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DVD Explained

DVD represents a quantum leap forward in digital media. Not only does it offer business users many times more capacity than CD-ROM, it also brings spectacular interactive audio and visual entertainment to the home. The advanced technologies that DVD employs, together with the medium's extensive functionality, mean that authoring and mastering discs is comparatively more complex than is the case for CD.

These pages give a brief overview of DVD and the processes used in creating it.

DVD Authoring line

Philips DVD Authoring demonstration facility in Eindhoven

The DVD Disc

DVD looks like a conventional CD, but hidden in its structure is the capacity to hold up to 26 times more information. Not only that, but DVD will run at a much faster data rate, enabling stunning graphics, widescreen video, surround sound, interactivity, multiple languages, multiple language subtitles.... the list goes on.

The vast increase in data storage capacity is achieved by reducing the size and space between the individual storage elements, allowing much more data to be fitted into a given area. In addition to this, DVDs can have two layers of information in a single side of a disc. The two layers are separated by a semi-transparent layer, allowing both layers to be read by the player. The laser reading the disc focuses to read the appropriate layer. The dual layer gives an almost twofold increase in information capacity compared to a single layer disc.

DVD Capacity (GB)

Disc Type	8 cm Disc	12 cm Disc
Single Sided, Single Layer	1.4	4.7
Single Sided, Dual Layer	2.6	8.5
Double sided, Single Layer	2.9	9.4
Double sided, Dual Layer	5.3	17

CD has a maximum storage capacity of around 730 MB. DVD's can be produced with eight different capacities. They can have one or two layers of data, and can be single or double sided. Like CD, DVD can also also be produced in two diameters; 8 cm and 12 cm. DVD players will be able to read all of the disc types, and existing CDs.

DVD has a so-called Lead-in area which acts like an index in a book, telling the player what is contained on the disc. The actual information is contained in the Program area. At the end of the recorded information a Lead-out area, which signifies to the player the disc is finished, fills the rest of the disc.

Dual layer discs can be constructed in two ways: Opposite track path and Parallel track path. In Parallel track path discs, the first layer is read from the inside of the disc to the outside, then the second layer is read in the same direction.

Opposite track path discs also have a Middle area, which bridges the point where the first layer ends and the second layer starts. The second layer is read from the outside of the disc to the inside

. Dual Layer DVD construction

Audio and Video Preparation/Encoding

DVD's increased complexity means additional authoring processes have to be carried out compared to CD. For DVD-Video, (and audio-visual segments of DVD-ROM) the digital video and audio is compressed using a technique called MPEG2 encoding. This process reduces the amount of video and audio information to put on the disc without reducing quality. DVD also allows inclusion of different versions of the movie soundtrack with different language dialogue.

Subtitling/Disc Definition

Subtitles in several different languages can be included. These are formatted in the disc definition phase along with basic information about the disc being authored.

Multiplexing

When all of this information has been prepared, the data streams are applied to a multiplexer, which combines all of the audio, video and subtitle data into a single digital data stream. Data storage between the processes can be on hard disk, or for portability where the disc is not mastered in one location, digital tape.

Disc Building

The encoded and multiplexed data stream is used by Disc Building software to create an image of the final disc. This process combines information such as file structures, synchronization timing codes, disc branching details for interactivity, disc contents etc. as specified during disc definition.

Emulation

For pre-production checking, before the the volume pressing of the discs is carried out, a computerised emulation of the disc playback is carried out to verify that the contents of the disc is correct.

Mastering DVD

Once the disc image has been created, information such as error correction data must be added to the data stream. This process is called channel encoding, which writes the DVD formatted data stream to the Laser Beam Recorder to cut the glass master.

The glass master is used to manufacture the DVD in volume production. A substrate (disc blank), made from transparent polycarbonate, is pressed onto a stamper created from the glass master, then coated with a layer of reflective material. This layer is then bonded with another substrate layer. For dual layer discs, the reflective layer is pressed first, then a second reflective layer is added, with a semi-transparent adhesive space layer in between.

Authoring DVD