Working of DVD's

How DVDs Work

It wasn't really that long ago that VHS tapes dominated the home video market, but now, DVDs have all but wiped them out completely. Going from tape to disc gave the home theater experience a major upgrade, and ushered in an era of feature-packed special edition home video.

In this article, you will learn what a DVD consists of, how a DVD player reads a disc, what to look for when buying a DVD player, a little DVD history and much more.

DVD Disks

DVD Fact The first DVD player hit the market in March 1997.

A DVD is very similar to a CD, but it has a much larger data capacity. A standard DVD holds about seven times more data than a CD does. This huge capacity means that a DVD has enough room to store a full-length, MPEG-2-encoded movie, as well as a lot of other information.

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 surrounding sound
• Subtitles in up to 32 languages

DVD can also be used to store almost eight hours of CD-quality music per side.

The format offers many advantages over VHS tapes:

• 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.

DVD Layers

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

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.

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.

DVD Storage Capacity

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	CD	DVD
Track Pitch	1600 nanometers	740 nanometers
Minimum Pit Length (single-layer DVD)	830 nanometers	400 nanometers
Minimum Pit Length (double-layer DVD)	830 nanometers	440 nanometers

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	Capacity	Approx. Movie Time
Single-sided/single-layer	4.38 GB	2 hours
Single-sided/double-layer	7.95 GB	4 hours
Double-sided/single-layer	8.75 GB	4.5 hours
Double-sided/double-layer	15.9 GB	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. This helps to avoid interference between the layers, which would cause errors when the disc is played.

DVD Video

DVD Fact If an average DVD movie were uncompressed, it would take at least a year to download it over a normal phone line.

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 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 (see How Video Formatting Works for details on these formats). 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.

Did you know? DVDs often have special features hidden on the disc. These "Easter eggs" can be previews of other movies, computer software or music. DVD Review has a listing of some great Easter eggs that viewers have found on DVDs.

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	CD Audio	DVD Audio
Sampling Rate	44.1 kHz	192 kHz
Samples Per Second	44,100	192,000
Sampling Accuracy	16-bit	24-bit
Number of Possible Output Levels	65,536	16,777,216

In an audio CD or DVD, each bit represents a digital command telling the DAC what voltage level to output (see How Analog and Digital Recording Works for details). 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.

Did you know? Some DVDs carry commentary tracks, in which the filmmaker talks about the movie while it is running. This can be very exciting for true film buffs. DVDs can also contain extra, previously unreleased scenes. And a DVD is sometimes a director's cut -- the film as the director originally intended it.

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

DVD Fact The Sony PlayStation 2 was the first video game system able to play DVDs.

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 (see How CDs Work for details). The DVD player decodes the MPEG-2 encoded movie, turning it into a standard composite video signal (see How Television Works for details). The player also decodes the audio stream and sends it to a Dolby decoder, where it is amplified and sent to the speakers.

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.

DVD Player Features

DVD Extras Because DVDs are so durable, film aficionados can watch a favorite movie repeatedly without the disc losing its quality. This is also good for parents whose children like to watch the same movies over and over again. DVD players allow you to find scenes, play them in slow motion or freeze a scene, and the video quality will remain the same. Software loaded from DVD, as opposed to CD-ROM, can contain more information. An entire encyclopedia can fit onto one DVD, whereas other formats would require multiple discs.

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 surrounding 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.)

Audio and Video Outputs

Video Outputs

• Component-video outputs
  These outputs provide the highest quality video signal to your TV. They are quite rare right now; only the newest high-end TVs can support them. But, if you have such a TV, you'll definitely want a DVD player with component video outputs.

  There are three separate connectors for component video output. The player pictured below has one component video output.

  DVD player video outputs from left to right: composite video, s-video, component video

• S-video outputs
  TVs with this type of connection are more common. S-video provides a very good picture quality, and every DVD player has at least one of these outputs. The player pictured above has two of them.

• Composite-video outputs
  These are the most common type of output, and they provide adequate picture quality. Usually, they have a yellow plastic insert. The player pictured above has two of these outputs.

Audio Outputs

• Coaxial digital output and optical digital output
  These outputs provide the highest-quality audio. They send the digital sound information to the receiver for decoding. You can use either one of these if you have a Dolby Digital receiver.

  DVD player audio outputs

• 5.1 channel outputs
  5.1 channel is a set of six analog outputs, one for each of the Dolby Digital channels (left front, center front, right front, left rear, right rear and subwoofer). The DVD player decodes the Dolby Digital signal and uses its own DAC to output an analog signal. These are the outputs you'll need to use if you are hooking the DVD player up to a "Dolby Digital ready" receiver.

  DVD players with 5.1 channel outputs will always have Dolby Digital decoders, and they may or may not have DTS decoders. If you have a "Dolby Digital ready" receiver and you want DTS sound, you will need a DVD player with a built-in DTS decoder.

• Stereo outputs
  These outputs carry only the stereo music signal. You would use these if you were hooking your DVD player up to a TV that has only two speakers.

Connecting the DVD Player

Connecting a DVD player to your stereo receiver (or television, if you don't have a receiver) involves making two basic connections: audio and video.

Audio
The first connection to make is for the audio portion of the signal. There will be several options depending on the receiver you have.

• The best choice (if available) is either to use an optical (also called Tos-link) or coaxial (RCA) digital connection. These two choices are equal in quality. In order to use either of these, you will need to have both an output on the DVD player, and an input on the receiver. Only receivers with built-in Dolby Digital decoders will have this type of input.

  The audio outputs on a DVD player

• If your receiver does not have a built-in Dolby Digital or DTS decoder, but is "Dolby Digital ready," look for the 5.1-channel Dolby or 5.1-channel DTS. This connection involves six cables, corresponding to different speaker channels: left front, center front, right front, left rear, right rear and subwoofer.

• The last option to connect the two components is with analog RCA outputs. This is a two-cable connection, with one cable delivering the left speaker sound, and the other cable delivering the right. This connection will deliver only stereo sound, but it may be your only option if you are hooking up directly to a television, or if you have an old receiver with only two channels.

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 (see How Analog and Digital Recording Works). 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.

Now let's take a look at the video connection.

• The best quality choice is to use component connection. This connection consists of three cables: color-labeled red, blue and green. The quality is superb. However, these connections only exist on extremely high-end receivers and television sets.

  The video outputs on a DVD player

• The next option is s-video. One cable connects the DVD player to the receiver in this setup.

• The last option, similar to the audio setup, is to use the analog RCA video output, usually color-labeled yellow on both ends. This will deliver the lowest quality, but will suffice for most older, analog televisions.

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 (see How Analog and Digital Recording Works). 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.

DVD FAQ

• What does "DVD" stand for?
  "DVD" stands for digital versatile disc, but some sources declare that it doesn't stand for anything anymore.

• Can I record television shows or movies on a DVD player?
  Yes, you can -- if your DVD player is also a DVD recorder.

• Can I play CDs on a DVD player?
  Yes. DVD players are completely compatible with audio compact discs. And music will become increasingly available in DVD format. See What is the difference between DVD-audio and CDs? for more discussion of the DVD audio format.

• What is the difference between DVD+R and DVD-R?
  DVD+ ("plus") and DVD- ("dash") are two competing DVD formats. You may remember the "war" between the Betamax and VHS formats for domination of the VCR market. The big difference with DVD+R vs. DVD-R is that there are hybrid (dual-format) drives capable of reading both types. Many companies have taken sides -- the DVD Forum is a group of manufacturers that support DVD-R, while the DVD+RW Alliance supports that format. Consumers have yet to make either format the winner.

• What are region codes?
  Movie studios use region Codes on DVDs to thwart unauthorized copying, and to control the release dates of DVD movies. The actual region code is stored in one byte on the DVD. The DVD player or drive has a region code in its firmware. Personal computer DVD-ROM players often have the code in the software or in the MPEG-2 decoder.

  For the player or drive to play the movie, the two codes must match. The code is also printed on the back of a DVD package, superimposed on a small image of the globe. If you have a DVD that was made for release in Asia, you won't be able to play it on a DVD player intended for use in Australia.

• What is the difference between DVD-audio and CDs?
• If you have read the article How CDs Work (as well as How Analog and Digital Recording Works), then you know that CDs store music as digital samples. The sampling rate is 44,100 samples per second, and the analog signal is converted to a 16-bit binary number at each sample. The sampling is done on two channels (one for each speaker in a stereo system).

  If you have read How DVDs Work, you know that DVDs are very similar to CDs, but they hold more data -- 4.7 gigabytes for a DVD compared to 650 megabytes for a CD. Two-layer DVDs can store twice that.

  The DVD-audio standard uses the extra data space on a DVD to do two things:

• Increase the sampling rate and quantization levels dramatically
  Although there are a number of options, DVD-audio typically uses 96,000 samples per second and 24 bits per sample. In other words, there are more than twice as many samples per second and 256 times more quantization levels.

• Record in surrounding sound (six channels instead of two)

­ DVD-audio has the potential to replace CDs because of the higher quality of the sound. However, there are several things that might prevent widespread acceptance of DVD-audio:

• The standard has taken some time to gel, and it came some time after DVD-video and its players were released. Many people with DVD players today would have to replace their players to listen to DVD-audio discs.
• It is not clear that most consumers care about the difference in sound quality, since it is fairly subtle.
• There is a competing standard from Sony/Philips called the Super Audio CD that claims to have even better sound quality.