A DVD works exactly the same way, but it can hold a lot more information -- about 4.7 gigabytes (about seven times as much as a CD). DVDs can hold more data than CDs because the bumps are smaller and the tracks are closer together, giving DVDs more storage space. Here are the typical contents of a movie stored on a DVD:
- Up to 133 minutes of high-resolution video in letterbox or pan-and-scan format, at 720 dots of horizontal resolution (The video compression ratio is typically 40:1 under MPEG-2.)
- Soundtrack presented in up to eight languages using 5.1 channel Dolby digital surround sound
- Subtitles in up to 32 languages
Storing Data on a DVD
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, totaling about 1.2 millimeters thick. 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. More on the bumps later.
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 nonreadable side. Double-sided discs are printed only on the nonreadable area near the hole in the middle. Cross sections of the various types of completed DVDs (not to scale) look like this:
 DVD formats |
 Data tracks on a DVD |
What the image to the right cannot impress upon you is how incredibly tiny the data track is -- just 740 nanometers separate one track from the next (a nanometer is a billionth of a meter). And the elongated bumps that make up the track are each 320 nanometers wide, a minimum of 400 nanometers long and 120 nanometers high. The following figure illustrates looking through the polycarbonate layer at the bumps.
 DVD pit layout |
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 (48 km) 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
- Multi-layer storage
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.
| Specification | ||
| Track Pitch | ||
| Minimum Pit Length (single-layer DVD) | ||
| Minimum Pit Length (double-layer DVD) |
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 much 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:
| Format | ||
| Single-sided/single-layer | ||
| Single-sided/double-layer | ||
| Double-sided/single-layer | ||
| Double-sided/double-layer |
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
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 use 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.
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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.
DVD Audio
DVD audio and DVD video are different formats. DVD audio discs and players are relatively rare right now, but they will become more common, and the difference in sound quality should be noticeable. In order to take advantage of higher-quality DVD audio discs, you will need a DVD player with a 192kHz/24-bit digital-to-analog converter (DAC). Most DVD players have only a 96kHz/24-bit digital-to-analog converter. So if you want to be able to listen to DVD audio discs, be sure to look for a DVD audio player with a 192kHz/24-bit digital-to-analog converter.DVD audio recordings can provide far better sound quality than CDs. The chart below lists the sampling rate and accuracy for CD recordings and the maximum sampling rate and accuracy for DVD recordings. CDs can hold 74 minutes of music. DVD audio discs can hold 74 minutes of music at their highest quality level, 192kHz/24-bit audio. By lowering either the sampling rate or the accuracy, DVDs can be made to hold more music. A DVD audio disc can store up to two hours of 6-channel, better than CD quality, 96kHz/24-bit music. Lower the specifications further, and a DVD audio disc can hold almost seven hours of CD-quality audio.
| Specification | ||
| Sampling Rate | ||
| Samples Per Second | ||
| Sampling Accuracy | ||
| Number of Possible Output Levels |
In an audio CD or DVD, each bit represents a digital command telling the DAC what voltage level to output . While an ideal recording would follow the raw waveform exactly, digital recordings sample the sound at different frequencies, and therefore lose some of the data.
 Comparison of a raw audio signal to the CD audio and DVD audio output |
The graph above shows how the highest quality DVD audio compares to CD audio. You can see that DVD follows the signal more closely, but it's still a long way from perfect.
To get the full experience of the Dolby Digital sound used on many DVDs, you need a home theater system with five speakers, a subwoofer, and a receiver that is either "Dolby Digital ready" or has a built-in Dolby Digital decoder.
If your receiver is Dolby Digital ready, then it does not have a Dolby Digital decoder, so you need to buy a DVD player with its own Dolby Digital decoder and 5.1 channel outputs. If you also want your system to be compatible with DTS sound, then your DVD player will need a DTS decoder, too.
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If your receiver has its own Dolby Digital decoder and DTS decoder, then you don't need a DVD player with 5.1 channel outputs, and you can save some money on cables by using the digital outputs.
The DVD Player
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:
- A drive motor to spin the disc - The drive motor is precisely controlled to rotate between 200 and 500 rpm, depending on which track is being read.
- A laser and a lens system to focus in on the bumps and read them - The light from this laser has a smaller wavelength (640 nanometers) than the light from the laser in a CD player (780 nanometers), which allows the DVD laser to focus on the smaller DVD pits.
- A tracking mechanism that can move the laser assembly so the laser beam can follow the spiral track - The tracking system has to be able to move the laser at micron resolutions.
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.
The hardest part of reading a DVD is keeping the laser beam centered on the data track. This centering is the job of the tracking system. As the DVD is played, the tracking system has to move the laser continually outward. As the laser moves outward from the center of the disc, the bumps move past the laser at an increasing speed. This happens because the linear, or tangential, speed of the bumps is equal to the radius times the speed at which the disc is revolving. So, as the laser moves outward, the spindle motor must slow the spinning of the DVD so that the bumps travel past the laser at a constant speed, and the data comes off the disc at a constant rate.
An interesting thing to note is that if a DVD has a second layer, the start of that layer's data track can be at the outside of the disc instead of the inside. This allows the player to transition quickly from one layer to the next, without a delay in data output, because it doesn't have to move the laser back to the center of the disc to read the next layer.
Supported Formats
- DVD movies
Just about all players play DVD movies. - Music CDs
Most players also play music CDs. - Video CDs
Some players can handle this format, which is used mostly for music videos and some movies from foreign countries. - CD-Rs
Some players can play content that you create on your own computer. - Audio DVDs
A few players can handle this format for high-quality audio.
Other Features
- Dolby Digital decoder
This feature allows the DVD player to decode the Dolby Digital information from a DVD and convert it to six separate analog channels. This feature is not necessary if you have a Dolby Digital receiver, which has a digital input that carries all of the audio information. - DTS decoder
This feature allows the DVD player to decode the DTS information from a DVD and convert it to six separate analog channels. Again, this feature is not necessary if you have a receiver with a DTS decoder. - DTS compatible
All DVD players are DTS compatible. They pass the digital audio information on to the receiver, which then decodes it. - Simulated surround
If you are going to hook the DVD player up to a TV or a stereo system with only two speakers, a DVD player with simulated surround processing will give you some sense of surround sound without the extra speakers. - Disc capacity
Some DVD players can hold three, five or even several hundred discs. Since most DVD players can also play audio CDs, if you buy a player with a high disc capacity you could store your whole CD collection in the machine. - 96kHz/24-bit DAC
This is the speed and accuracy of the digital-to-analog converter (DAC), which converts the audio information into an analog signal. Most movie soundtracks are encoded in this format, so this is really a required feature, and most DVD players will have at least a 96kHz/24-bit DAC. - 192kHz/24-bit DAC
This is an upcoming format for audio-only DVDs that are recorded at speeds of up to 192kHz and 24-bits. Only the newest DVD audio players have the 192kHz/24-bit DAC required to play these audio discs. - Remote-control type
DVD players may come with three types of remotes:- A dedicated remote, which only runs the DVD player
- A multibrand remote, which can control other components, like VCRs and TVs made by other manufacturers (Usually, they only support the more common brands.)
- A learning remote, which can learn the signals from other remotes and assign them to a button (This feature is useful if you have uncommon brands of components to control.)
- A dedicated remote, which only runs the DVD player
DVDs and Laser Discs
Laser disc is an older technology. It offered a better picture and better sound than videotapes, and it is comparable to DVD. But the laser disc format is analog; DVDs are digital. Laser discs are only used for prerecorded movies, and they are larger, about 12 inches in diameter, instead the 5-inch diameter of DVDs. The two formats usually can't be played on the same machine.Laser discs, like DVDs, allow viewers to go to the exact scene they wish to see, and to freeze a frame or slow the picture. Laser discs can only hold an hour on each side, so you have to flip the disc to watch the second half of the movie.
Because of DVD compression techniques, DVDs can hold more data. You rarely have to flip a DVD to watch a whole movie. Laser disc players are noisier than DVD players, and they can sometimes suffer "laser rot" -- the aluminum side of the disc oxidizes, and the quality of the disc deteriorates. DVDs are less likely to have this problem, because manufacturing techniques have improved. As the popularity of DVD grows, laser discs are becoming harder to find.
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