The purpose of this page is to provide an overview of DVD by explaining the terminology and technology used. The differences between Digital Versatile Discs (DVD), Compact Discs (CD) and their players will be explained as well as the data formats used. This will clarify the global variations of DVD and explain why, for example, a DVD-Video bought in the USA cannot be played in an UK system.

DVD is the next generation of optical disc storage technology. It's essentially a bigger, faster CD that can hold video as well as audio and computer data. DVD aims to encompass home entertainment, computers, and business information with a single digital format, eventually replacing audio CD, videotape, laserdisc, CD-ROM, and perhaps even video game cartridges.

Because DVD's have a much greater capacity than CD's and initial discs contained video data, DVD was mistakenly taken to stand for Digital Video Disc. This is not so; as indicated above, DVD stands for Digital Versatile Disc. There will not be a test on this later...

It's important to understand the terminology used in this emerging technology. Like CD, DVD is the generic term given to the disc. We need to understand the difference between DVD-Video and DVD-ROM. A DVD-Video is the disc that holds video programs and is played in a DVD player connected directly to a TV monitor; or via a music centre. A DVD-ROM is the disc that holds computer data and is read by a DVD-ROM drive in a computer. The difference is similar to that between Audio CD and CD-ROM. To complicate manners slightly, a DVD-Video can actually be played in a DVD-ROM drive by adding extra hardware in the form of an MPEG-2 Decoder into the computer. A DVD-ROM drive maybe able to read a DVD-Video, it's the MPEG-2 decoder that makes it work.

CD vs DVD Structure

While both DVD's and CD's are physically similar in size, their data structure is very different as shown in the table.

Comparison of CD and DVD Formats
	CD	DVD
Disc Diameter	120 mm	120 mm
Disc Thickness	1.2 mm	1.2 mm
Disc Structure	Single Substrate	Two Bonded 0.6 mm Substrates
Laser Wavelength	780 nm (Infrared)	650 nm & 635 nm (Red)
Numerical Aperture	0.45	0.60
Track Pitch	1.6 um	0.74 um
Shortest Pit/Land Length	0.83 um	0.4 um (0.44 um Double Layer)
Reference Speed	1.2 m/s CLV	4.0 m/s CLV
Data Layers	1	1 or 2
Data Capacity	Approx 680 MB	4.7 GB (Single Layer) 8.5 GB (Two Layers)
Reference User Data Rate	153.6 KBps (Mode1) 176.4 KBps (Mode2)	1108 KBps (Nominal)
	Video CD	DVD-Video
Video Data Rate	1.44 Mbps (Video, Audio)	1-10 Mbps Variable (Video, Audio, Subtitles)
Video Compression	MPEG1	MPEG2
Sound Tracks	2-Channel MPEG	Mandatory NTSC; 2-Channel Linear PCM and/or 2-Channel/5.1 Channel Dolby Digital™ (AC-3) Optional Up To 8 Streams of Data
Subtitles	Open Caption Only	Up to 32 Languages

DVD increases its capacity by using higher resolution optics. DVD uses a shorter wavelength laser with red light around 650 nm compared to a CD with infrared light at 780 nm. In addition, better focusing optics allows closer tracks and smaller pits; as shown in the diagrams below

TheSingle-Sided, Dual Layer configuration increases the capacity by 3.8 GB to 8.5 GB on one side by using a second layer. The laser has to read "through" the outer layer to the inner layer. To reduce the possibility of interference between layers, the minimum pit length of both layers is increased from .4 um to .44 um. Also, the reference scanning velocity is slightly faster from 3.49 m/s for a single layer discs to 3.84 m/s. These longer pits, being spaced further apart, are easier to read correctly. However, their increased length means fewer pits per revolution; hence the reduced capacity per layer and the reason why a Single-Sided, Dual Layer disc is only 8.5 GB and not 2 x 4.7 = 9.4 GB. This is more than 13 times the capacity of a CD-ROM.

The Double-Sided, Single Layer configuration gives a capacity of 9.4 GB, with 4.7 GB on each side.

However, as data is on both sides, the disc must be either 'flipped-over' or used in a drive that is capable of reading both sides.

Note: There is an unfortunate confusion with capacity labelling in the DVD world; which uses the notation of mega and giga to be multiples of 1000, whereas the computer uses mega and giga to be multiples of 1024. Therefore, a 4.7 GB DVD actually holds 4 700 000 000 or 4.7 billion bytes; which is only 4.38 GB in computer talk.

DVD-Video offers picture quality comparable with D1, the CCIR-601 TV Studio production standard; which offers a resolution of 720x480 at 30 frame per second NTSC. To achieve this, DVD-Video takes advantage of the two-pass variable bit rate encoding process of MPEG-2.

With the first pass, the encoder scans the video source for complexity; detecting changes and determining the optimum bit rate for each frame. The second pass then assigns higher bit rates to complex picture sequences and lower bit rates to simple sequences. This two-pass approach optimises the picture quality for a given sequence. The average bit rate for digital video is usually quoted at 3.5 Mbps; although the actual value is dependant on the complexity of the picture sequence being encoded. The maximum allowable bit rate is 9.8 Mbps.

MPEG-2 also supports encoding at various viewing angles and aspect ratios such as 4:3 for standard TV and 16:9 for wide-screen TV. Combine this video with surround sound and you can rival that of theatre quality. DVD-Video also supports up to 8 different languages and 32 subtitles.

It is important not to confuse the DVD-Audio formats with the Audio formats supported within the DVD-Video specification. The DVD-Audio specification was scheduled to be released in October 1998. Since this specification introduces new features and formats that are not supported by existing DVD players, some DVD-Audio discs may not playable on older DVD players. The introduction of DVD-Audio discs has necessitated new DVD players capable for playing both DVD-Video and DVD-Audio discs.

The DVD-Video specification supports three audio formats, which are shown in the table below.

Dolby Digital™ is a multi-channel digital audio using lossy compression. Dolby Digital™ supports both 2-channel stereo and 5.1 channel surround sound. The six completely separate channels are Left, Centre, Right, Left-Rear and Right-Rear, with an additional low frequency sub-woofer channel; hence the 5.1 designation. The average bitrate for 5.1 channels is 384 Kbps.

MPEG-2 Audio is also a multi-channel digital audio using lossy compression. MPEG-2audio supports up to 8 channels with the 7.1 format by adding Left-Centre and Right- Centre channels. The average bitrate is 384 Kbps. The MPEG-2 audio standard provides for backward compatibility with the MPEG-1 standard; by adding surround sound channels to the 2 channel limit of MPEG-1 audio. Hence, the Left and Right main channels of MPEG-2 audio remain backwards compatible with MPEG-1; whilst new feature use the extra channels.

MPEG-1 audio has 3 compression methods, called audio Layer I, II and III. These layers provide increasing compression ratios with a corresponding increase in complexity and processing power. Layer I is the simplest; Layer II adds more advanced techniques and accuracy; whilst Layer III is very compute intensive to the point of being impractical for DVD. The specification states that a Layer III decoder must be able to decoder any Layer I, II or III audio stream; whilst a Layer II decoder must decoder both Layer I & II streams.

When creating a DVD-Video the producer has to decide upon which video and audio format to use. DVD-Video suffers the same NTSC v PAL problem as VCR's and TV's. Whilst the MPEG-2 video on the DVD is stored in a compressed digital format, the producer has to decide which one of the two mutually incompatible formats should be used for the targeted market. In general, the USA and Japan uses NTSC TV with 525 lines at 60 Hz; whilst Europe uses PAL TV with 625 lines at 50 Hz.

There is also the audio format to consider. Effectively, there are three major alternatives between discs, which the producer has to decide upon. These are Picture Size, should it be 720x480 or 720x576; Display Frame Rate, should it be 30 or 25; and Audio format, should it be Dolby Digital™ or MPEG.

Before you create a disc, you have to capture and edit the data. To do this you will need a real time MPEG-2 encoder and both MPEG-2 editing and DVD Authoring software.

For Personal Computers, there are now several CD Writers on the market and a CD-R is capable of holding approximately 14 minutes of MPEG-2 encoded video. However, you will still need to install an MPEG-2 decoder to play it. The market is now begins to see relatively low cost DVD Writers and DVD-R's, which can hold approximately 2 hours of encoded video. Again, you will still need an MPEG-2 decoder as well as a DVD-ROM drive to play it.

Film Studios want to control the release date of a movie in the different countries that it will be shown. Generally, movies are released in the USA well before Europe and the movie industry wants to prevent early releases from appearing too soon in other markets. Therefore, the industry requested that the DVD specification included codes that would prevent playback of a particular DVD in a particular geographical region.

Each DVD player has a regional code for the geographical region in which it is intended to operate and will not play a DVD that is not allowed in its region. It is optional for DVD-Video producers to use regional codes and those with non, will play in any country. The code is just a permanent byte of information burnt onto the DVD, which the player checks.

There are 6 geographical regions, with players and discs being identified by their region number. Discs that will play in several regions will have the corresponding region numbers burnt on them. The region numbers are shown in the table below.