DVD File Systems

One of the major achievements of DVD is that it has brought all the conceivable uses of CD for data, video, audio, or a mix of all three, within a single physical file structure called UDF, the Universal Disc Format. Promoted by the Optical Storage Technology Association (OSTA), the UDF file structure ensures that any file can be accessed by any drive, computer or consumer video. It also allows sensible interfacing with standard operating systems as it includes CD standard ISO 9660 compatibility. UDF overcomes the incompatibility problems from which CD suffered, when the standard had to be constantly rewritten each time a new application like multimedia, interactivity, or video emerged.

The version of UDF chosen for DVD which – to suit both read-only and writable versions – is a subset of the UDF Revision 1.02 specification known as MicroUDF (M-UDF).

Because UDF wasn’t supported by Windows until Microsoft shipped Windows 98, DVD providers were forced to use an interim format called UDF Bridge. UDF Bridge is a hybrid of UDF and ISO 9660. Windows 95 OSR2 supports UDF Bridge, but earlier versions do not. As a result, to be compatible with Windows 95 versions previous to OSR2, DVD vendors had to provide UDF Bridge support along with their hardware.

DVD-ROM discs use the UDF Bridge format. (Note, Windows95 was not designed to read UDF but can read ISO 9660). The UDF Bridge specification does not explicitly include the Joliet extensions for ISO 9660, which are needed for long filenames. Most current Premastering tools do not include the Joliet extensions but it is expected that this feature will be added in due course. Windows98 does read UDF so these systems have no problem with either UDF or long filenames.

DVD-Video discs use only UDF with all required data specified by UDF and ISO 13346 to allow playing in computer systems. They do not use ISO 9660 at all. The DVD-Video files must be no larger than 1 GB in size and be recorded as a single extent (i.e. in one continuous sequence). The first directory on the disc must be the VIDEO_TS directory containing all the files, and all filenames must be in the 8.3 (filename.ext) format.

DVD-Audio discs use UDF only for storing data in a separate DVD-Audio zone on the disc, specified as the AUDIO_TS directory.