LaserDisc

DVD's immediate predecessor, the LaserDisc system stores high quality audio (analogue or digital) and video (analogue) on a 12" or 5" optical disc. Physically similar to an oversized CD, virtually every LaserDisc player manufactured since 1986 will also play conventional CDs.

One 12" LaserDisc is capable of storing approximately sixty minutes of video on each side of a CLV (Constant Linear Velocity) disc, or 30 minutes on each side of a CAV (Constant Angular Velocity) disc. Because of these playback limitations, movies over two hours in length are usually recorded on two or more discs. Many players include an automatic side change function, which allows up to two hours of continuous viewing (albeit with a slight pause of up to fifteen seconds as the side is changed). Both Dolby Digital and DTS were first used domestically on LaserDisc. Dolby Digital LaserDiscs require a LaserDisc player with an 'AC-3 RF Digital Out', while DTS discs may be played on any LaserDisc player with an S/PDIF digital output (TosLink or coaxial).

DVD

The same size as a CD, a DVD is capable of holding significantly more data (17.08 GB compared to ~650 MB on CD). DVD's features include: Dolby Digital, PCM, MPEG or DTS soundtracks, multiple aspect ratios (fullscreen and/or letterbox depending on the DVD), up to 8 language tracks, up to 32 subtitle languages, alternate camera angles, and parental lockout functions. Many discs also contain extra features such as production material and director's commentary. DVD is capable of approximately 500 lines of horizontal resolution, compared to VHS's 240, Super VHS's 400 and LaserDisc's 425.

As a side-note, DVD is often regarded as an abbreviation of 'Digital Video Disc' or 'Digital Versatile Disc'. Neither definition is correct, and the DVD Forum's official stance on the matter is that the letters do not stand for anything: 'DVD' is complete in and of itself. For those wondering, this was the result of a little infighting between forum camps. The original proposal was for the letters to stand for 'Digital Video Disc'. However, it was rightly pointed out that DVD's capabilities extend far beyond video and that the abbreviation didn't address this flexibility. 'Digital Versatile Disc' was eventually mooted as a more general purpose and inclusive term, but was regarded as so vague the term received only lukewarm support at best. Consequently, and undoubtedly after some pushing by those sick of the whole thing, the letters were officially decreed to stand for nothing: DVD. Monty Python would be proud.

For more be sure to read Jim Taylor's excellent DVD FAQ Page.

Dolby Pro Logic/Pro Logic II

Introduced in 1987 by Dolby Laboratories, Dolby Pro Logic is essentially the domestic version of Dolby's theatrical 'Dolby Stereo' system. Currently the defacto standard for two-channel home theatre systems, Pro Logic extracts four channels of information (left, centre, right and surround) from 'Dolby Surround' encoded stereo soundtracks on DVD, LaserDisc, VHS, CD, MTS etc.

Dolby Surround soundtracks processed by a Pro Logic decoder include full-range left, centre and right channels, and a limited range mono surround channel (100Hz to 7kHz). Due to the matrixing technique employed by the system, these channels are unable to simultaneously reproduce identical frequencies, and channels are forced to interact with one another to varying degrees. To overcome these limitations, fully discrete systems such as Dolby Digital and DTS Digital Surround were developed.

Dolby Pro logic 2, developed by Jim Fosgate and demonstrated in late 1998, offers enhanced channel steering and the reproduction of discrete full-range surround channels. Although a significant improvement over Dolby Pro Logic, it does not offer the absolute quality achievable with the major discrete digital formats.

Dolby Digital

Formerly known as 'AC-3' or 'Dolby Digital AC-3', Dolby Digital is a versatile, high-quality audio compression system. Dolby Digital allows five discrete full-range (20Hz to 20kHz) channels (left, centre, right, left surround, right surround), and a limited frequency (20Hz to 120Hz) LFE or Low Frequency Effects channel to fit into one quarter of the space previously needed to reproduce CD's two channels. Dolby Digital's first theatrical appearance was in the 1992 release Batman Returns; its first domestic appearance was on the 1995 LaserDisc release of Clear And Present Danger. Dolby Digital is one of DVD-Video's audio standards, and is used on virtually all DVDs.

DTS Digital Surround

Digital Theater Systems' DTS Digital Surround allows up to six discrete full-range (20Hz to 24kHz) channels and a limited frequency LFE or Low Frequency Effects channel. Although functionally similar to Dolby Digital, DTS relies on a fundamentally different data reduction scheme, and uses significantly less audio compression than Dolby Digital. Whether these differences are detectable is uncertain. DTS requires a datarate of 754, 1235 or 1509kbps, significantly more than Dolby Digital's common datarates or 384 and 448kbps. DTS first appeared on LaserDisc in 1997 with the release of Jurassic Park. DTS Digital Sound was first used in the 1993 release Jurassic Park, but is a different audio format than that used for the domestic DTS Digital Surround system.

THX (Tomlinson Holman's Experiment)

THX's domestic video and hardware certification programmes are designed to provide minimum production standards that must be met in order to achieve certification.

The hardware (speakers, subwoofers, amplifiers, processors, receivers and DVD players) programme, established in 1991, includes two primary certification ranges: THX Select and the recently introduced premium THX Ultra 2 (replacing THX Ultra). These standards specify minimum speaker performance, dispersion characteristics, power output and post-processing systems.

The THX video programme was created to ensure LaserDiscs were presented in their optimum state, beginning with the release of The Abyss: SE in 1993. Unfortunately, while THX's video certification programme enjoyed much success and respect when used on LaserDisc, its translation to DVD has been less successful. THX certification of a given DVD is no longer indicative of, nor does it assure, superior quality.

Component Video

A DVD's picture information is stored as separate picture elements: Luminance, or brightness (Y), and colour difference information (R-Y B-Y). Transmitting these picture elements separately eliminates artifacts that might occur were they stored together as a composite signal, and bypasses a monitor's comb filter and PAL/NTSC converter. Composite video signals must be separated into their separate picture components by the television, with variable and unpredictable results. Component video inputs/outputs use three conventional RCA/coaxial cables, and are currently the highest quality analogue video connection available between a DVD player and television. If your television or DVD player does not include component video inputs or outputs, S-Video cabling should be used instead, with composite RCA/RF as a last resort.




S-Video

S-Video cable carries luminance, or brightness (Y) and chroma, or color (C) information separately, rather than as a composite signal (in which all brightness and color information is blended together). When transmitted together, color and brightness information must be extracted from one another by the television, often resulting in picture artifacts such as 'dot crawl' and color bleeding. When used with a DVD player, S-Video cables provide a dramatic improvement in picture quality over composite RCA connections, and should be used if component video connection is not possible.




Rear Projection TV (RPTV)

Large televisions, ranging in size from 40-80 inches (diagonal screen size), RPTVs are ideally suited for those wanting a large picture without the hassle and expense of a front projection system. Higher quality RPTVs include 3-D digital comb filters, which are used to eliminate dot crawl and hanging dots. Some form of digital comb filter is highly recommended. If your RPTV is to be used primarily for movie watching, you may want to consider a Widescreen RPTV (see Anamorphic).




High Definition Television (HDTV)

The FCC describes HDTV as follows: "High Definition Television offers approximately twice the vertical and horizontal resolution of NTSC, which is a picture quality approaching 35 mm film and has a sound quality approaching that of a compact disc."

ATSC digital television specifications allow for numerous video variants, but generally only three standards are regarded as high definition: 720p, 1080i and 1080p. 720p draws non-interlaced 1.78 horizontal unit by 1 vertical unit (ie.16x9) frames with 720 vertical lines or resolution up to 30 times per second, while 1080i draws interlaced 1.78:1 frames with 1080 lines of vertical resolution up to 30 times per second. 1080p is allowed for under ATSC specifications, but has yet to be used broadly. All three variants offer significantly better image quality than conventional NTSC (480i) and PAL (576i) transmissions.




Anamorphic Enhancement

Anamorphically-enhanced DVD transfers currently offer the highest picture quality possible from the DVD format. Essentially, a conventional widescreen image is compressed horizontally when transferred to DVD, and uncompressed upon playback by a widescreen television or video projector. This process increases the image's vertical resolution by 33%, and nearly eliminates visible scan lines. On a standard 4:3 television, the film will be horizontally compressed, with actors and scenery appearing tall and thin. To compensate for this effect on 4:3 televisions, DVD players remove every fourth vertical line and place black bars above and below the resultant active image (this technique is called anamorphic downconversion). The result is an image that appears normal on standard televisions. Different players perform this task with varying degrees of success. Simply discarding every fourth line will result in a sharp image, but with obvious aliasing and video artifacts, while more sophisticated digital interpolation techniques eliminate these artifacts, but may produce an image that appears subjectively soft.




DVD-18

A DVD-18 DVD is a dual-sided, dual-layered DVD, permitting up to eight hours of high quality audio and video to fit on a single disc. A DVD-18 disc is functionally identical to two DVD-9 (single-sided, dual layer) RSDL discs glued back-to-back, but requires a much more delicate construction technique.

Conventional dual-layer DVDs store one layer of data on each side of the disc; the DVD player's laser simply changes focus at the end of the first side, and begins reading the second side (through the transparent glue used to bond the two layers together). Unlike DVD-9 discs, the two layers on each side of a DVD-18 are not located on opposite sides of the disc. The first layer of data is actually embedded within the disc's upper polycarbonate substrate (plastic surface layer), while the second layer is located where the first layer would normally be found on a conventional DVD-9 disc.

This process requires much more care when manufacturing than conventional DVDs, making DVD-18s more expensive to produce than conventional single-sided single-layer (DVD-5) or single-sided dual-layer (DVD-9) DVDs. The first DVD-18 released in North America was Artisan's The Stand.




RSDL

RSDL stands for 'Reverse Spiral Dual Layer'. An RSDL disc utilises two data layers stored on opposite sides of a DVD disc; both of which can be read from only one side of the disc. The DVD player reads the first layer (layer 0) from the centre of the disc out. When the laser approaches the outer edge of the disc, it adjusts focus onto the second layer, and begins to read the second layer (layer 1) from the disc's outer edge toward the centre of the disc. RSDL discs permit much more information to be stored on one 'side' of a DVD, allowing longer films to play uninterrupted, and higher video and audio bit-rates to be used. When a DVD player changes layer, there may be a slight pause as the player locates and buffers the beginning of the second layer. The first RSDL disc released in North America was Artisan's (then Live Entertainment, now acquired and subsumed by Lion's Gate) Terminator 2: Judgment Day.




Progressive Scan

Conventional interlaced television systems display one low quality video 'field' 50 to 60 times every second. These fields are shown so rapidly that the eye is fooled into believing it is viewing a high quality moving image, not a succession of low resolution still images. A progressive scan DVD player reconstructs or interpolates complete frames from a DVD, and does not output individual low quality fields. The result is a more natural, stable image, with fewer interlacing artefacts such as shimmer, flicker and 'combing'.