Frequently Asked Questions About DVD Discs

Please contact Media Sciences if your questions are not answered on these pages.

Is UDF required for all DVD discs?

No, it is not required. OSTA’s Universal Disc Format (UDF) for optical disc volume and file structure has many versions. UDF ver. 1.01, or 1.02, is mandatory for DVD-Video discs, and can also be used for DVD-ROM. ISO 9660, and its various extensions such as Joliet and El Torito, are perfectly acceptable for DVD data discs, and may be preferable, because readability is assured in almost all platforms. UDF requires a special driver that may not be present in some operating systems.

ISO 9660 has limitations, such as logical addressing that is not suited to file addition and deletion and a maximum directory depth of eight, that may be overcome by the use of UDF. Otherwise, little is to be gained by exclusive use of UDF, although hybrid discs can contain non-conflicting ISO 9660 and UDF volume and file structures.

A simple sequence mounts an ISO 9660 volume. First, the Volume Descriptors beginning in sector 16 are read, followed by the path tables and root directory.

UDF sequences are more complex. The UDF Volume Recognition Sequence (VRS) is in sector 16, or immediately following any ISO 9660 sequences. The VRS points to an Anchor Volume Descriptor Pointer (AVDP) that identifies a Volume Descriptor Sequence consisting of a Primary Volume Descriptor, Logical Volume Descriptor (LVD), Partition Descriptor, Unallocated Space Descriptor, and Implementation Use Descriptor. The LVD also provides the virtual address of a File Set Descriptor that provides the virtual address of the Root Directory Entry Information Control Block that either contains or points to File Identifier Descriptors. These provide indirect virtual addresses of File Entry Information Control Blocks containing virtual addresses for each file and directory. Virtual addresses are then translated into logical addresses by a Virtual Allocation Table. Needless to say, many things can go wrong in such a complex structure.

Multiple UDF versions can also be confusing. UDF version 2.01 added requirements for DVD-RW, DVD-R, and support for real time DVD incremental recording. UDF version 2.50 added similar requirements that support incremental recording for DVD+R, DVD+RW, and Mount Rainier formats.

Incremental recording to DVD recordable or rewritable discs may require specific UDF versions. Always confirm that the proper version has been applied.

ISO 9660 and its extensions are stable, and are recognized by almost all operating systems. Use it for DVD data discs to assure optimum probability for successful interchange