DVD

DVD is a digital optical storage format.

In the mid-1990s, consumer electronics and computer companies merged the previously competing High-Definition Disc and Multimedia Disc proposals into a unified standard. The DVD specification was finalized in 1995, and the first DVD players and DVD-Video discs went on sale in Japan in 1996. Both “Digital Video Disc” and “Digital Versatile Disc” have been used to explain the name; the DVD Forum’s specifications typically use “DVD” directly as the format name and do not require the acronym to be expanded on products.

A standard DVD has a diameter of 120 millimeters and a thickness of 1.2 millimeters; there is also a smaller disc with a diameter of 80 millimeters. The disc is typically composed of two polycarbonate substrates approximately 0.6 millimeters thick that are bonded together; the data layer can be located on one or both sides, and each side can be configured as single-layer or dual-layer. The reading system uses a red laser with a wavelength of approximately 650 nanometers; compared to CDs, DVDs have shorter recording marks and narrower track spacing, and employ EFMPlus modulation with corresponding error-correction structures, thereby accommodating more data within the same diameter. A single-sided, single-layer disc has a nominal capacity of 4.7 GB and is known in the industry as a DVD-5; a single-sided, dual-layer disc has a capacity of approximately 8.5 GB and is known as a DVD-9. A double-sided, single-layer DVD-10 has a capacity of approximately 9.4 GB, while a double-sided, dual-layer DVD-18 has a capacity of approximately 17 GB. These figures are approximate names based on decimal capacity; when displayed in binary units by an operating system, the values will be smaller. The second layer of a dual-layer disc may use parallel track paths or opposite track paths; the latter allows the read head to continue reading in the opposite direction after switching layers, which is often used for long programs requiring continuous playback. A brief pause may occur while the player refocuses.

DVD is a family of media formats and does not refer exclusively to movie discs.

DVD-ROM is used for factory-pressed read-only data; DVD-Video specifies video programs, audio tracks, subtitles, and navigation structures; and DVD-Audio is designed for high-fidelity audio. DVD-R, DVD-RW, DVD+R, DVD+RW, and DVD-RAM use different write-once or rewritable technologies; dual-layer recordable discs are further divided into DVD-R DL and DVD+R DL. Although the physical form factor is consistent, there is no guarantee that early drives or players will be able to recognize all variants.

The primary video codec for DVD-Video is MPEG-2 Video, though several low-resolution MPEG-1 modes are also permitted. The full frame for 525/60 systems is typically 720×480, while for 625/50 systems it is typically 720×576, and may be labeled as 4:3 or 16:9. Widescreen programs are generally stored with non-square pixel distortion and are expanded to the display aspect ratio during playback. Content sourced from films may use progressive encoding with repeat field flags, or it may be converted to interlaced video during the authoring stage; therefore, a DVD’s scanning and motion processing methods cannot be determined solely by its resolution label. A typical DVD-Video uses a file system compatible with UDF and ISO 9660, with the program stored in the VIDEO_TS directory. VOB files multiplex video, audio, subtitles, and navigation data; IFO files store title, program chain, chapter, menu, and stream attributes; and BUP files serve as backups of key IFO information. Individual VOB files are subject to file system compatibility requirements; long programs are typically split into consecutively numbered files, but can still be combined into a single title during playback.

Audio formats may include linear PCM, Dolby Digital, DTS, or MPEG Audio; specific mandatory capabilities vary by region and television system standards. The disc can simultaneously contain multiple languages, commentary tracks, and different audio channel versions; video, audio, subtitles, and navigation data share the DVD-Video system bitrate and disc capacity. Subtitles are typically stored as bitmap images, rather than as plain text with fonts freely selectable by the player. The multi-angle feature is implemented by interleaving multiple video streams, which significantly increases data usage.

Commercial DVD-Video discs can be assigned region codes to divide discs and players into different sales regions; Region 0 or “All” typically indicates that the disc is not restricted to a single region. CSS is used to scramble pre-recorded video data, while mechanisms such as Macrovision were once used to prevent analog copying. Region codes and encryption belong to the access control layer and do not alter disc capacity, video standards, or encoding specifications; differences in standards such as PAL and NTSC are not equivalent to region codes.

Recordable DVDs gave rise to distinct industry ecosystems from the late 1990s through the 2000s. The DVD Forum promoted DVD-R, DVD-RW, and DVD-RAM, while the DVD+RW Alliance promoted DVD+R and DVD+RW. Later multi-format optical drives typically supported both types of recordable discs, but differences remained in writing strategies, lead-in sectors, defect handling, and sealing methods. DVD-RAM employs sector management more akin to random-access media, and some discs are housed in protective cases, resulting in different compatibility ranges compared to standard DVD-R/RW discs.