krssn wrote:But they can be harder to read and may need to be re-read more, which makes the lens work longer for the same result. Thus putting extra strain on the lens.
The lens can't be stressed. A CD-R is about 20% less reflective ("shiny") than a pressed disc, which means if your laser is weak, your DC may not be able to read a CD-R, even though GD-ROMs still work. This doesn't make CD-Rs worse than GD-ROMs, though, as your laser will get weaker simply from use, and the kind of use doesn't matter. As your DC ages, the laser will get too weak to read GD-ROMs, too. If the DC has trouble reading the disc, it will have to seek more to re-read the same data, which in turn will put extra wear on the motors and gears responsible for moving the laser assembly around. Eventually GD-ROMs will have this same problem, and once you pass below that threshold, your games will randomly freeze when the DC fails to read your discs (and eventually it won't let you boot games at all). You'll cross this threshold sooner with CD-Rs sooner than you will with GD-ROMs, because as your laser's output decreases, you'll pass the threshold where the laser doesn't have enough power to read a CD-R first, since it's less reflective than a GD-ROM, but as long as you have a strong laser, it will degrade equally fast regardless of which disc type you use.
krssn wrote:Genuine question, what exactly makes a disc MIL-CD? Is it a form of code, or is there actually a physical difference between MIL-CD and other compact disc formats?...
A genuine MIL-CD was created with Sega's permission back in 1999/2000 or so. The idea was that a music CD could contain special bonus content, much as some CDs come with screensavers, wallpapers, or music videos that you can watch on your PC. Sega wanted to make the DC an entertainment hub by putting special DC applications on music CDs, so you could play a game or watch a video on your DC while you listened to a CD, or something. The format never caught on, though, and much to Sega'd dismay, somebody displayed the ability to boot any
unlicensed code on the DC by exploiting this format. Soon after, hackers discovered that they could use this exploit to release bootleg copies of commercial games.
MIL-CD uses a standard CD with specially formatted data. The first session contains audio tracks (the music), while the second session contains data -- specifically, a DC game. The problem is that a MIL-CD doesn't have the security ring of a GD-ROM, so you don't need a stamped disc to make one, and the "encryption" method Sega created was super easy to crack: a MIL-CD only requires the game's executable to be scrambled in a rather simple way that was easy to reproduce, and if you make your own disc using that format, you can boot your own code on the DC. By not doing anything to prevent burned discs from meeting the MIL-CD format's specifications, the DC had no way to block CD-Rs while allowing stamped officially licensed discs. Official MIL-CDs have actual albums on them along with their DC game content, but bootlegs and homebrew games usually include a single music track of the minimum length required to have a valid audio session, so that the rest of the disc's space can be devoted to storing game content.
Sega should have immediately halted DC production and made a new BIOS that wouldn't boot the MIL-CD format, but they waited a stupidly long time to do it, so the damage was done: almost any DC could boot unapproved discs without needing a modchip. Most DCs can also boot discs that contain two data sessions instead of one audio and one data session, but those are technically not MIL-CDs, since they don't contain CDDA; even so, they get lumped in with MIL-CDs, since the DC BIOS boots them in the same way as it boots MIL-CDs.
Homebrew game developers don't have the ability to stamp GD-ROMs, so they exploit the MIL-CD functionality to make their games boot, just as pirates exploit the functionality to boot their bootlegs.