Working of Digital Versatile Disc (DVD)

What is a DVD?

A Digital Versatile Disc/Digital Video Disc [DVD], is an optical disc storage medium like a compact disc [CD], but with greater data storage and high quality audio and video formats. The clarity, when comparison with a CD is almost six times higher.

A DVD can be termed in different ways according to their applications.

• If they are used for storing data that can only be used to read and cannot be written, it is called a DVD-ROM, where ‘ROM’ stands for ‘Read Only Memory’.
• If a DVD is used to record data [audio, video or a combination of both] and then work as a DVD-ROM, it is termed as DVD-R, where ‘R’ stands for ‘recordable’. In a DVD-R, the data is can be inputted only once.
• If a DVD can be read and again be written over it, it will be called a DVD-RW, where ‘RW’ stands for’re-writable’. The data in a DVD-RW can be erased and re-written and also can be written on the remaining space in a DVD.

TAKE A LOOK : WORKING OF DVD PLAYER

Specifications of DVD

• DVD has a total capacity of 4.7 GB.
• It can run a high-quality video for a maximum of 133 minutes.
• DVD has a video compression ratio of 40:1 with an MPEG-2 compression.
• DVD can use subtitles in almost 32 languages with a soundtrack in 8 languages.
• Wavelength of a DVD is almost 650 nanometres laser diode light.
• DVD’s can be written at a speed of almost 18x to 20x. [1x = 1318 Kbps].

Advantages of DVD

• The picture quality is far better than a CD.
• Most DVD’s have Dolby Digital or DTS. Thus the clarity of the sound will be nearly equal to that in a theatre.
• Though the technology of CD is gone, they were made with a compatibility with audio CD’s.
• The DVD has a special on-screen index quality which helps you to take a closer look at important scenes of a movie which was earlier indexed by the publisher of the movie. This method is not common.
• Unlike a CD player, the DVD player is specialised in taking you to the correct part which you prefer to see. Thus, there will be no need of fast-forwarding.
• The format of a DVD can be changed according to the type of view you need. It can be a standard TV size format and also a wide-screen TV format.

Structure of a DVD

The manufacturing methods and materials used for manufacture as the same as that of a CD. The DVD has a number of layers of plastic pieces with a total thickness of 1.2 millimetres. Clear poly-carbonate plastic is basically used for making these layers. Apart from the original plastic, there will be some impressions of microscopic bumps on the layers. Naturally, with the formation of bumps, there will also be pits. Thus a long spiral of data can be created which is together called a track. The track in any DVD is only 740 nanometers thick. There are several tracks in a DVD. Thus the bumps will be smaller than the track with a width of 320 nanometers, a length of 400 nanometers a height of and 120 nanometers. Although the bumps are often mistaken for pits, it should be noted that they only appear as pits when seen from the aluminium side.  They are actually bumps when read from the laser side.

At the inner layers of the plastic, the bumps and pits are covered with the help of a reflective aluminium layer. But their outer layers are protected with the help of a very thin layer of semi-reflective gold. Thus the lasers that hit the DVD are made to completely focus on the outer and also into the inner layers. On top of all these layers a coating of lacquer is also put. The lacquer is strongly bonded with the help of infrared light.

There are DVD discs which are single-sided as well as double-sided. For the former, the outer label is silk-screened onto the side that is not readable. For the latter there are no labels except at the area which is not readable near the middle hole.

In the case of DVD’s that are writable, each track in the DVD is spiral and has data stored in it. These tracks also have a unique direction. For single-sided discs, the circling will be from the inside to the outside. Take a look at the diagrams illustrated below.

Thus, it is clear why DVD’s are capable of storing more data than a CD. The high density of tracks and smaller size of the pits and bumps is the main reason. In a CD, a lot of valuable space is simply wasted in codes, so as to avoid errors in the information. These codes are actually a repetition of the information that is already present in the disc. This shows that the method is not only efficient, but a wastage of storage space as well. But DVD’s does not need such error correction codes.

TAKE A LOOK : WORKING OF DVD PLAYER

DVD Video Format

It must be noted that a movie video can never be stored in a DVD without being compressed. The movies are usually compresses with the help of MPEG-2 format, which is accepted internationally. Though the coding is done to compress the video, it also has to be decoded, which is actually done by the DVD.

Decrease in Data size using MPEG-2 Format

When a movie is being filmed, they use either a PAL format or a NTSC format. These formats are distinguished according to the number of frame rates that are displayed on a movie screen for a particular second. For NTSC format the frame rate is 30 and for NTSC the frame rate is 50. Thus, it is clear that there will be a difference in the resolution as well. Thus the MPEG-2 format must be compatible to both the above said formats.

While using an MPEG format, each frame is carefully studied by the compressing file and a possible solution of encoding is finally decided. There are mainly three ways of encoding a file.

1. One method is called the intra-frame format. This format contains all the image data for that particular frame. This type of coding provides lesser compression than the other two.
2. The second method is the predicted frame format. The most recently displayed frame [either intra-frame or predicted frame] will be shown to the DVD player and will be told to predict the frame based on it.
3. The third method is called bi-directional frame, in which a method of interpolation is carried out in such a way that the exact position and colour of each pixel is given from the nearest frames. For this frame the help of the other two frames are needed.

DVD Audio Format

The audio and video formats in a DVD are completely different. Audio DVD discs are not much in sale because, to play such a disc you will need a 192 kHz/24-bit digital-to-analog converter (DAC) compatible DVD player. This is rarely sold. Our usual DVD players have only a 96 kHz/24-bit DAC.

Though an audio DVD is not as perfect as the original sound, it is far better than the sound from an audio CD. Whether it is a DVD or an audio CD, each bit is represented by a digital response from the DAC according to the voltage level it needs to output. As the sound is sampled at different frequencies, it is sure to lose small pieces of data and hence the difference.

To get the best sound from an audio DVD, you will have to use a 5.1 home theatre along with a Dolby digital decider implanted receiver.