Frequently Asked Questions About DVD Discs

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

I hear of many different DVD recordable or rewritable discs. Are they all acronyms for a single format? Which one is best?

A confusing variety of DVD discs have been announced. Initially, two different read-only formats existed. Toshiba, Matsushita, and Times-Warner, backed by Hitachi and Samsung, promoted a double sided structure. Philips and Sony, supported by Mitsumi, NEC, Nokia, and Aiwa, fostered dual layer construction. These two formats were incompatible, and even their single sided, single layer discs were not interchangeable.

A compromise in 1995 created the DVD disc, offered today in DVD-Video, DVD-Audio, and DVD-ROM formats. Molded pits similar to those of CD-ROM discs contain data. Shorter pit and land lengths and smaller track spacings result in data capacities more than seven times those of CD-ROM discs. A red laser provides a narrower spot that can resolve the smaller data structures of DVD.

The DVD-ROM format has four standardized options. DVD-5 is a single sided, single layer structure having a capacity of 4.7 GB, while the single sided, dual layer structure having a capacity of 8.5 GB is known as DVD-9. The double sided, single layer structure having a capacity of 9.4 GB is referred to as DVD-10, and DVD-18 is a double sided, dual layer structure having a capacity of 17.0 GB. DVD-VIDEO and DVD-AUDIO have a DVD-ROM structure but add specialized application layers.

A variety of incompatible DVD rewritable and recordable structures have appeared since the 1995 DVD-ROM compromise that led to the DVD Forum. Toshiba, Pioneer, Matsushita, and Hitachi introduced a DVD-RAM format in 1997 using a rewriteable phase change layer. The format supports random access read and write operations. Single or double sided version 1 discs contained up to 2.6 GB of information on each side. Version 2 discs have a capacity of 4.7 GB on each side. Information is recorded into both land and groove areas. Pre-embossed header pits contain address information. These headers are pits molded in short regions between every 2K sector. Land and groove positions are switched once every revolution. Defect management is contained on the disc that has a claimed life of 100,000 overwrite cycles. DVD-RAM discs must be written in a DVD-RAM recorder, but cannot be read in many DVD drives.

Pioneer announced a DVD-R write-once format in 1997 capable of recording up to 3.95 GB per side into the dye layer of a single or double sided disc. Cyanine dye similar to that used in CD-R discs is modified, or "tuned," for the DVD 635/650 nm red laser. The resulting disc appears violet in color when viewed from the readout side. A 4.7 GB version 2 was introduced in 2000 with two incompatible formats. The General format requires a 650 nm recording laser, and is intended for computer applications. It cannot record copy-protected discs. The Authoring format requires a 635 nm recording laser, and is intended for mastering applications by professional DVD content producers and publishers. It accomodates the Cutting Master Format, but cannot record copy-protected discs. Information is recorded in a wobbled pre-groove. Unrecorded address information is contained in molded pits and lands, called pre-pits. DVD-R General discs must be written in a DVD-R General recorder. Likewise, DVD-R Authoring discs can only be written in a DVD Authoring recorder. DVD-R discs can be read in most DVD drives.

Pioneer also introduced a DVD-RW format in 2001 using a 4.7 GB single-sided, high reflectivity phase change recording layer that is rewriteable. The format supports sequential re-recording and streaming audio/video. It is intended for consumer audio/video applications, or as a scratch pad for content developers. Information is recorded in a wobbled pre-groove. Unrecorded address information is contained in molded pits and lands, called land pre-pits, that are in the areas between grooves. Defective sectors are not replaced. Pioneer claims a disc life of at least 1,000 overwrites. Discs must be written in a DVD-RW recorder, and can be read in most DVD drives.

A powerful consortium of Sony, Philips, Hewlett-Packard, Ricoh, Yamaha, and Mitsubishi announced a 3 GB recordable, rewriteable DVD+RW format in 1998 that was then withdrawn. A 4.7 GB format was introduced in 2001 using rewritable a phase change layer. Recording occurs in a wobbled pre-groove containing unrecorded address information. Use of a high wobble frequency supports lossless linking, enabling recording pauses caused by variable bit rate MPEG-2 compression to occur without introducing uncorrectable errors in linking blocks. Discs must be writted in DVD+RW Mount Rainier drives that support drag and drop recording, defect management, and background formatting. Compatible DVD+R discs are available for write-once use. DVD+RW and DVD+R discs are readable in most DVD drives.

Winners in the recordable and rewritable DVD race have not yet been determined. Each format has its own advantages and limitations. Hopefully, either the marketplace or a neutral body will moderate a compromise resulting in a single format that offers acceptable price and performance. Archival storage of 4.7 GB on a widely interchangeable disc that can be recorded, or rewritten, having access times of a few tenths of one second could have broad application as an alternative to the popular video tape recorder as well as to linear and helical scan tape.