DVD-Video Review Paper

CONTENTS

1. INTRODUCTION

The use of digital storage has taken a sharp increase since the early 1980's, thanks to the widespread use of compact disc. First, CD Audio for music was released in 1982, followed by CD-ROM in 1985 (Sony, 2000). However, until the 1990's, digital video was virtually impossible (Goldberg, 1998). There was simply too much information to be digitised and stored. Advanced compression techniques changed all this (Goldberg, 1998), and helped pave the way for a new storage medium: DVD.

This review paper will discuss the DVD technology, focussing mainly on one of the DVD standards: DVD-Video. First, the history of DVD will be outlined. Second, the paper will detail the features and specifications of the DVD-Video format. Thirdly, the paper will compare DVD-Video with other forms of storage, analysing the advantages and disadvantages of each. Finally, the future of DVD will be discussed.

2. DVD BACKGROUND

Since Sony and Philips introduced the compact disc, it has been possible to store almost anything digitally: audio, text, photographs and animations (Parker, 1999, p.80). Unfortunately, video was unable to be stored successfully on CD. One of the main reasons was that CD had a limited storage capacity (Parker, 1999, p.80), of 650MB (Ely & Block, 1998, p.3). MPEG-1 compression allowed two hours of video to be stored on a double-density CD (Ely & Block, 1998, p.3), but the picture quality of MPEG-1 is not sufficient for video (Goldberg, 1998). A higher quality compression technique, MPEG-2, was developed, but MPEG-2 files are larger than MPEG-1 files (Goldberg, 1998), and the capacity of a CD was too small to hold a movie of any great length.

Additionally, according to Parker (1999, p.80), there were a number of other problems with CD. These included:

• Awkward random accessibility and rewritability;
• Cross-platform failings;
• Overlapping and sometimes mutually incompatible standards.

This is why, in 1993, Sony, Philips and several other companies began work on a "new, higher density version of the CD" (Parker, 1999, p.80) to answer all the problems of the CD. In January 1995, two proposed, competing formats for the new DVD were announced: the Multimedia Compact Disc (MMCD), backed by Sony, Philips and several other companies, and the Super Disc (SD), backed by Toshiba, Time Warner, and several others (Taylor, 1999, p.86). A group of computer companies, including Microsoft, Intel, Apple and IBM (The PC Technology Guide, 2000a), insisted the companies agree on a single standard, to avoid a costly and confusing repeat of the Betamax vs. VHS videotape battle (Taylor, 2000). The combined format was announced in late 1995, although it relied heavily on the SD format (The PC Technology Guide, 2000a). It was the work of ten companies, which formed the DVD Consortium (now known as the DVD Forum): Hitachi, JVC, Matsushita, Mitsubishi, Philips, Pioneer, Sony, Thomson, Time Warner and Toshiba (Taylor, 2000).

The DVD format was further broken down into five specifications, or "books": DVD-ROM, DVD-Video, DVD-Audio, DVD Recordable and DVD RAM (Ely & Block, 1998, p.4).

DVD-ROM and DVD-Video were released in late 1996 (Taylor, 1999, p.86). The relationship between DVD-ROM and DVD-Video is similar to that of CD-ROM and CD Audio (Taylor, 1999, p.86). DVD-ROM holds computer data, and can only be read by a DVD-ROM drive connected to a computer, similar to a CD-ROM. DVD-Video, on the other hand, holds video programs, and can be read by a DVD player hooked up to either a TV or a computer (in the same way a CD Audio disc can be read by a CD player, or a computer) (Taylor, 1999, p.86).

The aim of DVD is to eventually replace other forms of storage: audio CD, CD-ROM, videotape, laser disc and video game cartridges (Taylor, 1999, p.86). In 1997, the number of DVD-Video players in the U.S. alone was 349,000; by June 2000, this figure had risen to 7.47 million, with 2 million of these sold during the first half of 2000 (Taylor, 2000). It is expected that DVD will eventually be the main form of storage for both home entertainment (e.g. video and audio), as well as information (e.g. computer games and business documents) (Taylor, 1999, p.86).

3. FEATURES AND SPECIFICATIONS

A DVD looks almost like a CD. Both discs are plastic, 120mm in diameter, 1.2mm thick, and rely on laser to read the data, which is stored in a spiral track on the disc (The PC Technology Guide, 2000a). However, their physical and data structures are completely different. A CD is made of a single 1.2mm platter, while a DVD is made of two 0.6mm polycarbonate substrate platters bonded together (Parker, 1999, p.83). During manufacture, the disc may warp slightly. This isn't a problem for CD. With DVD, however, the pits and lands that store the data are closer together. Therefore, warping may adversely affect the ability to read the disc. The solution is to bond two 0.6mm "half-discs" together. This results in greater rigidity, and any warping is offset by the bonding (Ely & Block, 1998, p.6).

DVD has greater data capacity than CD. This is because the pits and lands that store information are smaller, and compressed closer together, as Figure 1 demonstrates:

Figure 1: Pits and lands on CD and DVD. (The PC Technology Guide, 2000a).

On a DVD, the track pitch (width) is 0.74um (micrometers), compared with 1.6um on a CD. On a DVD, the minimum pit length is 0.4um, compared with 0.83um on a CD (Ely & Block, 1998, p.5).

Of the five DVD formats, or "books" specified by the DVD Consortium, three are read-only formats: DVD-ROM, DVD-Video and DVD-Audio (Technicolor, 2000, p.3). The other two - DVD-RAM and DVD-R (or DVD-RW) - are recordable formats (Technicolor, 2000, p.3), and are beyond the scope of this paper. DVD-Video and DVD-Audio are extensions of the DVD-ROM format. There are three layers, or levels, to each format, which are mentioned by Parker (1999, pp.80-81):

• Physical level - describes the physical properties of the disc.
• File system (logical) level - defines the logical format and file structure of the disc.
• Application level - defines the actual placement and type of video, audio, images, navigation and presentation.

All three read-only formats share the same physical and logical levels. DVD-ROM has no application layer; thus DVD-Video and DVD-Audio are merely extensions of the DVD-ROM format.

The physical data structure (physical layer) determines how data is organised and placed physically on the disc. Data is stored sequentially in a physically contiguous order (Taylor, 1999, p.87). According to Ely & Block (1998, pp.14-15) and Taylor (1999, pp.87-89), the physical layer defines the following:

• Video Title Set (VTS) - stores internal information about the title (the logical construct of the disc - whether it's a movie, a collection of songs, an episode, etc). There are up to 99 Video Title Sets on one disc - they contain all the video and audio elements. Although there can be up to 99 VTSs on a disc, most feature films only include one, for the movie. The Video Title Sets are made up of the Video Title Set Menu, and one or more Video Titles. Titles are then broken into Part of Titles (PTT) (i.e. chapters). The data within the Video Title is made up of Video Object Sets (VOBSs).
• Video Object Set (VOBS) - contains information about the data, including attributes for video, audio, and subpictures. Subpictures are full-screen graphic overlays for subtitles, captions, text effects, highlighting, pointers, etc. The first VOBS may be an optional menu, followed by VOBSs that contain actual file content.
• Main Menu - if present, this is the first thing the viewer sees when the disc is inserted. If it isn't present, then a special autoplay piece automatically begins playback.
• Video Object (VOB) - each Video Object Set consists of one or more Video Objects. A Video Object makes up all or part of an MPEG-2 program stream. It is comprised of the video, audio, subpictures and navigation data of a program. The VOB is the fundamental media element of the disc.
• Cells - the smallest addressable chunk. Each VOB consists of one or more cells. A cell is a group of pictures or audio blocks. A cell can be any length, from one second, to the length of the entire movie.
• Video Object Units (VOBU) - a cell is further divided into Video Object Units. These may or may not contain video, and last between 0.4 to 1 second.
• Packs and Packets - finally, VOBUs are broken into packs of packets. Packs include system clock reference information for timing and synchronisation. Each packet identifies which stream (pack) it belongs to, and carries a chunk of data for that stream. Different packs contain different data for video, audio, subpicture and navigation.

The logical data structure (also known as the presentation data structure) defines the file structure for the disc. As discussed by Ely & Block (1998, pp.14-18), Taylor (1999, p.89) and The PC Technology Guide (2000a), the logical layer defines the following (in order):

The application layer of a DVD-Video disc is clearly defined as to the type of video codec (compression/decompression), audio compression, navigational commands, auxiliary files, and other information it may contain (Parker, 1999, p.82). If a DVD-ROM disc is not defined according to the above information, it will not play (Parker, 1999, p.82).

Menus interact with the program content. Ely & Block (1998, p.21), discuss 3 types of menus, known as Systems Menus, available through the press of a button on the remote control:

• Title Menu - resides in the Video Manager and accesses Titles on the disc.
• Root Menu - resides in each Video Title Set, and can access Video Titles.
• Audio, Angle and Subpictures Menus - also reside within each Video Title Set, and are used to change the audio stream, video angle and subpicture stream.

When a systems menu is accessed, the DVD player remembers exactly where in the video stream it was playing, and can return to that location after the viewer has finished using the menu (Ely & Block, 1998, p.21).

DVDs can be dual-sided and dual-layered. A dual-sided disc has data on both sides. A dual-layered disc can have two layers of data on the one side. The second layer can be read by changing the wavelength (focus) of the laser. The first layer of the disc is semi-reflective, usually gold, which allows the laser beam to either be focussed on it, or pass through it to the second layer, which is fully reflective aluminium (Parker, 1999, p.84). There are 4 main types of pre-recorded DVDs, as Table 1 shows:

Table 1: Types of DVD discs and their capacities for data and video (adapted from Technicolor, 2000, p.5).

All DVD players can read all DVD capacities (Ely & Block, 1998, p.7). Note that a dual-layered disc is not exactly double the capacity of a single-layered disc. Because the layers are so close together, and no disc is perfectly flat (due to the warpage endured during the manufacturing process), there exists a greater potential for errors and cross-talk between layers. Therefore, the pits and lands on both layers of a dual-layered disc are slightly larger, to reduce errors, thus the capacity is reduced slightly (Parker, 1999, p.84). Figure 2 shows the structure of each type of disc.

Figure 2: Structure of each disc size. (The PC Technology Guide, 2000a).

The second layer can be read from the inside of the disc to the outside, as well as outside to in. In other words, data can be stored "backwards" in a reverse spiral track. The PC Technology Guide (2000a), outline two ways of reading data on a disc:

• Parallel Track Path (PTP) - both layers are read from the inside of the disc to the outside. When the player has stopped playing one layer, there is a break for the laser to refocus, and find the inside of the disc again.
• Opposite Track Path (OTP) - the outer layer is read from the inside to the outside of the disc, and the inner layer is read from the outside to the inside. The drive can read both layers almost continuously, with only a short break to refocus the lens. This is good for long movies (i.e. movies longer than 133 minutes).

In order for a movie to fit onto a DVD, some form of compression is needed. Despite the large capacities of DVDs, it's still not enough to hold a full, uncompressed movie. One single digitised video frame (image) takes up approximately 1MB. Motion video is 30 frames-per-second, which means a single second of video would take up 30MB. Multiply that by the length of the average movie, say, 120 mins, and we would need 216 000MB (211GB) to store the entire movie (Goldberg, 1998). Therefore, since DVD-Video can only hold a maximum of 17GB, some form of compression is needed.

Lossy compression was selected by the governing body, MPEG (Motion Picture Experts Group), as the method of compression to be used. With lossy compression, data that is deemed unnecessary or redundant (i.e. data that it is assumed will not be missed by human perception) is removed, in order to reduce the size of the image or sound (Goldberg, 1998). The first method of lossy compression was developed by MPEG, and was called MPEG-1. MPEG-1 was fairly poor quality, with a maximum playback rate of 1.856 Mbps, but an average of 1.15 Mbps (Taylor, 2000). The result is that the motion is jerky (i.e. not smooth), and complex pictures turn out blocky-looking (Goldberg, 1998). MPEG-1 is now used mostly for CD-ROM games (Goldberg, 1998), information DVDs and training videos which are stored on DVD (Ely & Block, 1998, p.8).

MPEG-2 compression is less lossy than MPEG-1 (therefore files are larger), but is better quality (Goldberg, 1998). It is now typically used for movie playback on DVD-Video. It allows 133 minutes of high quality video to be stored on a single-layered, single-sided DVD. For MPEG-2, a full motion picture lasting one second requires 3500 Kbits (437.5 KB) of storage space. Digital surround sound requires a further 384 Kbits (48 KB) per second of storage. Additional storage for dialogue tracks in different languages, subtitles, etc, increases the total required storage for one second of high quality movie to 4692 Kbits (586.5 KB) (The PC Technology Guide, 2000a). For a 133-minute movie, this works out to 4.68 GB. The maximum playback bit rate for both NTSC and PAL displays is 9.8 Mbps (Ely & Block, 1998, p.8). The average playback rate is 3.5 to 4 Mbps, but this will vary, depending on the length and quality of video, and the amount of audio (Taylor, 2000).

Taylor (2000) mentions two main video formats for television displays:

• NTSC - capable of displaying 720 horizontal pixels, * 480 vertical pixels, at a frame rate of 29.97 frames per second.
• PAL - capable of displaying 720 horizontal pixels * 576 vertical pixels, at a frame rate of 25 frames per second.

Depending on the country of release, the disc will be in either of these formats (Ely & Block, 1998, p.7).

The aspect ratio is the ratio of the width of the display to the height of the display. According to Taylor (2000), there are two aspect ratios for DVD-Video:

• 4:3 - the display (i.e. TV screen) is 4/3 (1.33) times as wide as it is high.
• 16:9 - the display is 16/9 (1.78) times as wide as it is high.

DVD players can support both displays. According to Taylor (2000), all players are capable of four playback modes (one for 4:3 video and three for 16:9 video):

• Full frame - both the video and display are 4:3, therefore no adjustment is required.
• Auto letterbox - video is 16:9, display is 4:3. The player squeezes the picture horizontally, creating black bars at the top and bottom of the screen. This leaves exactly � of the height remaining. The picture is then squeezed vertically, combining every four lines into three. This vertical squeezing exactly compensates for the horizontal squeezing; therefore the movie is shown in full width.
• Auto pan and scan - video is 16:9, display is 4:3. The thinner TV "window" (display) is panned and zoomed across the wider picture, using a "centre of interest" offset that is encoded in the video streams, chopping off the sides. The "window" is now 75% the full width, reducing the horizontal pixels from 720 to 540. The picture cannot be squeezed vertically, therefore the sides are chopped off, and information is lost.
• Widescreen - both the video and display are 16:9. The video is stored on the disc anamorphically - the picture is squeezed to 4:3 format for storage, and unsqueezed to 16:9 for playback.

As discussed by Taylor (2000), DVD-Video discs can contain one or more of the following audio formats:

• Pulse Code Modulation (PCM) - uncompressed (i.e. lossless) digital audio. This is the same format used on CD. Sounds are sampled at 48 or 96 kHz (i.e. sound is sampled 48000 or 96000 times per second), with 16, 20 or 24 bits per sample. The maximum bit-rate is 6.144 Mbps, and there are 1-8 channels.
• Dolby Digital - lossy digital audio. Sounds are sampled at 48kHz, with up to 24 bits per sample. The bit rate is 64-448 Kbps. The two channel stereo version has a maximum bit rate of 192 Kbps, while the 5.1 channel surround sound version (5 channels - left rear, right rear, left front, right front, and centre; 0.1 channel is the low-bandwidth sub-woofer) has a bit rate of 384-448 Kbps.
• Digital Theatre Systems (DTS) - is an optional 5.1 channel format, with lossy compression. Sounds are sampled at 48kHz, with up to 24 bits per sample. Bit rates of 64-1536 Kbps are typical. The 5.1 channel format has bit rates of 768 and 1536 Kbps, while the 2 channel format has bit rates of 384 and 768 Kbps.
• MPEG Audio - is a multi-channel digital audio format, with lossy compression. Sounds are sampled at 48kHz, with 16 or 20 bits per sample. This format contains both MPEG-1 and MPEG-2 formats. Bit rates vary between 32-912Kbps, with 384Kbps being the average rate. MPEG-1 is limited to 384Kbps. There is also an optional 7.1 channel version, which adds left centre and right centre channels.
• Sony Dynamic Digital Sound (SDDS) - is an optional 5.1 or 7.1 channel audio format. Sounds are sampled at 48kHz. The data rate can be up to 1280 Kbps.

DVD-Video has many features, which are analysed by Ely & Block (1998, pp.11-13), Taylor (1999, pp.89-90) and The PC Technology Guide (2000b):

4. COMPARISON OF DVD-VIDEO WITH OTHER STORAGE DEVICES

Advantages
DVD-Video has many advantages over other forms of storage, as outlined by Taylor (1999, pp.90-91):

• Low Cost - production costs less than videotape, and less than CD when you factor in the large capacity of DVD-Video.
• Simple, cheap and reliable distribution - it easier and cheaper to mail discs than tapes or books.
• Ubiquity - the number of DVD players was expected to exceed the number of VCRs in 1999 or 2000.
• High capacity.
• Self-contained ease of use - DVD can provide instruction tutorials and pop-up help. The disc can also start with a menu, "how-to" sections, and background information.
• Portability - a portable DVD-Video player fits into a briefcase and can be connected to any television or video monitor.
• Desktop production - ability to complete digital video production with inexpensive desktop equipment.
• Merged media - a single disc can contain information normally provided by several different media: newspapers, videotapes, databases, audiotapes, CD, etc.
• Copyright protection - DVD-Video provides encryption to prevent copying of movies.

Disadvantages
There are, however, a number of disadvantages to DVD, as mentioned by Parker (1999, p.84) and Taylor (1999, p.91):

• It will take years for DVD-Video to catch up to the popularity of VCR. DVD has received a lot of hype, and time will tell whether or not it can live up to this hype.
• Vagueness of DVD specification, and inadequate testing means that some discs don't function fully on some players.
• DVD cannot record video yet. Recordable formats only work with computer data. However, the DVD Forum is developing new specifications to allow video recording.
• Regional management complicates DVD-Video.
• DVD is not a true worldwide standard, with regard to regions. Manufacturers also need to consider whether to encode the disc for NTSC or PAL video displays.
• Poorly compressed digital video looks blocky or fuzzy.
• DVD doesn't fully support HDTV (High Definition Television) - first generation DVD doesn't carry high-definition signals.
• Some DVD players and drives may not be able to read CD-Rs.
• Current DVD-Video players and DVD-ROM drives can't read some recordable discs (i.e. DVD-RAM and DVD+RW).

Advantages
There are a number of advantages VCR has over other forms of storage, as examined by Taylor (2000):

• Easy to record - from TV, etc.
• Blank tapes are cheap, as opposed to blank DVD discs. VCRs are also cheap compared to DVD-Video players.
• Don't need to worry about compression, encoding audio and video into digital audio and video.

Disadvantages
There are, however, a number of disadvantages of VCR, according to Taylor (2000):

• Searching (i.e. fast forward and rewind) is a slow process.
• Tapes are not as durable as discs.
• Although discs themselves are far more expensive than tapes, discs hold more.

Video CD is another digital video format, which is basically a primitive version of DVD-Video (Wvong, 2000).

Advantages
As mentioned by Sanderse (1999), Taylor (2000) and Wvong (2000), VideoCD has a number of advantages over other forms of storage:

• Video CDs can be played in some (about 2/3) DVD drives.
• Video CDs can be played in a VCD player connected to either a TV, or a PC with a CD-ROM drive.
• It is easy to create your own Video CDs (e.g. for home movies) using a CD-ROM burner.
• Video CD is hugely popular in Asia; therefore there are many Asian titles on Video CD. This is good for fans of Asian movies.
• Video CD has many of the features of DVD-Video - random access, longevity, both discs and players are relatively cheap, and durability of discs.

Disadvantages
There are, however, also a number of disadvantages of VideoCD, which are discussed by Sanderse (1999), Taylor (2000) and Wvong (2000):

• Video CD players cannot play DVD-Video discs.
• Lower resolution than DVD-Video (352*240 pixels versus 720*480 pixels for NTSC, and 352*288 pixels versus 720*576 pixels for PAL). This is comparable to VHS.
• Lower quality MPEG-1 compression is used, as opposed to MPEG-2 for DVD-Video.
• Video CDs can only hold 74 minutes of video, so two discs would be needed for a full-length movie.
• It is very easy to pirate Video CDs.

Laserdisc is an analog video format (Taylor, 2000).

Advantages
Taylor (2000) outlines a number of advantages LaserDisc has over other forms of storage:

• A lot of the features of laserdisc are the same as DVD-Video, including: multiple camera angles, parental control, menus, interactivity, scanning, pause, search, freeze, and slow-motion.
• Can have better audio quality than DVD-Video on Dolby Surround soundtracks.
• There are thousands of titles on laserdisc that will probably never appear on DVD-Video.
• No regional coding, therefore less complications and confusion.
• It is still popular for education and training videos.
• Both discs and players are now very cheap.

Disadvantages
There are, however, a number of disadvantages as well, which are discussed by Taylor (2000):

• Less capacity than DVD-Video - only � to 1 hour per side.
• Players make a whirring noise, which can be distracting during a movie.
• Can have poorer audio quality than DVD-Video on Dolby Digital.
• Uses analog audio storage.
• Lower resolutions (567*480 as opposed to 720*480 with DVD-Video. Both figures are for NTSC displays).
• Players and discs are becoming hard to find.
• Laserdiscs are more expensive than DVD-Video discs.

Divx (Digital Video Express) is "a limited-use, pay-per-view DVD technology", (The PC Technology Guide, 2000a) with a heavy emphasis on rental. Users purchase a disc at minimal cost, and view it for an unlimited number of times within 48 hours (The PC Technology Guide, 2000a). After this, the user is charged $3.25 for each additional use - this is billed to an account (Dunnill, 2000). Users can also pay a flat rate each month, and receive unlimited viewing each month (Taylor, 2000). This technology was only available in the USA and Canada, and production of players ceased on 16th June, 1999 (Taylor, 2000). The billing service will continue to operate until 30th June, 2001, after which it is assumed that all Divx discs will become unplayable (Taylor, 2000).

Advantages
According to Dunnill (2000), Taylor (2000) and The PC Technology Guide (2000a), Divx has a number of advantages over other forms of storage:

• Divx players are capable of playing DVD-Video discs.
• Movie studios have greater control over use of their content - against copying, etc.
• High quality, in comparison to some other players.
• Players and titles became very cheap after the format was axed in June, 1999.
• There are numerous titles available on Divx not available on DVD-Video.

Disadvantages
There are, however, a number of disadvantages of Divx, as discussed by Taylor (2000) and The PC Technology Guide (2000a):

• Pay-per-view - an internal modem within the player calls a central billing service, and reports each player use.
• Not available outside the USA or Canada.
• Limited lifespan - the central billing service will operate until 30th June, 2001, then it is assumed that Divx discs will become unusable.
• Divx discs do not play in DVD-Video players.
• The format was axed in 1999, so players and discs are becoming hard to find.
• Unlimited viewing is no longer available.
• The Divx player won't work unless it is hooked to a phone line, to contact the central billing service.

5. FUTURE OF DVD-VIDEO

DVD has come a long way in only a short space of time, and looks set to become more popular than ever, as prices of both discs and players continue to fall. Add to this the superior quality DVD has over other forms of storage, such as videotape, CD and laserdisc, and DVD looks set to become one of, if not the primary source of file, audio and video storage.

DVD will probably eventually be used in conjunction with the Internet. Already, news, music, games, educational programs and other information is available via the Internet. In the future, movies will also be available online. The Internet will be primarily responsible for bringing us this media, while DVD will be used for storage. However, the bandwidth (capacity) of the Internet will need to increase enormously before this can happen; at the present time, on a typical 56K modem, it would take over a week to download the contents of a single-layered DVD (Taylor, 2000). As well as this, other problems with DVD, such as those mentioned above, will need to be fixed. I feel it will be at least five to ten years before all the problems are ironed out, and Internet bandwidth can become sufficient enough to handle DVD movies. After this, it is almost certain that DVD will become the primary source of storage for information and multimedia.

REFERENCE LIST