All you need to know about DVDs
What does ‘DVD’ stand for?
‘DVD’ stands for Digital Versatile Disc, but some sources say that it doesn’t stand for anything anymore.
Typical contents of a DVD movie

• Up to 133 minutes of high-resolution video, in letterbox or pan-and-scan format, with 720 dots of horizontal resolution. (The video compression ratio is typically 40:1 using MPEG-2 compression.)
• Soundtrack presented in up to eight languages using 5.1 channel Dolby digital surround sound
• Subtitles in up to 32 languages
  A DVD can also be used to store almost eight hours of CD-quality music per side.

Advantages

• DVD picture quality is better, and many DVDs have Dolby Digital or DTS sound, which is much closer to the sound you experience in a movie theatre.
• Many DVD movies have an on-screen index, where the creator of the DVD has labelled many of the significant parts of the movie, sometimes with a picture. With your remote, if you select the part of the movie you want to view, the DVD player will take you right to that part, with no need to rewind or fast-forward.
• DVD players are compatible with audio CDs.
• Some DVD movies have both the letterbox format, which fits wide-screen TVs, and the standard TV size format, so you can choose which way you want to watch the movie.
• DVD movies may have several soundtracks on them, and they may provide subtitles in different languages.

How they are made

DVDs are of the same diameter and thickness as CDs, and they are made using some of the same materials and manufacturing methods. Like a CD, the data on a DVD is encoded in the form of small pits and bumps in the track of the disc.

A DVD is composed of several layers of plastic, totalling about 1.2 millimetres thick. Each layer is created by injection moulding polycarbonate plastic. This process forms a disc that has microscopic bumps arranged as a single, continuous and extremely long spiral track of data.

Once the clear pieces of polycarbonate are formed, a thin reflective layer is sputtered onto the disc, covering the bumps. Aluminum is used behind the inner layers, but a semi-reflective gold layer is used for the outer layers, allowing the laser to focus through the outer and onto the inner layers. After all of the layers are made, each one is coated with lacquer, squeezed together and cured under infrared light. For single-sided discs, the label is silk-screened onto the non readable side. Double-sided discs are printed only on the non-readable area near the hole in the middle.

The data tracks on DVD are incredibly small - just 740 nanometres separate one track from the next (a nanometre is a billionth of a metre) and the lengthened bumps that make up the track are each 320 nanometres wide, a minimum of 400 nanometres long and 120 nanometres high.

The microscopic dimensions of the bumps make the spiral track on a DVD extremely long. If you could lift the data track off a single layer of a DVD, and stretch it out into a straight line, it would be almost 7.5 miles long! That means that a double-sided, double-layer DVD would have 30 miles (48 km) of data!

How do DVDs outperform CDs?
Higher density data storage
Single-sided, single-layer DVDs can store about seven times more data than CDs. A large part of this increase comes from the pits and tracks being smaller on DVDs. Let’s try to get an idea of how much more data can be stored due to the physically tighter spacing of pits on a DVD.

The track pitch on a DVD is 2.16 times smaller, and the minimum pit length for a single-layer DVD is 2.08 times smaller than on a CD. By multiplying these two numbers, we find that there is room for about 4.5 times as many pits on a DVD. So where does the rest of the increase come from?

Less overhead, more area
On a CD, there is a lot of extra information encoded on the disc to allow for error correction. This information is really just a repetition of information that is already on the disc. The error correction scheme that a CD uses is quite old and inefficient compared to the method used on DVDs. The DVD format doesn’t waste as much space on error correction, enabling it to store more real information. Another way that DVDs achieve higher capacity is by encoding data onto a slightly larger area of the disc than is done on a CD.

Multi-layer storage
To increase the storage capacity even more, a DVD can have up to four layers, two on each side. The laser that reads the disc can actually focus on the second layer through the first layer. Here is a list of the capacities of different forms of DVDs:

Single-sided/single-layer
Capacity - 4.38 GB
Approx. Movie Time - 2 hours
Single-sided/double-layer
Capacity - 7.95 GB
Approx. Movie Time - 4 hours
Double-sided/single-layer
Capacity - 8.75 GB
Approx. Movie Time - 4.5 hours
Double-sided/double-layer
Capacity - 15.9 GB

Approx. Movie Time - Over 8 hours
You may be wondering why the capacity of a DVD doesn’t double when you add a whole second layer to the disc. This is because when a disc is made with two layers, the pits have to be a little longer on both layers than when a single layer is used, thus reducing the total number of pits that can be etched onto the disk. This helps to avoid interference between the layers, which would cause errors when the disc is played.

Sent in by Thushara Perera
Source: Hardware Jungle

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