DVD Video

DVD video

DVD-Video discs require a DVD-drive with a MPEG-2 decoder (eg. a DVD-player or a DVD computer drive with a software DVD player). Commercial DVD movies are encoded using a combination of MPEG-2 compressed video and audio of varying formats (often multi-channel formats as described below). Typical data rates for DVD movies range from 3-10 Mbit/s, and the bitrate is usually adaptive. A high number of audio tracks and/or lots of extra material on the disk will usually result in a lower bitrate (and image quality) for the main feature.

The audio data on a DVD movie can be of the format PCM, DTS, MPEG audio, or Dolby Digital (AC-3). In countries using the NTSC standard any movie should contain a sound track in (at least) either PCM or Dolby AC-3 formats, and any NTSC player must support these two; all the others are optional. This ensures any standard compatible disc can be played on any standard compatible player. The vast majority of commercial NTSC releases today employ AC-3 audio.

Initially, in countries using the PAL standard (e.g. most of Europe) the sound of DVD was supposed to be standardized on PCM and MPEG-2 audio, but apparently against the wishes of Philips, under public pressure on December 5, 1997, the DVD Forum accepted the addition of Dolby AC-3 to the optional formats on discs and mandatory formats in players. The vast majority of commercial PAL releases employ AC-3 audio by now.

DVDs can contain more than one channel of audio to go together with the video content. In many cases, sound tracks in more than one language track are present (for example the film's original language as well as a dubbed track in the language of the country where the disc is being sold).

With several channels of audio from the DVD, the cabling needed to carry the signal to an amplifier or TV can occasionally be somewhat frustrating. Most systems include an optional digital connector for this task, which is then paired with a similar input on the amplifier. The selected audio signal is sent over the connection, typically over RCA jacks or TOSLINK, in its original format to be decoded by the audio equipment. When playing compact discs, the signal is sent in S/PDIF format instead.

Video is another issue which continues to present problems. Current players typically output analog video only, both composite video on an RCA jack, as well as S-Video in the standard connector. However neither of these connectors were intended to be used for progressive video, so yet another set of connectors has started to appear in the form of component video, which keeps the three components of the video, one luminance signal and two color difference signal, as stored on the DVD itself, on fully separate wires (whereas s-video uses two wires, uniting and degrading the two color signals, and composite only one, uniting and degrading all three signals). Additionally, the connectors are further confused by using a number of different physical connectors on different player models, RCA or BNC, as well as using VGA cables in a non-standard way (VGA is normally analog RGB, not component). Even worse, there are often two sets of component outputs, one carrying interlaced video, and the other progressive. In Europe and other PAL areas, SCART connectors are typically used, which carry both composite and analog RGB intelaced video signals, as well as analog 2-channel sound on a single multiwire cable, and which offer a reasonable compromise between video quality -- which is superior to S-Video though inferior to progressive component video -- and cost.

DVD Video may also include one or more subtitle tracks in various languages, including those made especially for the hearing impaired. They are stored as images with transparent background which are overlayed over the video during playback. Subtitles are restricted to four colors (including transparency) and thus tend to look cruder than permanent subtitles on film.

DVD Video may contain Chapters for easy navigation (and continuation of a partially watched film). If space permits, it is also possible to include several versions (called "angles") of certain scenes, though today this feature is mostly used -- if at all -- not to show different angles of the action, but as part of internationalization to e.g. show different language versions of images containing written text, if subtitles won't do.

A major selling point of DVD Video is that its storage capacity allows for a wide variety of extra features in addition to the feature film itself. This can include audio commentary that is timed to the film sequence, documentary features, unused footage, trivia text commentary, simple games and film shorts.

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