Once the video is formatted to full-frame or widescreen form, it's encoded and stored on DVD discs. DVD players have four playback modes, one for 4:3 video and three for 16:9 video:

Video stored in 4:3 format is not changed by the player. It will appear normally on a standard 4:3 display. Widescreen systems will either enlarge it or add black bars to the sides. 4:3 video may have been formatted with letterboxing or pan & scan before being transferred to DVD. All formatting done to the video prior to it being stored on the disc is transparent to the player. It merely reproduces it as a standard 4:3 TV picture. Video that is letterboxed before being encoded can be flagged so that the player will tell a widescreen TV to automatically expand the picture. Unfortunately, some discs (such as Fargo) do not flag the video properly. And worse, some players ignore the flags.

The beauty of anamorphosis is that less of the picture is wasted on letterbox mattes. DVD has a frame size designed for 1.33 display, so the video still has to be made to fit, but because it's only squeezed horizontally, 33% more pixels (25% of the total pixels in a video frame) are used to store active picture instead of black. Anamorphic video is best displayed on widescreen equipment, which stretches the video back out to its original width. Alternatively, many new 4:3 TV's can reduce the vertical scan area to restore the proper aspect ratio without losing resolution (an automatic trigger signal is sent to European TVs on SCART pin 8). Even though almost all computers have 4:3 monitors, they have higher resolution than TVs so they can display the full widescreen picture in a window (854x480 pixels or bigger for NTSC; 1024x576 or bigger for PAL).

Anamorphic video can be converted by the player for display on standard 4:3 TVs in letterbox or pan & scan form. If anamorphic video is shown unchanged on a standard 4:3 display, people will look tall and skinny as if they have been on a crash diet. The setup options of DVD players allow the viewer to indicate whether they have a 16:9 or 4:3 TV. In the case of a 4:3 TV, a second option lets the viewer indicate a preference for how the player will reformat anamorphic video. The two options are detailed below.

For automatic letterbox mode, the player generates black bars at the top and the bottom of the picture (60 lines each for NTSC, 72 for PAL). This leaves 3/4 of the height remaining, creating a shorter but wider rectangle (1.78:1). In order to fit this shorter rectangle, the anamorphic picture is squeezed vertically using a letterbox filter that combines every 4 lines into 3, reducing the vertical resolution from 480 scan lines to 360. (If the video was already letterboxed to fit the 1.78 aspect, then the mattes generated by the player will extend the mattes in the video.) The vertical squeezing exactly compensates for the original horizontal squeezing so that the movie is shown in its full width. Some players have better letterbox filters than others, using weighted averaging to combine lines (scaling 4 lines into 3 or merging the boundary lines) rather than simply dropping one out of every four lines. Widescreen video can be letterboxed to 4:3 on expensive studio equipment before it's stored on the disc, or it can be stored in anamorphic form and letterboxed to 4:3 in the player. If you compare the two, the letterbox mattes will be identical but the picture quality of the studio version may be slightly better. (See 1.38 for more about letterboxing.)

For automatic pan & scan mode, the anamorphic video is unsqueezed to 16:9 and the sides are cropped off so that a portion of the image is shown at full height on a 4:3 screen by following a center of interest offset that's encoded in the video stream according to the preferences of the people who transferred the film to video. The pan & scan "window" is 75% of the full width, which reduces the horizontal pixels from 720 to 540. The pan & scan window can only travel laterally. This does not duplicate a true pan & scan process in which the window can also travel up and down and zoom in and out. Auto pan & scan has three strikes against it: 1) it doesn't provide the same artistic control as studio pan & scan, 2) there is a loss of detail when the picture is scaled up, and 3) equipment for recording picture shift information is not widely available. Therefore, no anamorphic movies have been released with auto pan & scan enabled, although a few discs use the pan & scan feature in menus so that the same menu video can be used in both widescreen and 4:3 mode. In order to present a quality full-screen picture to the vast majority of TV viewers, yet still provide the best experience for widescreen owners, some DVD producers choose to put two versions on a single disc: 4:3 studio pan & scan and 16:9 anamorphic.

Playback of widescreen material can be restricted by the producer of the disc. Programs can be marked for the following display modes:
- 4:3 full frame
- 4:3 LB (for sending letterbox expand signal to widescreen TV)
- 16:9 LB only (player not allowed to pan & scan on 4:3 TV)
- 16:9 PS only (player not allowed to letterbox on 4:3 TV)
- 16:9 LB or PS (viewer can select pan & scan or letterbox on 4:3 TV)

You can usually tell if a disc contains anamorphic video if the packaging says "enhanced for 16:9 widescreen" or something similar. If all it says is "widescreen," it may be letterboxed to 4:3, not 16:9. The Laserviews Web site has a list of anamorphic DVD titles.

Additional explanations of how anamorphic video works can be found at Greg Lovern's What's an Anamorphic DVD? page, Bill Hunt's Ultimate Guide to Anamorphic Widescreen DVD, David Lockwood's What Shape Image?, and Dan Ramer's What the Heck Is Anamorphic?. There are excellent animated illustrations at DVD Web (requires Flash). More information can be found at the Anamorphic Widescreen Support Page. See 1.38 for further discussion of letterboxing.

Anamorphosis causes no problems with line doublers and other video scalers, which simply duplicate the scan lines before they are stretched out by the widescreen display.

For anamorphic video, the pixels are fatter. Different pixel aspect ratios (none of them square) are used for each aspect ratio and resolution. 720-pixel and 704-pixel sizes have the same aspect ratio because the first includes overscan. Note that "conventional" values of 1.0950 and 0.9157 are for height/width (and are tweaked to match scanning rates). The table below uses less-confusing width/height values (y/x * h/w).

      720x480   720x576

      704x480   704x576   352x480   352x576

4:3     0.909     1.091     1.818     2.182

16:9    1.212     1.455     2.424     2.909

[3.6] What are the audio details?

There are two home-entertainment flavors of DVD: DVD-Video and DVD-Audio. Each supports high-definition multichannel audio. DVD-Audio includes higher-quality PCM audio.

[3.6.1] Details of DVD-Audio and SACD

LPCM is mandatory in DVD-Audio discs, with up to 6 channels at sample rates of 48/96/192 kHz (also 44.1/88.2/176.4 kHz) and sample sizes of 16/20/24 bits. This allows theoretical frequency response of up to 96 kHz and dynamic range of up to 144 dB. Multichannel PCM is downmixable by the player, although at 192 and 176.4 kHz only two channels are available. Sampling rates and sizes can vary for different channels by using a predefined set of groups. The maximum data rate is 9.6 Mbps.

The WG4 decided to include lossless compression (it's about time!), and on August 5, 1998 approved Meridian's MLP (Meridian Lossless Packing) scheme, already licensed by Dolby. MLP removes redundancy from the signal to achieve a compression ratio of about 2:1 while allowing the PCM signal to be completely recreated by the MLP decoder (required in all DVD-Audio players). MLP allows playing times of about 74 to 135 minutes of 6-channel 96kHz/24-bit audio on a single layer (compared to 45 minutes without packing). Two-channel 192kHz/24-bit playing times are about 120 to 140 minutes (compared to 67 minutes without packing).

Other audio formats of DVD-Video (Dolby Digital, MPEG audio, and DTS, described below) are optional on DVD-Audio discs, although Dolby Digital is required for audio content that has associated video. A subset of DVD-Video features (no angles, no seamless branching, etc.) is allowed. It's expected that shortly after DVD-Audio players appear, new universal DVD players will also support all DVD-Audio features.

DVD-Audio includes specialized downmixing features for PCM channels. Unlike DVD-Video, where the decoder controls mixing from 6 channels down to 2, DVD-Audio includes coefficent tables to control mixdown and avoid volume buildup from channel aggregation. Up to 16 tables can be defined by each Audio Title Set (album), and each track can be identified with a table. Coefficients range from 0dB to 60dB. This feature goes by the horribly contrived name of SMART (system-managed audio resource technique). (Dolby Digital, supported in both DVD-Audio and DVD-Video, also includes downmixing information that can be set at encode time.)

DVD-Audio allows up to 99 still images per track (at typical compression levels, about 20 images fit into the 2 MB buffer in the player), with a set of limited transitions (cut in/out, fad in/out, dissolve, and wipe). On-screen displays can be used for synchronized lyrics and navigation menus. A special simplified navigation mode can be used on players without a video display.

Sony and Philips are promoting SACD, a competing DVD-based format using Direct Stream Digital (DSD) encoding with sampling rates of up to 100 kHz. DSD is based on the pulse-density modulation (PDM) technique that uses single bits to represent the incremental rise or fall of the audio waveform. This supposedly improves quality by removing the brick wall filters required for PCM encoding. It also makes downsampling more accurate and efficient. DSD provides frequency response from DC to over 100 kHz with a dynamic range of over 120 dB. DSD includes a lossless encoding technique that produces approximately 2:1 data reduction by predicting each sample and then run-length encoding the error signal. Maximum data rate is 2.8 Mbps.

SACD includes a physical watermarking feature. Pit signal processing (PSP) modulates the width of pits on the disc to store a digital watermark (data is stored in the pit length). The optical pickup must contain additional circuitry to read the PSP watermark, which is then compared to information on the disc to make sure it's legitimate. Because of the requirement for new watermarking circuitry, SACD discs are not playable in existing DVD-ROM drives.

SACD includes text and still graphics, but no video. Sony says the format is aimed at audiophiles and is not intended to replace the audio CD format. 

See 1.12 for more general info on DVD-Audio and SACD.

[3.6.2] Audio details of DVD-Video

The following details are for audio tracks on DVD-Video. Some DVD manufacturers such as Pioneer are developing audio-only players using the DVD-Video format. Some DVD-Video discs contain mostly audio with only video still frames.

A DVD-Video disc can have up to 8 audio tracks (streams). Each track can be in one of three formats:

Two additional optional formats are provided: DTS and SDDS. Both require external decoders and are not supported by all players.

The ".1" refers to a low-frequency effects (LFE) channel that connects to a subwoofer. This channel carries an emphasized bass audio signal.

Linear PCM is uncompressed (lossless) digital audio, the same format used on CDs and most studio masters. It can be sampled at 48 or 96 kHz with 16, 20, or 24 bits/sample. (Audio CD is limited to 44.1 kHz at 16 bits.) There can be from 1 to 8 channels. The maximum bit rate is 6.144 Mbps, which limits sample rates and bit sizes when there are 5 or more channels. It's generally felt that the 120 dB dynamic range of 20 bits combined with a frequency response of around 22,000 Hz from 48 kHz sampling is adequate for high-fidelity sound reproduction. However, additional bits and higher sampling rates are useful in audiophile applications, studio work, noise shaping, advanced digital processing, and three-dimensional sound field reproduction. DVD players are required to support all the variations of LPCM, but some of them may subsample 96 kHz down to 48 kHz, and some may not use all 20 or 24 bits. The signal provided on the digital output for external digital-to-analog converters may be limited to less than 96 kHz and less than 24 bits.

Dolby Digital is multi-channel digital audio, using lossy AC-3 coding technology from original PCM with a sample rate of 48 kHz at up to 24 bits. The bitrate is 64 kbps to 448 kbps, with 384 or 448 being the normal rate for 5.1 channels and 192 being the typical rate for stereo (with or without surround encoding). (Most Dolby Digital decoders support up to 640 kbps.) The channel combinations are (front/surround): 1/0, 1+1/0 (dual mono), 2/0, 3/0, 2/1, 3/1, 2/2, and 3/2. The LFE channel is optional with all 8 combinations. For details see ATSC document A/52 <www.atsc.org/document.html>. Dolby Digital is the format used for audio tracks on almost all DVDs.

MPEG audio is multi-channel digital audio, using lossy compression from original PCM format with sample rate of 48 kHz at 16 or 20 bits. Both MPEG-1 and MPEG-2 formats are supported. The variable bit rate is 32 kbps to 912 kbps, with 384 being the normal average rate. MPEG-1 is limited to 384 kbps. Channel combinations are (front/surround): 1/0, 2/0, 2/1, 2/2, 3/0, 3/1, 3/2, and 5/2. The LFE channel is optional with all combinations. The 7.1 channel format adds left-center and right-center channels, but will probably be rare for home use. MPEG-2 surround channels are in an extension stream matrixed onto the MPEG-1 stereo channels, which makes MPEG-2 audio backwards compatible with MPEG-1 hardware (an MPEG-1 system will only see the two stereo channels.) MPEG Layer III (MP3) and MPEG-2 AAC (aka NBC or unmatrix) are not supported by the DVD-Video standard.

DTS (Digital Theater Systems) Digital Surround is an optional multi-channel (5.1) digital audio format, using lossy compression from PCM at 48 kHz at up to 24 bits. The data rate is from 64 kbps to 1536 kbps, with typical rates of 768 and 1536 for 5.1 channels and 384 or 768 for 2 channels. (The DTS Coherent Acoustics format supports up to 4096 kbps variable data rate for lossless compression, but this isn't supported by DVD. DVD also does not allow sampling rates other than 48 kHz.). Channel combinations are (front/surround): 1/0, 2/0, 3/0, 2/1, 2/2, 3/2. The LFE channel is optional with all combinations. DTS ES support 6.1 channels in two ways: 1) a Dolby Surround EX compatible matrixed rear center channel, 2) a discrete 7th channel. DTS also has a 7.1-channel mode (8 discrete channels), but no DVDs have used it yet. The 7-channel and 8-channel modes require a new decoder. The DVD standard includes an audio stream format reserved for DTS, but many older players ignore it. The DTS format used on DVDs is different from the one used in theaters (Audio Processing Technology's apt-X, an ADPCM coder, not a psychoacoustic coder). All DVD players can play DTS audio CDs, since the standard PCM stream holds the DTS code. See 1.32 for general DTS information. For more info visit <www.dtstech.com> and read Adam Barratt's article for Movie Sound Page.

SDDS (Sony Dynamic Digital Sound) is an optional multi-channel (5.1 or 7.1) digital audio format, compressed from PCM at 48 kHz. The data rate can go up to 1280 kbps. SDDS is a theatrical film soundtrack format based on the ATRAC compression format that is also used by Minidisc. Sony has not announced any plans to support SDDS on DVD.

THX (Tomlinson Holman Experiment) is not an audio format. It's a certification and quality control program that applies to sound systems and acoustics in theaters, home equipment, and digital mastering processes. The LucasFilm THX Digital Mastering program uses a patented process to track video quality through the multiple video generations needed to make a final format disc or tape, setup of video monitors to ensure that the filmmaker is seeing a precise rendition of what is on tape before approval of the master, and other steps along the way. THX-certified "4.0" amplifiers enhance Dolby Pro Logic: crossover sends bass from front channels to subwoofer; re-equalization on front channels (compensates for high-frequency boost in theater mix designed for speakers behind the screen); timbre matching on rear channels; decorrelation of rear channels; bass curve that emphasizes low frequencies. THX-certified "5.1" amplifiers enhance Dolby Digital and improve on 4.0: rear speakers are now full range, so crossover sends bass from both front and rear to subwoofer; decorrelation is turned on automatically when rear channels have the same audio, but not during split-surround effects, which don't need to be decorrelated. More info at Home THX Program Overview.

Discs containing 525/60 video (NTSC) must use PCM or Dolby Digital on at least one track. Discs containing 625/50 video (PAL/SECAM) must use PCM or MPEG audio or Dolby Digital on at least one track. Additional tracks may be in any format. A few first-generation players, such as those made by Matsushita, can't output MPEG-2 audio to external decoders.

The original spec required either MPEG audio or PCM on 625/50 discs. There was a brief scuffle led by Philips when early discs came out with only two-channel MPEG and multichannel Dolby Digital, but the DVD Forum clarified in May 1997 that only stereo MPEG audio was mandatory for 625/50 discs. In December 1997 the lack of MPEG-2 encoders (and decoders) was a big enough problem that the spec was revised to allow Dolby Digital as the only audio track on 625/50 discs.

Because of the 4% speedup from 24 fps film to 25 fps PAL display, the audio must be adjusted to match. Unless the audio is digitally processed to shift the pitch back to normal it will be slightly high (about one half of a semitone).

For stereo output (analog or digital), all players have a built-in 2-channel Dolby Digital decoder that downmixes from 5.1 channels (if present on the disc) to Dolby Surround stereo (i.e., 5 channels are phase matrixed into 2 channels to be decoded to 4 by an external Dolby Pro Logic processor). PAL players also have an MPEG or MPEG-2 decoder. Both Dolby Digital and MPEG-2 support 2-channel Dolby Surround as the source in cases where the disc producer can't or doesn't want to remix the original onto discrete channels. This means that a DVD labeled as having Dolby Digital sound may only use the L/R channels for surround or "plain" stereo. Even movies with old monophonic soundtracks may use Dolby Digital -- but only 1 or 2 channels. Sony players can optionally downmix to non-surround stereo. If surround audio is important to you, you will hear significantly better results from multichannel discs if you have a Dolby Digital system.

The new Dolby Digital Surround EX (DD-SEX?) format, which adds a rear center channel, is compatible with DVD discs and players, and with existing Dolby Digital decoders. The new DTS Digital Surround ES (DTS-ES) format, which likewise adds a rear center channel, works fine with existing DTS decoders and with DTS-compatible DVD players. However, for full use of both new formats you need a new decoder to extract the rear center channel, which is phase matrixed into the two standard rear channels in the same way Dolby Surround is matrixed into standard stereo channels. Without a new decoder, you'll get the same 5.1-channel audio you get now. Because the additional rear channel isn't a full-bandwidth discrete channel, it's appropriate to call the new formats "5.2-channel" digital surround.

The Dolby Digital downmix process does not usually include the LFE channel and may compress the dynamic range in order to improve dialog audibility and keep the sound from becoming "muddy" on average home audio systems. This can result in reduced sound quality on high-end audio systems. The dynamic range compression (DRC) feature, often called midnight mode, reduces the difference between loud and soft sounds so that you can turn the volume down to avoid disturbing others yet still hear the detail of quiet passages. Some players have the option to turn off DRC. The downmix is auditioned when the disc is prepared, and if the result is not acceptable the audio may be tweaked or a separate L/R Dolby Surround track may be added. Experience has shown that minor tweaking is sometimes required to make the dialog more audible within the limited dynamic range of a home stereo system, but that a separate track is not usually necessary.

Dolby Digital also includes a feature called dialog normalization, which could more accurately be called volume standardization. DN is designed to keep the sound level the same when switching between different sources. This will become more important as additional Dolby Digital sources (digital satellite, DTV, etc) become common. Each Dolby Digital track contains loudness information so that the receiver can automatically adjust the volume, turning it down, for example, on a loud commercial. (Of course the commercial makers can cheat and set an artificially low DN level, causing your receiver to turn up the volume during the commercial!) Turning DN on or off on your receiver has no effect on dynamic range or sound quality, its effect is no different than turning the volume control up or down.

All five DVD-Video audio formats support karaoke mode, which has two channels for stereo (L and R) plus an optional guide melody channel (M) and two optional vocal channels (V1 and V2).

A DVD-5 with only one surround stereo audio stream (at 192 kbps) can hold over 55 hours of audio. A DVD-18 can hold over 200 hours.

Many people complain that the audio level from DVD players is too low. In truth the audio level is too high on everything else. Movie soundtracks are extremely dynamic, ranging from near silence to intense explosions. In order to support an increased dynamic range and hit peaks (near the 2V RMS limit) without distortion, the average sound volume must be lower. This is why the line level from DVD players is lower than from almost all other sources. So far, unlike on CDs and LDs, the level is much more consistent between discs. If the change in volume when switching between DVD and other audio sources is annoying, you can adjust the output signal level on some players, or the input signal level on some receivers, but other than that, there's not much you can do.

For more information about multichannel surround sound, see Bobby Owsinski's FAQ at <www.surroundassociates.com/fqmain.html>.

[3.6.3] Can you explain this Dolby Digital, Dolby Surround, Dolby Pro Logic, DTS stuff in English?

Almost every DVD contains audio in Dolby Digital format. DTS is an optional audio format that can be added to a disc in addition to Dolby Digital audio. Dolby Digital and DTS can store mono, stereo, and multichannel audio (usually 5.1 channels).  

Every DVD player in the world has an internal Dolby Digital decoder. The built-in 2-channel decoder turns Dolby Digital into standard analog stereo audio, which can be fed to almost any type of audio equipment (receiver, TV, boombox, etc.) using a pair of stereo audio cables. See 3.2 for more information.

There's a standard audio mixing technique, called Dolby Surround, that "piggybacks" a rear channel and a center channel onto a 2-channel signal. A Dolby Surround signal can be played on any stereo system (or even a mono system), in which case the rear- and center-channel sounds remain mixed in with the left and right channels. When a Dolby Surround signal is played on a multichannel audio system that knows how to handle it, the extra channels are extracted to feed center speakers and rear speakers. The old technique of decoding Dolby Surround, called simply Dolby Surround, extracts only the rear channel. The improved decoding technique, Dolby Pro Logic, also extracts the center channel. Then there's a brand new decoding technology, Dolby Pro Logic II, that extracts both the center channel and the rear channel and also processes the signals to create more of a 3D audio environment. Dolby Surround is independent of the storage or transmission format. In other words, a 2-channel Dolby Surround signal can be analog audio, broadcast TV audio, digital PCM audio, Dolby Digital, DTS, MP3, audio on a VHS tape, etc.

The built-in, 2-channel Dolby Digital decoder in every DVD player handles multichannel audio by downmixing it to two channels using Dolby Surround (see 3.6.2). This allows the analog stereo outputs to be connected to just about anything, including TVs and receivers with Dolby Pro Logic capability. Most DVD players also output the downmixed 2-channel Dolby Surround signal in digital PCM format, which can be connected to a digital audio receiver, most of which do Dolby Pro Logic.

Most DVD players also output the "raw" Dolby Digital signal for connection to a receiver with a built-in Dolby Digital decoder. Some DVD players have built-in multichannel decoders to provide 6 (or 7) analog audio outputs to feed a receiver or amplifier with multichannel analog inputs. See 3.1 for more info.

DTS is handled differently. Many DVD players have a DTS Digital Out feature (also called DTS pass-through), which sends the raw DTS signal to an external receiver with a DTS decoder. A few players have a built-in 2-channel DTS decoder that downmixes to Dolby Surround, just like a 2-channel Dolby Digital decoder. Some players have a built-in multichannel DTS decoder with 6 (or 7) analog outputs. Some DVD players don't recognize DTS tracks at all (see 1.32).

If you have a POS (plain old stereo), a Dolby Surround receiver, or a Dolby Pro Logic receiver, you don't need anything special in the DVD player. Any model will connect to your system. If you have a Dolby Digital receiver, then you need a player with Dolby Digital out (all but the cheapest players have this). If your receiver can also do DTS, you should get a player with DTS Digital Out. The only reason to get a player with 6-channel Dolby Digital or DTS decoder output is if you want use multichannel analog connections to receiver (see the component analog section of 3.2).

[3.7] How do the interactive features work?

DVD-Video players (and software DVD-Video navigators) support a command set that provides rudimentary interactivity. The main feature is menus, which are present on almost all discs to allow content selection and feature control. Each menu has a still-frame graphic and up to 36 highlightable, rectangular "buttons" (only 12 if widescreen, letterbox, and pan & scan modes are used). Remote control units have four arrow keys for selecting onscreen buttons, plus numeric keys, select key, menu key, and return key. Additional remote functions may include freeze, step, slow, fast, scan, next, previous, audio select, subtitle select, camera angle select, play mode select, search to program, search to part of title (chapter), search to time, and search to camera angle. Any of these features can be disabled by the producer of the disc.

Additional features of the command set include simple math (add, subtract, multiply, divide, modulo, random), bitwise and, bitwise or, bitwise xor, plus comparisons (equal, greater than, etc.), and register loading, moving, and swapping. There are 24 system registers for information such as language code, audio and subpicture settings, and parental level. There are 16 general registers for command use. A countdown timer is also provided. Commands can branch or jump to other commands. Commands can also control player settings, jump to different parts of the disc, and control presentation of audio, video, subpicture, camera angles, etc.

DVD-V content is broken into "titles" (movies or albums), and "parts of titles" (chapters or songs). Titles are made up of "cells" linked together by one or more "program chains" (PGC). A PGC can be one of three types: sequential play, random play (may repeat), or shuffle play (random order but no repeats). Individual cells may be used by more than one PGC, which is how parental management and seamless branching are accomplished: different PGCs define different sequences through mostly the same material.

Additional material for camera angles and seamless branching is interleaved together in small chunks. The player jumps from chunk to chunk, skipping over unused angles or branches, to stitch together the seamless video. Since angles are stored separately, they have no direct effect on the bitrate but they do affect the playing time. Adding 1 camera angle for a program roughly doubles the amount of space needed (and cuts the playing time in half). Examples of branching (seamless and non-seamless) include Kalifornia, Dark Star, and Stargate SE.

[3.8] What is the difference between interlaced and progressive video?

There are basically two ways to display video: interlaced scan or progressive scan. Progressive scan, used in computer monitors and digital television, displays all the horizontal lines of a picture at one time, as a single frame. Interlaced scan, used in standard television formats NTSC, PAL, and SECAM, displays only half of the horizontal lines at a time (the first field, containing the odd-numbered lines, is displayed, followed by the second field, containing the even-numbered lines). Interlacing relies on phosphor persistence of the TV tube to blend the fields together over time into a seemingly single picture. The advantage of interlaced video is that a high refresh rate (50 or 60 Hz) can be achieved with only half the bandwidth. The disadvantage is that the horizontal resolution is essentially cut in half, and the video is often filtered to avoid flicker (interfield twitter) and other artifacts.

It may help to understand the difference by considering how the source images are captured. A film camera shoots 24 frames per second, while a video camera alternately scans fields of odd and even lines in 1/60 of a second intervals. (Unlike projected film, which shows the entire frame in an instant, many progressive-scan displays trace a series of lines from top to bottom, but the end result is about the same.)

DVD is specifically designed to be displayed on interlaced-scan displays, which covers 99.9% of the more than one billion TVs worldwide. However, most DVD content comes from film, which is inherently progressive. To make film content work in interlaced form, the video from each film frame is split into two video fields —240 lines in one field, and 240 lines in the other— and encoded as separate fields in the MPEG-2 stream. A complication is that film runs at 24 frames per second, while TV runs at 30 frames (60 fields) per second for NTSC, or 25 frames (50 fields) per second for PAL and SECAM. For PAL/SECAM display, the simple solution is to show the film frames at 25 per second, which is a 4% speedup, and to speed up the audio to match. For NTSC display, the solution is to spread 24 frames across 60 fields by alternating the display of the first film frame for 2 video fields and the next film frame for 3 video fields. This is called 2-3 pulldown. The sequence works as shown below, where A-D represent film frames; A1, A2, B1, etc. represent the separation of each film frame into two video fields; and 1-5 represent the final video frames.

Film frames:   |  A  |   B    |  C  |   D    |

Video fields:  |A1 A2|B1 B2|B1 C2|C1 D2|D1 D2|

Video frames:  |  1  |  2  |  3  |  4  |  5  |

For MPEG-2 encoding, repeated fields (B1 and D2) are not actually stored twice. Instead, a flag is set to tell the decoder to repeat the field. (The apparently inverted order of  C2-C1 and D2-D1 are because of the requirement that top and bottom fields alternate. Since the fields are from the same film frame, the order doesn't matter.) MPEG-2 also has a flag to indicate when a frame is progressive (that the two fields come from the same instant in time). For film content, the progressive_frame flag should be true for every frame. See 3.4 for more MPEG-2 details.

As you can see, there are a couple of problems inherent in 2-3 pulldown: 1) some film frames are shown for a longer period of time than others, causing judder, or jerkiness, that shows up especially in smooth pans; and 2) if you freeze the video on the third or fourth video frame when there is motion in the picture you will see two separate images combined in a flickering mess. Most DVD players avoid the second problem by only pausing on coherent frames or by only showing one field, although some allow you to freeze on flicker-frames. (This is what the frame/field still option in the player's setup menu refers to.)

Most DVD players are hooked up to interlaced TVs, so there's not much that can be done about artifacts from film conversion. However, see 1.40 for information about progressive DVD players.

For more on progressive video and DVD, see part 5 and player ratings in the excellent DVD Benchmark series at Secrets of Home Theater and High Fidelity.

[3.9] What is edge enhancement?

When films are transferred to video in preparation for DVD encoding, they are commonly run through digital processes that attempt to clean up the picture. These processes include noise reduction (DVNR) and image enhancement. Enhancement increases contrast (similar to the effect of the "sharpen" or "unsharp mask" filters in PhotoShop), but can tend to overdo areas of transition between light and dark or different colors, causing a "chiseled" look or a ringing effect like the haloes you see around streetlights when driving in the rain.

Video noise reduction is a good thing, when done well, since it can remove scratches, spots, and other defects from the original film. Enhancement, which is rarely done well, is a bad thing. The video may look sharper and clearer to the casual observer, but fine tonal details of the original picture are altered and lost.

Note that ringing can also be caused by the player and by the TV. Scan velocity modulation (SVM), for example, causes ringing.

[3.10] Does DVD work with barcodes?

If your humble FAQ author and other long-time developers of laserdisc had prevailed, all DVD players would support barcodes. This would have made for really cool printed supplements and educational discs. But the rejection of our recommendations after an all-star meeting in August 1995 is another story for another day.

So the answer is "mostly no." A few industrial players, the Pioneer LD-V7200, Pioneer LD-V7400, and Philips ProDVD-170 support barcodes, including compatibility with the LaserBarCode standard. The DVD must be authored with one_sequential_PGC titles in order for timecode search to work. More info can be found in the Pioneer technical manuals.

[3.11] What is BCA?

BCA stands for burst cutting area, a zone near the hub of a DVD reserved for a barcode that can be etched into the disc by a YAG laser. Since barcode cutting is independent of the stamping process, each disc can have unique data recorded on it, such as a serialized ID. DVD readers can use the laser pickup head to read the BCA.

The BCA is used by CPRM (see 1.11) and Divx (see 2.10) to uniquely identify each disc.  


[4] DVD and computers

[4.1] Can I play DVD movies on my computer?

Yes, if your computer has the right stuff. The computer operating system or playback software must support regional codes and be licensed to descramble copy-protected movies. If the computer has TV video out, it must support Macrovision in order to play copy-protected movies. You may also need software that can read the UDF file system format used by DVDs. You don't need special drivers for Windows, since the existing CD-ROM drivers work fine with DVD-ROM drives. In addition to a DVD-ROM drive you must have extra hardware to decode MPEG-2 video and Dolby Digital or MPEG-2 audio, or your computer must be fast enough to handle software decoding. Good-quality software-only playback requires a 350-MHz Pentium II or a Mac G4. Less than 10% of new computers with DVD-ROM drives include decoder hardware, since software decoding is now possible on even the cheapest new models. Hardware upgrade kits can be purchased for existing computers (usually minimum 133 MHz Pentium or G3), starting at $150. See <www.brouhaha.com/~eric/video/dvd> for a list of drives and upgrade kits.

Mac OS X 1.0 (Cheetah) has no support for DVD playback, and also does not support Apple's DVD authoring applications (iDVD and DVD Studio Pro). Support for DVD playback is planned for a followup release in summer or fall. (More info at CNET.)

If you're having problems playing movies on your computer, see section 4.6.

Certain MPEG decoding tasks such as motion compensation, IDCT (inverse discrete cosine transform), IVLC (inverse variable length coding), and even subpicture decoding can be performed by additional circuitry on a video graphics chip, improving the performance of software decoders. This is called hardware decode acceleration, hardware motion comp, or hardware assist. Some card markers also call it hardware decode, even though they don't do all the decoding in hardware. All modern graphics cards also provide hardware colorspace conversion (YCbCr to RGB) and videoport overlay (some graphics card makers make a big deal about this even though all their competitors' cards have the same feature).

Microsoft Windows 98 and Windows 2000 includes DirectShow, which provides standardized support for DVD-Video and MPEG-2 playback. DirectShow can also be installed in Windows 95 (it's available for download). DirectShow creates a framework for DVD applications, but a third-party hardware or software decoder is required (see below). Windows NT 4.0 supports DVD-ROM drives for data, but has very little support for playing DVD-Video discs. Margi DVD-To-Go, Sigma Designs Hollywood Plus, and the related Creative Labs Dxr3 are among the few hardware decoders that work in NT 4.0. InterVideo WinDVD software works in NT 4.0 (Mediamatics DVD Express and MGI SoftDVD Max also work in NT 4.0, but they aren't available retail.) Windows 2000 uses essentially the same WDM drivers and DirectShow software as Windows 98, so it fully supports movie playback and DVD applications, including WebDVD. Internet Explorer 5.0 includes version 6.1 of Windows Media Player that enables scriptable DVD playback in an HTML page. Windows 98 and newer can read UDF discs. Adaptec provides a free filesystem driver, UDF Reader, for Windows 95/98/NT. Software Architects sells Read DVD for Windows 95.

Apple QuickTime 4 is partially ready for DVD-Video and MPEG-2 but does not yet have full decoding or DVD-Video playback support in place. Mac OS 8.1 or newer can read UDF discs. Adaptec provides a free utility, UDF Volume Access, that enables Mac OS 7.6 and newer to read UDF discs. Software Architects sells UDF reading software for Mac OS called DVD-RAM TuneUp. Intech's CD/DVD SpeedTools software allows most any DVD drive to be used with a Mac.

Note: The QuickTime MPEG Extension for Mac OS is for MPEG-1 only and does not play MPEG-2 DVD-Video.

Some DVD-ROMs and a few DVD-Videos use video encoded using MPEG-1 instead of MPEG-2. Most recent computers have MPEG-1 hardware built in or are able to decode MPEG-1 with software.

DVD player applications (using either software or hardware decoding) are virtual DVD players. They support most DVD-Video features (menus, subpictures, etc.) and emulate the functionality of a DVD-Video player remote control. Many player applications include additional features such as bookmarks, chapter lists, and subtitle language lists.

Software decoders and DVD player applications for Microsoft Windows PCs:

Software decoders need at least a 350 MHz Pentium II and a DVD-ROM drive with bus mastering DMA to play without dropped frames. Anything slower than a 400 MHz Pentium III will benefit quite a bit from hardware decode acceleration in the graphics card. An AGP graphics card (rather than PCI) also improves the performance of software decoders.

Hardware decoder cards and DVD-ROM upgrade kits for Microsoft Windows PCs:

All but the Sigma Designs decoder (including Creative Dxr3) have WDM drivers for DirectShow. The Sigma Designs decoder card is used in hardware upgrade kits from Hitachi, HiVal, Panasonic, Phillips, Sony, Toshiba, and VideoLogic. The advantage of hardware decoders is that they don't eat up CPU processing power, and they often produce better quality video than software decoders. The Chromatic Mpact2 chip does 3-field analysis to produce exceptional progressive-scan video from DVDs (unfortunately, Chromatic was bought by ATI and the chip is no longer supported, although some of the technology is now in ATI's Radeon). Hardware decoders use video overlay to insert the video into the computer display. Some use analog overlay, which takes the analog VGA signal output from the graphics card and keys in the video, while others use video port extension (VPE), a direct digital connection to the graphics adapter via a cable inside the computer. Analog overlay may degrade the quality of the VGA signal. See 4.4 for more overlay info.

Many Macintosh models come standard with DVD-ROM, DVD-RAM, or DVD-RW drives. The included Apple software DVD player uses hardware acceleration in the ATI graphics card. The still-unreleased QuickTime MPEG-2 decoder may use the Velocity Engine (AltiVec) portion of the PowerPC (G4) chip for video and audio decoding. Unfortunately, there are problems with Apple's software player. Apple released five updates in the first four months. Check MacFixit and MacInTouch for the latest info. DVD-ROM upgrade kits and decoder cards for Macintoshes are made by E4 (Elecede) (Cool DVD, C-Cube chip) [E4 has gone out of business], EZQuest (BOA Mac DVD), Fantom Drives (DVD Home Theater kit: DVD-ROM or DVD-RAM drive with Wired MPEG-2 card), and Wired (Wired 4DVD, Sigma EM8300 chip [same card as Hollywood plus]; MasonX [can't play encrypted movies]; DVD-To-Go [out of production]; Wired has been acquired by Media100). There's a beta version of a shareware DVD software player that can play unencrypted movies.

The Sigma Designs NetStream 2000 DVD decoder card will support Linux DVD playback. InterVideo and CyberLink have also announced commercial DVD players for Linux. In addition, there are free software players for Linux, Unix, BeOS, and other operating systems: MPlayer, OMS (LiViD), VideoLan, and Xine.

Computers have the potential to produce better video than settop DVD-Video players by using progressive display and higher scan rates, but many current systems don't look as good as a home player hooked up to a quality TV

If you want to hook a DVD computer to a TV, the decoder card or the VGA card must have a TV output (composite video or s-video). Video quality is much better with s-video. Alternatively, you can connect a scan converter to the VGA output. Scan converters are available from ADS Technologies, AITech, Antec, AverLogic, AVerMedia, Communications Specialties, Digital Vision, Focus Enhancements, Key Digital Systems, RGB Products, and others. Make sure the scan converter can handle the display resolution you have chosen: 640x480, 800x600, etc., although keep in mind that even 800x600 is beyond the ability of a standard TV, so higher resolutions won't make the TV picture better. 

The quality of video from a PC depends on the decoder, the graphics card, the TV encoder chip, and other factors, but will usually be a little inferior to a good consumer DVD player. The RGB output of the VGA card in computers is at a different frequency than standard component RGB video, so it can't be directly connected to most RGB video monitors. If the decoder card or the sound card has Dolby Digital or DTS output, you can connect to your A/V receiver to get multichannel audio.

A DVD PC connected to a progressive-scan monitor or video projector, instead of a standard TV, usually looks much better than a consumer player. See 2.9. Also see the Home Theater Computers forum at AVS.

For remote control of DVD playback on your PC, check out Animax Anir Multimedia Magic, Evation IRMan, InterAct WebRemote, Multimedia Studio Miro MediaRemote, Packard Bell RemoteMedia, RealMagic Remote Control, and X10 MouseRemote. Many remotes are supported by Visual Domain's Remote Selector software.

[4.2] What are the features and speeds of DVD-ROM drives?

Unlike CD-ROM drives, which took years to move up to 2x, 3x, and faster spin rates, faster DVD-ROM drives began appearing in the first year. 1x DVD-ROM drives provide a data transfer rate of 1.321 MB/s (11.08*10^6/8/2^20) with burst transfer rates of up to 12 MB/s or higher. The data transfer rate from a DVD-ROM disc at 1x speed is roughly equivalent to a 9x CD-ROM drive (1x CD-ROM data transfer rate is 150 KB/s, or 0.146 MB/s). DVD physical spin rate is about 3 times faster than CD (that is, 1x DVD spin ~ 3x CD spin), but most DVD-ROM drives increase motor speed when reading CD-ROMs, achieving 12x or faster performance.  A drive listed as "16x/40x" spins a DVD at 16 times normal, or a CD at 40 times normal. DVD-ROM drives are available in 2x, 4x, 4.8x, 5x, 6x, 8x, 10x, and 16x speeds, although they usually don't achieve sustained transfer at their full rating. The "max" in DVD and CD speed ratings means that the listed speed only applies when reading data at the outer edge of the disc, which moves faster. The average data rate is lower than the max rate. Most 1x DVD-ROM drives have a seek time of 85-200 ms and access time of 90-250 ms.

DVD drive speed Data rate Equivalent CD rate Actual CD speed
1x 11.08 Mbps (1.32 MB/s) 9x 8x-18x
2x 22.16 Mbps (2.64 MB/s) 18x 20x-24x
4x 44.32 Mbps (5.28 MB/s) 36x 24x-32x
5x 55.40 Mbps (6.60 MB/s) 45x 24x-32x
6x 66.48 Mbps (7.93 MB/s) 54x 24x-32x
8x 88.64 Mbps (10.57 MB/s) 72x 32x-40x
10x 110.80 Mbps (13.21 MB/s) 90x 32x-40x
16x 177.28 Mbps (21.13 MB/s) 144x 32x-40x

The bigger the cache (memory buffer) in a DVD-ROM drive, the faster it can supply data to the computer. This is useful primarily for data, not video. It may reduce or eliminate the pause during layer changes, but has no effect on video quality.

Rewritable DVD drives (see 4.3) write at about half their advertised speed when the data verification feature is turned on, which reads each block of data after it is written. Verification is usually on by default. Turning it off will speed up writing. Whether this endangers your data is a subject of debate.

In order to maintain constant linear density, typical CD-ROM and DVD-ROM drives spin the disc more slowly when reading near the outside where there is more physical surface in each track. (This is CLV, constant linear velocity.) Some faster drives keep the rotational speed constant and use a buffer to deal with the differences in data readout speed. (This is CAV, constant angular velocity.) In CAV drives, the data is read fastest at the outside of the disc, which is why specifications often list "max speed."

Note: When playing movies, a fast DVD-ROM drive gains you nothing more than possibly smoother scanning and faster searching. Speeds above 1x do not improve video quality from DVD-Video discs. Higher speeds only make a difference when reading computer data, such as when playing a multimedia game or when using a database.

Connectivity is similar to that of CD-ROM drives: EIDE (ATAPI), SCSI-2, etc. All DVD-ROM drives have audio connections for playing audio CDs. No DVD-ROM drives have been announced with DVD audio or video outputs (which would require internal audio/video decoding hardware). In order to hook a DVD-ROM PC to a television and a stereo receiver, the decoder card or the video card must have a TV video output and an audio output. Some cards have SP/DIF outputs to connect to digital audio receivers. If there's no video output, a TV scan converter can be connected to the VGA output.

Almost all DVD-Video and DVD-ROM discs use the UDF bridge format, which is a combination of the DVD MicroUDF (subset of UDF 1.02) and ISO 9660 file systems. The OSTA UDF file system will eventually replace the ISO 9660 system originally designed for CD-ROMs, but the bridge format provides backwards compatibility until more operating systems support UDF.

[4.3] What about recordable DVD: DVD-R, DVD-RAM, DVD-RW, DVD+RW, and DVD+R?

There are six recordable versions of DVD-ROM: DVD-R for General, DVD-R for Authoring, DVD-RAM, DVD-RW, DVD+RW, and DVD+R. All recordable drives can read DVD-ROM discs, but each uses a different type of disc for recording. DVD-R and DVD+R can record data once (sequentially only), while DVD-RAM, DVD-RW, and DVD+RW can be rewritten thousands of times. DVD-R was first available in fall 1997. DVD-RAM followed in summer 1998. DVD-RW came out in Japan in December 1999, but won't be available elsewhere until mid or late 2001. DVD+RW will be available in late 2001 or early 2002. DVD+R will be available in mid 2002.

Recordable DVD was first available for use on computers only (see 1.14). Home DVD video recorders began to be released worldwide near the end of 2000. Note that DVD video recorders can't copy encrypted DVD movie discs.

DVD-RAM is more of a removable storage device for computers than a video recording format. The other two erasable format families (DVD-R/RW and DVD+R/RW) are essentially in competition with each other. The market will determine which of them succeeds, or if they end up coexisting or merging.

A big problem is that none of the writable formats are fully compatible with each other or even with existing drives and players. As time goes by they will become more compatible and more intermixed. For example, home video recorders from Pioneer combine DVD-RW and DVD-R, while DVD-R/RW drives from Pioneer can also write to CD-R and CD-RW discs. New DVD-RAM drives also write to DVD-R discs as well as CD-R/RW. Toshiba, Panasonic, and others released combination DVD-ROM/CD-RW drives near the end of 1999. There are lists at DVDMadeEasy, YesVideo.com, HomeMovie.com, and Apple that indicate player compatibility with DVD-R discs. (Note: test results vary depending on media quality, handling, writing conditions, player tolerances, and so on. The indications of compatibility in these lists are often anecdotal in nature and are only general guidelines.)

A summary of compatibility:
DVD unit DVD-R(G) unit DVD-R(A) unit DVD-RW unit DVD-RAM unit DVD+RW unit
DVD-ROM disc reads reads reads reads reads reads
DVD-R(G) disc usually reads reads, writes reads, doesn't write reads, usually writes reads reads
DVD-R(A) disc usually reads reads, doesn't write reads, writes reads, doesn't write reads reads
DVD-RW disc usually reads reads reads reads, writes usually reads usually reads
DVD-RAM disc rarely reads doesn't read doesn't read doesn't read reads, writes doesn't read
DVD+RW disc usually reads usually reads usually reads usually reads usually reads reads, writes
DVD+R disc usually reads usually reads usually reads usually reads usually reads reads, writes

The DVD Forum has developed a DVD Multi logo that guarantees compatibility with DVD-R, DVD-RW, and DVD-RAM. A DVD Multi player can read all three formats, and a DVD Multi recorder can record using all three formats.  

Each writable DVD format is covered briefly below. See section 6.2.3 for hardware manufacturers. For more on writable DVD see Dana Parker's article at <www.emediapro.net/EM1999/parker1.html>. If you're interested in writable DVD for data storage, visit Steve Rothman's DVD-DATA page for FAQ and mailing list info.

DVD-R

DVD-R uses organic dye technology, like CD-R, and is compatible with most DVD drives and players. First-generation capacity was 3.95 billion bytes, later extended to 4.7 billion bytes. Matching the 4.7G capacity of DVD-ROM was crucial for desktop DVD production. In early 2000 the format was split into an "authoring" version and a "general" version. The general version uses a 650-nm laser (instead of 635-nm) so that DVD-RAM drives can use the same laser to write DVD-R discs. DVD-R(G) is intended for home use, while DVD-R(A) is intended for professional development. DVD-R(A) media is not writable in DVD-R(G) recorders, and vice-versa, but both kinds are readable in most DVD players and drives. The main differences, in addition to recording wavelength, are that DVD-R(G) uses decrementing pre-pit addresses, a pre-stamped (version 1.0) or pre-recorded (version 1.1) control area, CPRM (see 1.11), and allows double-sided discs. A third version for "special authoring" may be developed, allowing protected movie content to be recorded on DVD-R media.

Pioneer released 3.95G DVD-R(A) 1.0 drives in October 1997 (about 6 months late) for $17,000. New 4.7G DVD-R(A) 1.9 drives appeared in limited quantities in May 1999 (about 6 months late) for $5,400. Version 2.0 drives became available in fall 2000. Version 1.9 drives can be upgraded to 2.0 via downloaded software. (This removes the 2,500 hour recording limit.) New 2.0 [4.7G] media (with newer copy protection features), can only be written in 2.0 drives. 1.9 media (and old 1.0 [3.95G] media) can still be written in 2.0 drives. 

Pioneer's DVR-A03 DVD-R(G) drive was released in May 2001 for under $1000. It also writes CD-R/RW. It's the same drive (model DVR-103) that comes in certain Apple Macs and Compaq PCs. Panasonic announced a combination DVD-R(G)/DVD-RAM drive in June 2001 for under $600. VivaStar, Sharp, and others plan to release DVD-R(G) recorders in 2001.

Price for blank DVD-R(A) discs is $10 to $35, although cheaper discs seem to have more compatibility problems. Blank media is made by Eastman Kodak, Fuji, Hitachi Maxell, Mitsubishi, Mitsui, Pioneer, Ricoh, Ritek, TDK, Verbatim, and Victor. Price for blank DVD-R(G) discs is $10 to $25.

The DVD-R 1.0 format is standardized in ECMA-279. Andy Parsons at Pioneer has written a white paper that explains the differences between DVD-R(G) and DVD-R(A).

It's possible to submit DVD-R(A) and DVD-R(G) discs for replication, with limitations. First, not all replicators will accept submissions on DVD-R. Second, there can be problems with compatibility and data loss when using DVD-R, so it's best to generate a checksum that the replicator can verify. Third, DVD-R does not directly support CSS, regions, and Macrovision. Support for this is being added to DVD-R(A) with the new cutting master format (CMF), which stores DDP information in the control area, but it will take a while before most authoring software and replicators support CMF.

DVD-RW

DVD-RW (formerly DVD-R/W and also briefly known as DVD-ER) is a phase-change erasable format. Developed by Pioneer based on DVD-R, using similar track pitch, mark length, and rotation control, DVD-RW is playable in most DVD drives and players. (Some drives and players are confused by DVD-RW media's lower reflectivity into thinking it's a dual-layer disc. In other cases the drive or player doesn't recognize the disc format code and doesn't even try to read the disc. Simple firmware upgrades can solve both problems.) DVD-RW uses groove recording with address info on land areas for synchronization at write time (land data is ignored during reading). Capacity is 4.7 billion bytes. DVD-RW discs can be rewritten about 1,000 times.

In December 1999, Pioneer released DVD-RW home video recorders in Japan. The units cost 250,000 yen (about $2,500) and blank discs cost 3,000 yen (about $30). Since the recorder uses the new DVD-VR (video recording) format, the discs won't play in existing players (the discs are physically compatible, but not logically compatible). Recording time varies from 1 hour to 6 hours, depending on quality. A new version of the recorder was later released that also records on DVD-R(G) discs and can use DVD-Video format for better compatibility with existing players. The new model may be released in the U.S. and elsewhere around the end of 2001. Sharp announced a $2,200 DVD-RW recorder, and Zenith (LG) announced a $2,000 DVD-RW recorder, but neither appeared at the end of 2000 as expected. Sharp expects to have a DVD-RW video recorder that costs less than $1000 by March 2002.

New DVD-R(G) drives released in early 2001 by Pioneer (DVR-103 and DVR-A03, priced below $1000) are combination DVD-R/RW drives. Standalone drives write DVD-RW discs, but some drives that ship in PCs (such as the Apple SuperDrive G4 and early Compaqs) require a firmware upgrade before they will write DVD-RW discs. The drives also write CD-R and CD-RW discs.

Initial DVD-RW disc prices are around $25. Blank media is being made by Eastman Kodak, Hitachi Maxell, Mitsubishi, Mitsui, Pioneer, Ricoh, Ritek, TDK, Verbatim, and Victor.

DVD-RAM

DVD-RAM, with an initial storage capacity of 2.58 billion bytes, later increased to 4.7, uses phase-change dual (PD) technology with some MO features mixed in. DVD-RAM is the best suited of the writable DVD formats for use in computers, because of its defect management and zoned CLV format for rapid access. It is not compatible with most drives and players (because of defect management, reflectivity differences, and minor format differences). A wobbled groove is used to provide clocking data, with marks written in both the groove and the land between grooves. The grooves and pre-embossed sector headers are molded into the disc during manufacturing. Single-sided DVD-RAM discs come with or without cartridges. There are two types of cartridges: type 1 is sealed, type 2 allows the disc to be removed. Discs can only be written while in the cartridge. Double-sided DVD-RAM discs were initially available in sealed cartridges only, but now come in removable versions as well. Cartridge dimensions are 124.6 mm x 135.5 mm x 8.0 mm. DVD-RAM can be rewritten more than 100,000 times, and the discs are expected to last at least at least 30 years.

DVD-RAM 1.0 drives appeared in June 1998 (about 6 months late) for $500 to $800, with blank discs at about $30 for single-sided and $45 for double-sided. Disc prices were under $20 by August 1998, and retail drive prices were under $250 by November 1999. The first DVD-ROM drive to read DVD-RAM discs was released by Panasonic in 1999 (SR-8583, 5x DVD-ROM, 32x CD). Hitachi's GD-5000 drive, released in late 1999, also reads DVD-RAM discs. Blank DVD-RAM media is manufactured by Hitachi Maxell, Eastman Kodak, Mitsubishi, Mitsui, Ritek, and TDK.

The spec for DVD-RAM version 2.0, with a capacity of 4.7 billion bytes per side, was published in October 1999. The first drives appeared in June 2000 at about the same price as DVD-RAM 1.0 drives. Single-sided discs were priced around $20, and double-sided discs were around $30. DVD-RAM 2.0 also specifies 8-cm discs and cartridges for portable uses such as digital camcorders. Future DVD-RAM discs may use a contrast enhancement layer and a thermal buffer layer to achieve higher density.

Samsung and C-Cube made a technology demonstration (not a product announcement) in October 1999 of a DVD-RAM video recorder using the new DVD-VR format (see DVD-RW section above for more about DVD-VR). Panasonic demonstrated a $3,000 DVD-RAM video recorder at CES in January 2000. It appeared in the U.S. in September for $4,000 (model DMR-E10). At the beginning of 2001, Hitachi and Panasonic released DVD camcorders that use small DVD-RAM discs. The instant access and on-the-fly editing and deleting capabilities of the DVD camcorders are impressive.

The DVD-RAM 1.0 format is standardized in ECMA-272 and ECMA-273.

DVD+RW and DVD+R

DVD+RW is an erasable format based on CD-RW technology. It will become available in late 2001. DVD+RW is supported by Philips, Sony, Hewlett-Packard, Ricoh, and others. It is not supported by the DVD Forum (even though most of the DVD+RW companies are members), but the Forum has no power to set standards. DVD+RW drives read DVD-ROMs and CDs, and usually read DVD-Rs and DVD-RWs, but do not read or write DVD-RAM discs. DVD+RW drives also write CD-Rs and CD-RWs. DVD+RW discs, which hold 4.7 billion bytes per side, should be readable in about 70 to 80 percent of the existing DVD-Video players and DVD-ROM drives. (They run into the same reflectivity and disc format recognition problems as DVD-RW.)

DVD+RW backers claimed in 1997 that the format would be used only for computer data, not home video, but this was apparently a smokescreen intended to placate the DVD Forum and competitors. The original 1.0 format, which held 3 billion bytes (2.8 gigabytes) per side and was not compatible with any existing players and drives, was abandoned in late 1999.

The DVD+RW format uses phase-change media with a high-frequency wobbled groove that allows it to eliminate linking sectors. This, plus the option of no defect management, allows DVD+RW discs to be written in a way that is compatible with most existing DVD readers. DVD+RW discs can be recorded in either CLV format for sequential video access (read at CAV speeds by drive) or CAV format for random access. DVD+RW media can be rewritten about 1,000 times (down from 100,000 times in the original 1.0 version).

DVD+R is a write-once variation of DVD+RW, expected to appear in 2002. It's a dye-based medium, like DVD-R, so it will have similar compatibility as DVD-R. Most DVD+RW drives will be upgradeable via a firmware download to write DVD+R discs.

Philips announced a DVD+RW home video recorder for late 2001. The Philips recorder will use the DVD-Video format, so discs will play in many existing players.

DVD+RW media will be produced by MCC/Verbatim. Yamaha and Sony have also announced support for the DVD+RW format.

More DVD+RW information is at <www.dvdrw.org>. The obsolete DVD+RW 1.0 format is standardized in ECMA-274.

Others

Other upcoming potential competitors to recordable DVD include AS-MO (formerly MO7), which holds 5 to 6 billion bytes, and NEC's Multimedia Video Disc (MVDisc, formerly MMVF, Multimedia Video File), which holds 5.2 billion bytes and is targeted at home recording. ASMO drives are expected to read DVD-ROM and compatible writable formats, but not DVD-RAM. MVDisc is similar to DVD-RW and DVD+RW, using two bonded 0.6mm phase-change substrates, land and groove recording, and a 640nm laser, but contrary to initial reports, the drives won't be able to read DVD-ROM or compatible discs.

[4.3.1] How long do DVDs last?

Pressed discs (the kind that movies come on) last longer than you will, anywhere from 50 to 300 years.

DVD-R and DVD+R discs are expected to last anywhere from 40 to 250 years, about as long as CD-R discs.

The erasable formats (DVD-RAM, DVD-RW, and DVD+RW) are expected to last from 25 to 100 years.

See <www.ee.washington.edu/conselec/CE/kuhn/otherformats/95x9.htm> and <www.cd-info.com/CDIC/Technology/CD-R/Media/Kodak.html> for more info.

For comparison, magnetic media (tapes and disks) lasts 10 to 30 years; high-quality, acid-neutral paper can last a hundred years or longer; and archival-quality microfilm is projected to last 300 years or more. Note that computer storage media usually becomes technically obsolete within 20 to 30 years, long before it physically deteriorates. In other words, before the media becomes unviable it will become difficult or impossible to find equipment that can read it.

[4.4] Why can't I take a screenshot of DVD video? Why do I get a pink or black square?

Most DVD PCs, even those with software decoders, use video overlay hardware to insert the video directly into the VGA signal. This an efficient way to handle the very high bandwidth of full-motion video. Some decoder cards, such as the Creative Labs Encore Dxr series and the Sigma Designs Hollywood series, use a pass-through cable that overlays the video into the analog VGA signal after it comes out of the video display card. Video overlay uses a technique called colorkey to selectively replace a specified pixel color (often magenta or near-black) with video content. Anywhere a colorkey pixel appears in the computer graphics video, it's replaced by video from the DVD decoder. This process occurs "downstream" from the computer's video memory, so if you try to take a screenshot (which grabs pixels from video RAM), all you get is a solid square of the colorkey color.

Hardware acceleration must be turned off before screen capture will work. This makes some decoders write to standard video memory. Utilities such as Creative Softworx, HyperSnap, and SD Capture can then grab still pictures. Some player applications such as PowerDVD and the Windows Me player can take screenshots if hardware acceleration is turned off.

[4.5] Why can't I play movies copied to my hard drive?

Almost all movies are encrypted with CSS copy protection (see 1.11). Decryption keys are stored in the normally inaccessible lead-in area of the disc. If you copy the contents of an encrypted DVD to a hard drive, the keys will not be copied. If you try to play the VOB files, the decoder will request the keys from the DVD-ROM drive and will fail. You may get the message "Cannot play copy-protected files".

[4.6] Why do I have problems playing DVDs on my computer?

There are thousands of answers to this question, but here are some basic troubleshooting steps to help you track down problems such as jerky playback, pauses, error messages, and so on.

More information on specific graphics cards and driver updates:

[4.7] Can I stream DVD over a network or the Internet?

Short answer: Not if the disc is copy protected.

With a fast enough network (100 Mbps or better, with good performance and low traffic) and a high-performance server, it's possible to stream DVD-Video from a server to client stations. If the source on the server is a DVD-ROM drive (or jukebox), then more than one user simultaneously accessing the same disc will cause breaks in the video unless the server has a fast DVD-ROM drive and a very good caching system designed for streaming video.

A big problem is that CSS-encrypted movies (see 1.11) can't be remotely sourced because of security issues. The CSS license does not allow decrypted video to be sent over an accessible bus or network, so the decoder has to be on the remote PC. If the decoder has a secure channel to perform authentication with the drive on the server, then it's possible to stream encrypted video over a network to be decrypted and decoded remotely. (But so far almost no decoders can do this.)

One solution is the VideoLAN project which runs on GNU/Linux/Unix, BeOS, Mac OS X, and other operating systems. It includes a player with built-in CSS decryption. Although the code is different from DeCSS, it's an unlicensed implementation and is probably illegal in most countries (see 4.8).

An alternative approach is to decode the video at the server and send it to individual stations via separate cables (usually RF). The advantage is that performance is very good, but the disadvantage is that that DVD interactivity is