Hybrid disc

A Hybrid disc is a CD-ROM that has multiple file systems, so that it can be used on various system software, for example both Apple Macintosh and Microsoft Windows based operating systems.

A hybrid disc has multiple file systems installed on it, typically ISO 9660 and HFS+ (or HFS on older discs). The reason for the format is primarily that ISO 9660 has severe restrictions placed on filenames (initially only 8 characters per file, and a depths of 3 directories) and lacks key structures present in Apple Computer's HFS and HFS+ file systems. Another key factor is that ISO 9660 does not support resource forks, which is critical to Mac OS' software design (Mac OS X has removed much of the emphasis/need for resource forks in application design). Companies that released products for both DOS (later Windows) and Mac OS (later Mac OS X) could release a CD containing software for both, natively readable on either system. Data files can even be shared by both partitions, while keeping the platform specific data separate. In a true (or shared) hybrid HFS filesystem, files common to both the ISO 9660 and HFS partitions are stored only once, with the ISO 9660 partition pointing to file content in the HFS area (or vice versa). Blizzard Entertainment has released most of their computer games on hybrid CDs. By default, Mac OS 9 and Mac OS X burn hybrid discs.

A hybrid disc has an ISO 9660 primary volume descriptor, which makes it a valid ISO 9660 disc, and an Apple partition. It may also have an Apple partition map, although this is not necessary. The ISO 9660 portion of the disc can co-exist with an Apple partition because the header areas which define the contents of the disc are located in different places. The ISO 9660 primary volume descriptor begins 32,768 bytes (32KB) into the disc. If present, an Apple partition map begins 512 bytes into the disc; if there is no partition map, the header for an Apple HFS partition (known as a Master Directory Block, or MDB) begins 1,024 bytes into the disc.

Hybrid-CD also refers to the invention of a single CD with additional security measures. To replace the CD-soft R, such a CD combines in it a CD-ROM section and a CD-RW section. Utilizing this hardware, a single disk can have a software package written in the ROM (read only) section which cannot be changed. However, the RW (read-write) section can store user specific data.

Hybrid-CD also refers to audio CD which also includes a data track storing MP3 (or other digital audio compression format) copy of those CDDA tracks. Before the introduction and subsequent popularization of iTunes and the iPod, such discs were popular for sharing music on compact disc without requiring the recipient to extract and encode the CDDA themselves — a technical and perhaps time-consuming process on older computing hardware. However, with the advent of faster computing hardware and vastly simplified automated extraction and encoding tools (e.g. iTunes, Rhythmbox, etc.) and the lack of an automated hybrid feature in that very same software, popularity of such hybrid CD has subsequently declined. Furthermore, the legal status of such discs poses economic frictions in that they may be interpreted as two copies of a copyrighted work. However, such hybrid discs do remain in a commercial setting as a digital rights management enforcement technique, where encrypted compressed copies of the digital audio are provided with proprietary software for listening in a computer disc drive, while the CDDA is included for playback in stand-alone CD players.

See also

• Universal binary