Almost every movie produced today is available on DVD, and a lot of older movies are also being formatted. A movie usually comes out on DVD before it comes out on video tape, because the manufacturing and distribution costs for DVDs are so much lower!
By bringing incredible picture and sound to our favorite films, the DVD format is doing for movies exactly what CDs did for music.
A DVD looks pretty much like a CD -- it's just been designed to hold more data.
A DVD is very similar to a CD, but it has a much bigger data capacity. A standard DVD holds about 7x more data than a CD. This increased capacity means that a DVD has enough room to store a full-length, MPEG-2-encoded movie, as well as lots of cool extras.
Here are the 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
DVD can also be used to store almost eight hours of CD-quality music per side.
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 theater.
Many DVD movies have an on-screen index, where the creator of the DVD has labeled 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. Foreign movies may give you the choice between the version dubbed into your language, or the original soundtrack with subtitles in your language
Storing Data on a DVD:
DVDs are the same size as CDs, and they're 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 made of about 1.2 millimeters of plastic. Each layer is created by injection molding polycarbonate plastic. This process forms a disc that has microscopic bumps arranged as a single, continuous and extremely long spiral track of data. We'll get back to the little bumps later.
Once the clear pieces of polycarbonate are formed, a thin reflective layer is sputtered onto the disc, which covers 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 the layers are made, each one is coated with lacquer, squeezed together, and cured under infra-red light. For one-sided discs, the label is silk-screened onto the nonreadable side. Double-sided disc labels are printed only on the nonreadable area near the hole in the middle of the disc.
It's Really Small:
Data Tracks on a DVD
Each writable layer of a DVD has a spiral track of data. On single-layer DVDs, the track always circles from the inside of the disc to the outside. That the spiral track starts at the center means that a single-layer DVD can be smaller than 12 centimeters if desired.
The data track is incredibly tiny just 740 nanometers (a nanometer is a billionth of a meter) separate one track from the next. And the elongated bumps that make up the track are each 320 nanometers wide, a minimum of 400 nanometers long and 120 nanometers high!
You will often read about "pits" on a DVD instead of bumps. They appear as pits on the aluminum side, but on the side that the laser reads from, they are bumps.
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 of data!
To read bumps this small you need an incredibly precise disc-reading mechanism.
Data Storage: DVD vs. CD:
DVDs can store more data than CDs for a few reasons:
Higher-density data storage
Less overhead, more area
Higher Density Data Storage
As mentioned before, single-sided, single-layer DVDs can store about 7x more data than CDs. A large part of this increase comes from the fact that the pits and tracks are smaller on DVDs.
Let's try to get an idea of how much more data can be stored because of the physically tighter spacing of pits on a DVD. The track pitch on a DVD is 2.16x smaller, and the minimum pit length for a single-layer DVD is 2.08x smaller than on a CD. By multiplying these two numbers, we find theres room for about 4.5x as many pits on a DVD. So where does the rest of the increase come from?
Less Overhead, More Area:
On a CD, theres lots 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 much more 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.
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 lyer. Here is a list of the capacities of different forms of DVDs:
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. This helps to avoid interference between the layers, which would cause errors when the disc is played.
The DVD Video Format:
Even though its storage capacity is huge, the uncompressed video data of a full-length movie would never fit on a DVD. In order to fit a movie on a DVD, you need video compression. A group called the Moving Picture Experts Group (MPEG) establishes the standards for compressing moving pictures.
When movies are put onto DVDs, they are encoded in MPEG-2 format and then stored on the disc. This compression format is a widely accepted international standard. Your DVD player contains an MPEG-2 decoder, which can uncompress this data as quickly as you can watch it.
The MPEG-2 Format and Data Size Reduction:
A movie is usually filmed at a rate of 24 frames per second. This means that every second, there are 24 complete images displayed on the movie screen. American and Japanese television uses a format called NTSC, which displays a total of 30 frames per second; but it does this in a sequence of 60 fields, each of which contains alternating lines of the picture. Other countries use PAL format, which displays at 50 fields per second, but at a higher resolution. Because of the differences in frame rate and resolution, an MPEG movie needs to be formatted for either the NTSC or the PAL system.
The MPEG encoder that creates the compressed movie file analyzes each frame and decides how to encode it. The compression uses some of the same technology as still image compression does to eliminate redundant or irrelevant data. It also uses information from other frames to reduce the overall size of the file. Each frame can be encoded in one of three ways:
As an intraframe - An intraframe contains the complete image data for that frame. This method of encoding provides the least compression.
As a predicted frame - A predicted frame contains just enough information to tell the DVD player how to display the frame based on the most recently displayed intraframe or predicted frame. This means that the frame contains only the data that relates to how the picture has changed from the previous frame.
As a bidirectional frame - In order to display this type of frame, the player must have the information from the surrounding intraframe or predicted frames. Using data from the closest surrounding frames, it uses interpolation (something like averaging) to calculate the position and color of each pixel.
Depending on the type of scene being converted, the encoder will decide which types of frames to use. If a newscast were being converted, a lot more predicted frames could be used, because most of the scene is unaltered from one frame to the next. On the other hand, if a very fast action scene were being converted, in which things changed very quickly from one frame to the next, more intraframes would have to be encoded. The newscast would compress to a much smaller size than the action sequence.
If all of this sounds complicated, then you are starting to get a feeling for how much work your DVD player does to decode an MPEG-2 movie. A lot of processing power is required; even some computers with DVD players can't keep up with the processing required to play a DVD movie.
The DVD Player:
A DVD player is very similar to a CD player. It has a laser assembly that shines the laser beam onto the surface of the disc to read the pattern of bumps. The DVD player decodes the MPEG-2 encoded movie, turning it into a standard composite video signal. The player also decodes the audio stream and sends it to a Dolby Decoder, where it is amplified and sent to the speakers.
Inside a DVD Drive:
The DVD player has the job of finding and reading the data stored as bumps on the DVD. Considering how small the bumps are, the DVD player has to be an exceptionally precise piece of equipment. The drive consists of three fundamental components:
Reading a DVD:
Inside the DVD player, there is a good bit of computer technology involved in forming the data into understandable data blocks, and sending them either to the DAC, in the case of audio or video data, or directly to another component in digital format, in the case of digital video or data.
The fundamental job of the DVD player is to focus the laser on the track of bumps. The laser can focus either on the semi-transparent reflective material behind the closest layer, or, in the case of a double-layer disc, through this layer and onto the reflective material behind the inner layer. The laser beam passes through the polycarbonate layer, bounces off the reflective layer behind it and hits an opto-electronic device, which detects changes in light. The bumps reflect light differently than the "lands," the flat areas of the disc, and the opto-electronic sensor detects that change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes.