There are at least.... two whole albums released on the DataPlay format. It's an (phase change) optical medium with 500MB capacity, support for AAC, MP3, and DRM (
"ContentKey" allows you to pay a little bit of money at different times, to "unlock" encrypted data already stored on the physical media you have. It does some kind of challenge-response with a DataPlay server which no longer exists. Crypto mostly uses Tripple DES for everything. The purchased decryption keys for the media are burned onto your DataPlay disc with a laser. (Ok, reading through the patents, there's also a public/private keypair in a 'certificate' for talking to a website, and a key revocation list to check and stuff too, and I don't know if this is just SSL, or actually something specific to ContentKey... computer related patents are always written so obtusely. ).

Unlike CDs, DVDs, etc. the reflective layer is on the first surface (i.e. the part the laser is looking at, is just sitting in the open air with no thick layer of polycarbonate in front of it.) So it is packaged in a protective cartridge .

The patents are written to be both over specific and completely vague about the physical details. Like, it won't actually say what material the phase change layer is actually made out of, it just keeps suggesting that any of the dozen popular chemicals already being used in writable optical media. (Is it Indium Antimony Tin? Is it? It might be "Amorphous silicon"?) They won't even give a specific wavelength of laser light to use (but it's probably 650nm). It has an areal density of about 2.6 Gbit per square inch (slightly more than a single DVD layer) There is a 125KHz wobble for laser tracking while writing... which may or may not be present on the pre-recorded (stamped at the factory) music data.

The patents make a big deal about having both writable zones, and pre-mastered data physically stamped into the plastic at the DataPlay factory.... and being able to have both at the same time on the same surface... I think? (It's not like the top and bottom surfaces are different materials... the writable parts are right next to the stamped parts.)

There seem to, maybe?, be hard sectors (stamped at the factory in the plastic) on the writable portions of the surface. Like DVD-RAM (or the giant 14 inch WORM disks from the 1980's)

Like DVDs, it has a Burst Cutting Area... in fact, a lot of things about this medium are very similar to DVD. They claim to use a new and improved error correction code. (I haven't found any details yet.) They still use NRZI modulation, like CDs and DVDs do.

Oh, I found the ECC stuff (it's in TWI246059B), it's almost exactly like the ECC used in DVDs, except that DVDs use an array of 192 blocks of 172 bytes of data... plus 10 bytes of inner parity (per row), and 16 rows of outer parity. DataPlay only uses 88 blocks of 172 bytes of data... plus 10 bytes of inner parity (per row), and 16 rows of outer parity.
 
 

Yeah, so anyway... this is a thing that exists...

https://en.wikipedia.org/wiki/DataPlay

https://www.discogs.com/release/9178628-Avril-Lavigne-Let-Go

https://www.youtube.com/watch?v=uEISYaWgCRg

https://www.youtube.com/watch?v=aiNl1ZvFTmY

https://web.archive.org/web/20010612221719/http://www.dataplay.com/pdf/en/DataPlay_Overview.pdf

https://old.hotchips.org/wp-content/uploads/hc_archives/hc13/3_Tue/19dataplay.pdf

https://patents.google.com/?assignee=dataplay&oq=dataplay