In response to the ease of cloning a CD, Sega implemented a copy protection system - along with region locking - to control the distribution of its games.
For the copy protection itself, Sega deliberately deviated from the standard CD format, making it impossible for regular CD burners to produce a perfect copy of a Saturn game. Then, as part of its verification process, the Saturn's disc reader looks for these non-standard features.
To be more specific, Saturn discs were manufactured with an unusual data pattern pressed into the outer edge , creating a visible ring engraved with trademark labels. This ring sits outside the standard data area of a CD (known as the 'Program Area') and beyond the Lead-out section, which signals the end of readable data. As a result, conventional drives couldn't access or replicate that part of the disc.
Inside the Saturn's disc drive is a dedicated SH-1 processor that verifies the presence of the ring independently from the main CPUs. It uses obscure communication protocols to talk with the rest of the system. This check, however, is only performed once.
Defeat
To begin with, the classic method used for disabling the copy protection consisted in installing a mod chip, which could deceive the CD reader when any type of disc was inserted. Another technique, known as the 'swap trick', consisted in hot-swapping a genuine disc with a burned one immediately after the protection checks had passed... with the risk of damaging the drive.
My Saturn with the Fenrir chip installed. This is an Optical Drive Emulator (ODE) that replaces the original CD drive to execute games from an SD card instead. This is one of the many modern methods used for executing Homebrew.
After the turn of the century, more sophisticated methods for running unauthorised code were discovered, for instance:
- An exploit in the copy protection mechanism was identified, allowing to boot up any disc game without undergoing copy protection checks. This was subsequently implemented in the form of a cartridge called PseudoSaturn . As it relied on the cartridge medium, Action Replay cartridges were often re-flashed with PseudoSaturn (though the flasher program still needed to be bootstrapped somehow, most commonly through the swap trick).
- This method remains in use as of 2022, although a newer fork named 'Pseudo Saturn Kai' is installed instead.
- Another method emerged in 2016 (almost 20 years after the console's release) by exploiting the fact that the Video CD add-on can inject unencrypted code into the CD subsystem (bypassing the CD drive entirely). This finally allowed users to load Homebrew independently of the ageing drive. The Video CD exploit is commercially distributed as a product called 'Satiator' (I'm not sponsored, by the way).
- Lastly, there's a commercial alternative that replaces the CD reader with an SD or SATA adapter. The Saturn still thinks it's reading from a CD, but the 'CD' is being emulated by the adapter, which is in turn reading from a disc image . These products are known as Optical Drive Emulators (ODE).