CDs are everywhere these days. Whether they are used to hold music, data or computer software, they have become the standard medium for distributing large quantities of information in a reliable package. And if you have a computer and CD-R drive, you can create your own CDs. A CD is a fairly simple piece of plastic, about four one-hundredths (4/100) of an inch (1.2 mm) thick. Most of a CD consists of an injection-molded piece of clear polycarbonate plastic. During manufacturing, this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed onto the acrylic. A CD has a single spiral track of data, circling from the inside of the disc to the outside. The fact that the spiral track starts at the center means that the CD can be smaller than 4.8 inches (12 cm) if desired, and in fact there are now plastic baseball cards and business cards that you can put in a CD player. CD business cards hold about 2 MB of data before the size and shape of the card cuts off the spiral. What the picture on the right does not even begin to impress upon you is how incredibly small the data track is -- it is approximately 0.5 microns wide, with 1.6 microns separating one track from the next. (A micron is a millionth of a meter.) And the bumps are even more miniscule The elongated bumps that make up the track are each 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high. (A nanometer is a billionth of a meter.) Looking through the polycarbonate layer at the bumps, they look something like this: You will often read about "pits" on a CD instead of bumps. They appear as pits on the aluminum side, but on the side the laser reads from, they are bumps. The incredibly small dimensions of the bumps make the spiral track on a CD extremely long. If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost 3.5 miles (5 km) long! The CD player has the job of finding and reading the data stored as bumps on the CD. Considering how small the bumps are, the CD player is an exceptionally precise piece of equipment. The drive consists of three fundamental components: A drive motor spins the disc. This 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 focus in on and read the bumps. A tracking mechanism moves the laser assembly so that the laser's beam can follow the spiral track. The tracking system has to be able to move the laser at micron resolutions. Inside the CD 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 an audio CD) or to the computer (in the case of a CD-ROM drive). The fundamental job of the CD player is to focus the laser on the track of bumps. The laser beam passes through the polycarbonate layer, reflects off the aluminum layer and hits an opto-electronic device that detects changes in light. The bumps reflect light differently than the "lands" (the rest of the aluminum layer), 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 is keeping the laser beam centered on the data track. This centering is the job of the tracking system. The tracking system, as it plays the CD, has to continually move the laser outward. As the laser moves outward from the center of the disc, the bumps move past the laser faster -- 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 (rpm). Therefore, as the laser moves outward, the spindle motor must slow the speed of the CD. That way, the bumps travel past the laser at a constant speed, and the data comes off the disc at a constant rate. If you have a CD-R drive, and want to produce your own audio CDs or CD-ROMs, one of the great things you've got going in your favor is the fact that software can handle all the details for you. You can say to your software, "Please store these songs on this CD," or "Please store these data files on this CD-ROM," and the software will do the rest. Because of this, you don't need to know anything about CD data formatting to create your own CDs. However, CD data formatting is complex and interesting, so let's go into it anyway. Because the laser is tracking the spiral of data using the bumps, there cannot be extended gaps where there are no bumps in the data track. To solve this problem, data is encoded using EFM (eight-fourteen modulation). In EFM, 8-bit bytes are converted to 14 bits, and it is guaranteed by EFM that some of those bits will be 1s. Because the laser wants to be able to move between songs, data needs to be encoded into the music telling the drive "where it is" on the disc. This problem is solved using what is known as subcode data. Subcode data can encode the absolute and relative position of the laser in the track, and can also encode such things as song titles. Because the laser may misread a bump, there need to be error-correcting codes to handle single-bit errors. To solve this problem, extra data bits are added that allow the drive to detect single-bit errors and correct them. Because a scratch or a speck on the CD might cause a whole packet of bytes to be misread (known as a burst error), the drive needs to be able to recover from such an event. This problem is solved by actually interleaving the data on the disc, so that it is stored non-sequentially around one of the disc's circuits. The drive actually reads data one revolution at a time, and un-interleaves the data in order to play it. If a few bytes are misread in music, the worst thing that can happen is a little fuzz during playback. When data is stored on a CD, however, any data error is catastrophic. Therefore, additional error correction codes are used when storing data on a CD-ROM. There are several different formats used to store data on a CD, some widely used and some long-forgotten. The two most common are CD-DA (audio) and CD-ROM (computer data). A DVD works exactly the same as a CD, 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. 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 You can also use DVDs to store music. If you do, you can store almost eight hours of music per side! In 1997, a new technology emerged that brought digital sound and video into homes all over the world. It was called DVD, and it revolutionized the movie industry. The industry is set for yet another revolution with the introduction of Blu-ray Discs (BD) in 2006. With their high storage capacity, Blu-ray discs can hold and play back large quantities of high-definition video and audio, as well as photos, data and other digital content. A current, single-sided, standard DVD can hold 4.7 GB (gigabytes) of information. That's about the size of an average two-hour, standard-definition movie with a few extra features. But a high-definition movie, which has a much clearer image (see How Digital Television Works), takes up about five times more bandwidth and therefore requires a disc with about five times more storage. As TV sets and movie studios make the move to high definition, consumers are going to need playback systems with a lot more storage capacity. Blu-ray is the next-generation digital video disc. It can record, store and play back high-definition video and digital audio, as well as computer data. The advantage to Blu-ray is the sheer amount of information it can hold: A single-layer Blu-ray disc, which is roughly the same size as a DVD, can hold up to 27 GB of data -- that's more than two hours of high-definition video or about 13 hours of standard video. A double-layer Blu-ray disc can store up to 50 GB, enough to hold about 4.5 hours of high-definition video or more than 20 hours of standard video. And there are even plans in the works to develop a disc with twice that amount of storage. If you used to watch movies on videotape, you probably remember the first time you saw one on DVD. Suddenly, the video and sound were of much better quality. You could also pause without distorting the picture, skip from chapter to chapter and zoom in on the screen. When studios started adding commentary tracks, "extras" and multiple sound options on each disc, it seemed like the technology had reached its peak. People couldn't really imagine a better way to watch a recorded movie than on a DVD. Then, TVs got a whole lot bigger. DVDs look best on screens that are smaller than 36 inches, so they're not always up to the challenge of todays High-definition (HD) sets. To store and play HD movies, you need a disc that holds more information, like an HD-DVD. In this article, we'll explore how HD-DVDs differ from DVDs and what's happening in the struggle between HD-DVD and Blu-Ray. The basic idea behind the HD-DVD is really simple -- it looks like a DVD and acts like a DVD, but it holds more information. A DVD holds about two hours of standard definition video, but an HD-DVD can hold about 48 hours. If you already know how DVDs work, you already know a lot about HD-DVDs. A DVD stores information as a series of microscopic pits arranged in a very long spiral. A red laser reads these pits from the other side, so it sees them as bumps. The bumps reflect the laser's light to a sensor. Electronics within the DVD player read the information from the sensor as a digital signal. Check out How DVDs Work to learn more about how a DVD player does this. A simplified view of what happens in a DVD player. An HD-DVD player is a lot like this, but it can send the signal digitally rather than converting it to analog. An HD-DVD uses the same principles -- it contains a bumpy layer that reflects light from a laser to a sensor, creating a digital signal. HD-DVDs are even exactly the same size as DVDs (120 millimeters in diameter and 1.2 millimeters thick). But three important differences allow them to hold quite a bit more information than DVDs: They use 405 nanometer blue-violet lasers rather than 650 nanometer red lasers. The pits are smaller and the tracks are closer together. They use more efficient compression to cut down the size of the files they store. The color of the laser may seem like a trivial change to make. But the shorter wavelength of the blue-violet laser is what allows HD-DVDs pits to be smaller and arranged closer together. In other words, it allows the disc to have a much narrower track pitch. Regular DVDs have a track pitch of 0.74 micrometers, and HD-DVDs have a track pitch of 0.40 micrometers. You can imagine this as the difference between writing with a fine-tipped pen and a magic marker. The difference between a red laser and a blue laser is like the difference between a fine-tipped pen and a magic marker. The other big difference between DVDs and HD-DVDs involves how the information on the disc is compressed. Most DVDs use MPEG-2 compression. HD-DVDs can use MPEG-2, but they typically use the more efficient MPEG-4, which allows higher video quality with a smaller file size. HD-DVDs can also use VC-1 (or Windows Media) compression. Finally, because of general improvements in the technology, an HD-DVD player can read information from the disc and deliver it to the TV about three times as fast as a DVD player can. It can also send the signal to an HDTV digitally using a High Definition Multimedia Interface (HDMI), preventing the quality loss that conversion to analog causes. One of the first questions people ask about HD-DVD (besides "Is it better than Blu-ray?") is whether their old DVDs are about to become obsolete. Let's take a look at what is likely to happen with players and discs as people upgrade. Standard vs. High-definition Wondering what the difference is between standard and high definition? Standard definition (SD) uses 525 lines of pixels from top to bottom. High definition (HD) uses up to 1125 lines. Not all of these lines are visible on the screen, but they're included as part of the signal. If you rent a lot of DVDs, late fees are an unfortunate fact of life. And if you're not the most organized person around, they might even figure prominently in your monthly budget. Forget to return "Battlefield Earth" for a week, and you'll pay more in late fees than the DVD is worth. This, of course, was the video store's plan all along. If this sounds like you, then take heart -- the march of technology may have come to the rescue. A company called Flexplay Technologies has introduced a new type of DVD that sells at rental prices but never needs to be returned. A Flexplay® DVD takes care of the "rental period" itself -- it hits a chemical stopwatch when you open the package, and when your time is up (in 48 hours, say), the disc stops working. Recycle it or turn it into a coaster and consider it returned. And because you don't need a standard rental store set-up, just about any store (even restaurants) could potentially sell the DVDs. It's an interesting idea, and the technology behind it is pretty interesting, too. In this article, we'll find out how these DVDs do what they do. Compatibility and the Competition If you decided to buy an HD-DVD player the first day they hit the market, you'd still be able to play your DVDs on it. On the other hand, if you bought a new movie on HD-DVD, it wouldn't play in your old DVD player. Since an HD-DVD is exactly the same size and shape as a regular DVD, it's pretty easy to make new players that can handle both -- they just need a laser pickup that can read either format. The Toshiba HD-DVD player that released in April 2006 can read DVDs, HD-DVDs and CDs. If HD-DVDs gain widespread use, you should still be able to buy DVDs -- the majority of homes in the United States don't have HDTVs, and there's no point in upgrading to HD-DVD without one. In addition, HD-DVDs can store regular and high-definition content on the same disc. Twin format discs have two layers -- a DVD layer on top layer uses a red laser, and an HD-DVD layer on the bottom uses a blue laser. The outer layer is transparent to the blue laser. The blue laser sees through the outer layer, skipping straight to the high-definition content. The other option for including DVD and HD-DVD content on the same disc is a combination format, which uses a two-sided disc. A red laser can read the DVD side, and a blue laser can read the HD-DVD side. To access the DVD content on a combination HD-DVD, simply flip the disc over. With either option, you can buy one disc that will play in both DVD and HD-DVD players. If these discs become available in stores, they'll be a good choice if you plan to upgrade to high-definition at some point in the future. Twin or combination discs will also be useful for libraries and movie rental stores, since not everyone will be ready to upgrade their player and right away. HD-DVDs aren't the only option for high-definition video playback, though. The other likely candidate for DVD's successor is Blu-ray. Some people plan to put off upgrading their movies and players until there's a clear winner in the "format war" between the two. Up-conversion Regular DVDs generally have enough resolution to look good on 480-line analog displays, which isn't sufficient for new big-screen sets. The Toshiba HD-DVD player can automatically up-convert regular DVDs to accommodate 720- or 1080-line displays. The converted picture won't have all the resolution of an HD-DVD, but it will be better than an analog signal from a DVD player. HD-DVD vs. Blu-ray HD-DVD Facts HD-DVD Capacity: 15 GB single layer, 30 GB dual layer DVD Capacity: 4.7 GB single layer, 8.4 GB dual layer Compression: MPEG-2, MPEG-4, VC-1 High-definition Playback, 15 GB: 4 hours High-definition playback, 50 GB: 8 hours Several companies have developed alternatives to the existing DVD standard. The two forerunners are HD-DVD and Blu-ray. Competition between the two has escalated, drawing inevitable comparisons to the struggle between VHS and Betamax. Here are the highlights: Both formats use blue lasers rather than red. Both have the same options for video and audio compression. Blu-ray offers significantly more storage space -- 50 GB on a dual-layer disc versus HD-DVD's 30 GB. The DVD Forum, which creates DVD standards, has approved HD-DVD and has not approved Blu-ray. HD-DVD is less expensive than Blu-ray: HD-DVDs can be produced on existing equipment, and Blu-ray discs can't. HD-DVD players are selling for $499 (Toshiba HD-A1) to $799 (HD-XA1), and Blu-ray players are selling for around $1,000 (Samsung DB-P1000). HD-DVD players hit the market on April 18, 2006, two months before the first Blu-ray player hit the U.S. market in June, 2006. Along with other companies, Toshiba, Microsoft and Intel have sided with HD-DVD. Microsoft plans to release an add-on HD-DVD drive for its Xbox 360 in November 2006. The Blu-ray Disc Association, on the other hand, has electronics companies like Sony (which plans to release the Blu-ray-equipped PS3 in November) and Pioneer, computer companies like Dell and Apple and entertainment companies like Disney and Fox on its board of directors. Most of the motion picture industry seems to support Blu-ray, in part because the need for new manufacturing equipment might cut down on piracy. Even though Blu-ray seems to have the backing of more of the industry, the battle isn't over. Some companies, like Hewlett Packard, previously supported only Blu-ray but now support both formats. Critics of Blu-ray point out that it may have more capacity than any movie could ever use, even with special features. Many people theorize that HD-DVD will be the winner solely because it is so much cheaper. If you visit sites devoted to HD-DVD and Blu-ray, you might notice that something seems a little fishy with the supposed playback times. According to Toshiba, a 30 GB HD-DVD will hold 8 hours of HD video. According to the Blu-ray disc association, a 50 GB Blu-ray disc will hold 4.5 hours of HD video. So how could a disk that's twice as big hold about half as much information? The likeliest culprits for those numbers not adding up are bit rate and encoding. A file recorded at a low bit rate will take less space. In addition, many early Blu-ray tests used MPEG-2 encoding, which is less efficient than MPEG-4.