Official Sega Saturn games are loaded from the 2x CD-ROM drive. Its medium, a tailored variant of the Compact Disc (CD), has a capacity of 650 MB and adheres to the ISO 9660 standard for storing data . Additionally, many games include audio tracks next to the data tracks to stream uncompressed audio during gameplay.
The Compact Disc (CD)
Co-authored by the Dutch company Philips and the Japanese company Sony, the Compact Disc (CD) is an optical medium on which information is stored by engraving microscopic pits and lands into its polycarbonate surface . An infrared beam is then directed from the drive, and the reflection produced on the CD's surface is used to retrieve the information.
The process of converting digital information (ones and zeroes) into pits and lands, and vice versa, is by no means simple, especially since CDs must:
- Allow plenty of capacity to make them a competitive alternative to existing media.
- Notably, the first revision of the specification (CD-DA) only strived for 74 minutes of high-quality audio, while the later format (CD-ROM) allowed for 650 MB of general-purpose data.
- Be robust enough to sustain day-to-day damage and intensive use.
- Be reliable enough to store any kind of information without fear of data loss.
Thus, on the surface, data is encoded using the Non-Return-to-Zero inverted (NRZI) scheme . This means the bitstream consists of zeroes until a land-to-pit or pit-to-land transition occurs, at which point a 1 is appended instead.
Beneath the surface, however, things become more complex. NRZI looks manageable on paper, until the optical reader encounters a sequence of ones (continuous pit and land changes) or long sequences of zeroes (constant pits or lands), at which the sensor will struggle to detect or remain synchronised, respectively. To address this, a preliminary step called Eight-to-Fourteen Modulation (EFM) is added to the mix. EFM translates every possible combination of 8 bits into a 14-bit sequence that takes into account the constraints of the CD reader. Consequently, before data is engraved, groups of 8 bits are converted into a sequence 14 bits, after which three 'merging bits' are inserted between the encodings. All in all, the resulting engravings provide high data density while also keeping the CD reader synchronised.
Finally, with regards to reliability and robustness, the CD also employs Cross-Interleaved Reed-Solomon Coding (CIRC), which spreads data across the disc and adds redundancy so that damaged regions can be reconstructed.
I have to say, the evolution of optical storage is an interesting topic in itself, and as the CD evolved into the DVD and beyond, advanced mechanisms for sensing, encoding, and error correction made its way into consumer hardware - culminating in standards that dominated game distribution across multiple generations.
The Saturn's CD
So far, I've been discussing the wider standard, but in the case of the Saturn, Sega adopted the CD-ROM XA format (short for 'CD-ROM eXtended Architecture'). This is a superset of the CD-ROM format that allows to store data, uncompressed audio, and interleaved multimedia tracks . The latter is essential for slow-speed drives, allowing audio and images to be streamed at a reasonable rate, a capability particularly useful for video playback. Speaking of, video playback must still be decompressed, and for that Sega shipped an optional card called 'Video CD'.
Development
At first, Sega did not provide comprehensive software libraries or development tools (in fact, the initial documentation contained several inaccuracies), meaning the only way to achieve acceptable performance was through harsh assembly. Eventually, Sega shipped a complete Software Development Kit (SDK), hardware kits and additional support libraries to ease I/O and graphics operations.
That said, Sega Saturn games were ultimately written using a combination of C and various assembly languages targeting individual components.
I/O
Peripheral management and Real-Time Clock (RTC) are also provided by the aforementioned System Management and Peripheral Control (SMPC), which is controlled via commands sent by the SH-2s.
Expansion methods
This console bundles a considerable number of external connectors and interfaces, most of which only saw a handful of uses:
- Behind the drive, there is a cartridge slot, officially intended for additional storage (save data) or extra RAM. In Japan and the United States, a modem was also offered to provide online connectivity.
- At the rear of the console, there is a slot for a Video CD Card, which performs MPEG decompression for programs or games that support it.
- Finally, there is a mysterious socket at the back of the console called Communication Connector. Although Sega did not publish any developer documentation, reverse-engineering efforts revealed that it connects to the SCSP's MIDI pins and the Serial Interface (SCI) of two SH-2's . In any case, Sega released a floppy drive that made use of this interface.