek to a precise frame on the disc. They just have to be able to hit a 150 frame region. However, just because the standard says the pregap is supposed to be 2 seconds and silent doesn't mean it actually has to be. Players generally don't care and by the time the format was popular, even inexpensive players could seek precisely. This allows you to stick audio data before a track that will be skipped by the player when it's trying to seek to that track. If you stick it before track 01, it will be skipped even when just playing the disc through unless you rewind.
Some players permitted you to skip back from 1.1 to 1.0 to hear the lead-in as a hidden pseudo-track. Typically this was only possible with hardware index nav. buttons rather than the track nav. buttons, further obfuscating the presence of the hidden track.
The other means of "hiding" tracks is to have a bunch of short silent tracks until you get to track 99 (inconvenient to reach on a player without numeric track entry) or to have a long section of silence starting on the last track from index 1.
Holding the previous track button would "rewind" playback and get you into the pregap on all the CD players I remember using, but these would have been late 80s models onwards.
Was it limited to something negligible like a couple of seconds?
Or could you store a whole five-minute recording in there or something?
Edit: Son of a *, I've had a copy of Sister Machine Gun's Burn for almost 30 years and never knew there was a hidden track!
> These albums all had a pregap of either 32 or 33 “frames,” with a frame representing a length of about 1/75th of a second, per Hydrogen Audio’s Wiki.
> To offer a small correction to the original question now that we know we’re talking about 74 frames per second rather than 60 or 100
It's needlessly confusing and undermines my confidence of the entire article.
(Sometimes songs can also be hidden in tracks at the end of the CD like 99, but that feels less mysterious.)
Other CDs really experimented with the shuffle feature. They Might Be Giants’ Apollo 18 had a bunch of very short tracks that would usually play between songs when shuffle was used.
I think it was nine inch nails' Broken EP that had the hidden tracks on 98/99 rather than after a long silence, but I could be wrong.
My copy in the UK at the time didn’t have the individual tracks. Just one track.
The regular-sized CD looked about identical to the 99-track version, but had only 6 tracks.
(It was expensive to do this, and was never intended for long-term production. Later versions were generally as you describe.)
It was funny at first, but when every other CD I bought had one, it became tacky.