DVD
| DVD stands for digital versatile disc and in simplest terms is a high-capacity CD. In fact, every DVD-ROM drive is a CD-ROM drive; that is, they can read CDs as well as DVDs (many standalone DVD players can't read CD-R or CD-RW discs, however). DVD uses the same optical technology as CD, with the main difference being higher density. The DVD standard dramatically increases the storage capacity of, and therefore the useful applications for, CD-ROM-sized discs. A CD-ROM can hold a maximum of about 737MB (80-minute disc) of data, which might sound like a lot but is simply not enough for many up-and-coming applications, especially where the use of video is concerned. DVDs, on the other hand, can hold up to 4.7GB (single layer) or 8.5GB (dual layer) on a single side of the disc, which is more than 11 1/2 times greater than a CD. Double-sided DVDs can hold up to twice that amount, although you currently must manually flip the disc over to read the other side. Up to two layers of information can be recorded to DVDs, with an initial storage capacity of 4.7GB of digital information on a single-sided, single-layer disca disk that is the same overall diameter and thickness of a current CD-ROM. With Moving Picture Experts Groupstandard 2 (MPEG-2) compression, that's enough to contain approximately 133 minutes of video, which is enough for a full-length, full-screen, full-motion feature filmincluding three channels of CD-quality audio and four channels of subtitles. Using both layers, a single-sided disc could easily hold 240 minutes of video or more. This initial capacity is no coincidence; the creation of DVD was driven by the film industry, which has long sought a storage medium cheaper and more durable than videotape. Note It is important to know the difference between the DVD-Video and DVD-ROM standards. DVD-Video discs contain only video programs and are intended to be played in a DVD player connected to a television and possibly an audio system. DVD-ROM is a data storage medium intended for use by PCs and other types of computers. The distinction is similar to that between an audio CD and a CD-ROM. Computers might be capable of playing audio CDs as well as CD-ROMs, but dedicated audio CD players can't use a CD-ROM's data tracks. Likewise, computer DVD drives can play DVD-video discs (with MPEG-2 decoding in either hardware or software), but DVD video players can't access data on a DVD-ROM. This is the reason you must select the type of DVD you are trying to create when you make a writable or rewritable DVD. The initial application for DVDs was as an upgrade for CDs as well as a replacement for prerecorded videotapes. DVDs can be rented or purchased like prerecorded VCR tapes, but they offer much higher resolution and quality with greater content. As with CDs, which initially were designed only for music, DVDs have since developed into a wider range of uses, including computer data storage and high-quality audio. DVD HistoryDVD had a somewhat rocky start. During 1995, two competing standards for high-capacity CD-ROM drives were being developed to compete with each other for future market share. One standard, called Multimedia CD, was introduced and backed by Philips and Sony, whereas a competing standard, called the Super Density (SD) disc, was introduced and backed by Toshiba, Time Warner, and several other companies. If both standards had hit the market as is, consumers as well as entertainment and software producers would have been in a quandary over which one to choose. Fearing a repeat of the Beta/VHS situation that occurred in the videotape market, several organizations, including the Hollywood Video Disc Advisory Group and the Computer Industry Technical Working Group, banded together to form a consortium to develop and control the DVD standard. The consortium insisted on a single format for the industry and refused to endorse either competing proposal. With this incentive, both groups worked out an agreement on a single, new, high-capacity CD-type disc in September 1995. The new standard combined elements of both previously proposed standards and was called DVD, which originally stood for digital video disc, but has since been changed to digital versatile disc. The single DVD standard has avoided a confusing replay of the VHS versus Beta tape fiasco for movie fans and has given the software, hardware, and movie industries a single, unified standard to support. After agreeing on copy protection and other items, the DVD-ROM and DVD-Video standards were officially announced in late 1996. Players, drives, and discs were announced in January 1997 at the Consumer Electronics Show (CES) in Las Vegas, and the players and discs became available in March 1997. The initial players were about $1,000 each. Only 36 movies were released in the first wave, and they were available only in seven cities nationwide (Chicago, Dallas, Los Angeles, New York, San Francisco, Seattle, and Washington, D.C.) until August 1997 when the full release began. After a somewhat rocky start (much had to do with agreements on copy protection to get the movie companies to go along, and there was a lack of titles available in the beginning), DVD has become an incredible success. It will continue to grow as DVD moves from a read-only to a fully rewritable consumer as well as computer device. The organization that controls the DVD video standard is called the DVD Forum and was founded by 10 companies, including Hitachi, Matsushita, Mitsubishi, Victor, Pioneer, Sony, Toshiba, Philips, Thomson, and Time Warner. Since its founding in April 1997, more than 230 companies have joined the forum. Because it is a public forum, anybody can join and attend the meetings; the site for the DVD forum is www.dvdforum.org. Because the DVD Forum was unable to agree on a universal recordable format, its members who are primarily responsible for CD and DVD technology (Philips, Sony, and others) split off to form the DVD+RW Alliance in June 2000; their site is www.dvdrw.com. They have since introduced the DVD+RW format, which is the fastest, most flexible and backward-compatible recordable DVD format. DVD+RW might someday replace the VCR at home, and it's already well on its way to replacing the CD-RW and floppy drives in your PC. DVD Construction and TechnologyDVD technology is similar to CD technology. Both use the same size (120mm diameter, 1.2mm thick, with a 15mm hole in the center) discs with pits and lands stamped in a polycarbonate base. Unlike a CD, though, DVDs can have two layers of recordings on a side and be double-sided as well. Each layer is separately stamped, and they are all bonded together to make the final 1.2mm-thick disc. The manufacturing process is largely the same, with the exception that each layer on each side is stamped from a separate piece of polycarbonate plastic and are then bonded together to form the completed disc. The main difference between CD and DVD is that DVD is a higher-density recording read by a laser with a shorter wavelength, focused more closely to the disc, which enables more information to be stored. Also, whereas CDs are single-sided and have only one layer of stamped pits and lands, DVDs can have up to two layers per side and can have information on both sides. As with CDs, each layer is stamped or molded with a single physical track in a spiral configuration starting from the inside of the disc and spiraling outward. The disc rotates counterclockwise (as viewed from the bottom), and each spiral track contains pits (bumps) and lands (flat portions) just as on a CD. Each recorded layer is coated with a thin film of metal to reflect the laser light. The outer layer has a thinner coating to allow the light to pass through to read the inner layer. If the disc is single-sided, a label can be placed on top; if it's double-sided, only a small ring near the center provides room for labeling. Just as with a CD, reading the information back on a DVD is a matter of bouncing a low-powered laser beam off one of the reflective layers in the disc. The laser shines a focused beam on the underside of the disc, and a photosensitive receptor detects when the light is reflected back. When the light hits a land (flat spot) on the track, the light is reflected back; when the light hits a pit (raised bump), the phase differential between the projected and reflected light causes the waves to cancel and no light is reflected back. The individual pits on a DVD are 0.105 microns deep and 0.4 microns wide. Both the pits and lands vary in length from about 0.4 microns at their shortest to about 1.9 microns at their longest (on single-layer discs). Refer to the section "CD-ROM Construction and Technology," earlier in this chapter, for more information on how the pits and lands are read and converted into actual data, as well as how the drives physically and mechanically work. DVD uses the same optical laser read pit and land storage that CDs do. The greater capacity is made possible by several factors, including the following:
The DVD disc's pits and lands are much smaller and closer together than those on a CD, allowing the same physical-sized platter to hold much more information. Figure 11.6 shows how the grooved tracks with pits and lands are just over four times as dense on a DVD as compared to a CD. Figure 11.6. DVD data markings (pits and lands) versus those of a standard CD. DVD drives use a shorter wavelength laser to read these smaller pits and lands. A DVD can have nearly double the initial capacity by using two separate layers on one side of a disc and double it again by using both sides of the disc. The second data layer is written to a separate substrate below the first layer, which is then made semireflective to enable the laser to penetrate to the substrate beneath it. By focusing the laser on one of the two layers, the drive can read roughly twice the amount of data from the same surface area. DVD Tracks and SectorsThe pits are stamped into a single spiral track (per layer) with a spacing of 0.74 microns between turns, corresponding to a track density of 1,351 turns per millimeter or 34,324 turns per inch. This equates to a total of 49,324 turns and a total track length of 11.8km or 7.35 miles in length. The track is comprised of sectors, with each sector containing 2,048 bytes of data. The disc is divided into four main areas:
The center hole in a DVD is 15mm in diameter, so it has a radius of 7.5mm from the center of the disc. From the edge of the center hole to a point at a radius of 16.5mm is the hub clamp area. The lead-in zone starts at a radius of 22mm from the center of the disc. The data zone starts at a radius of 24mm from the center and is followed by the lead-out (or middle) zone at 58mm. The disc track officially ends at 58.5mm, which is followed by a 1.5mm blank area to the edge of the disc. Figure 11.7 shows these zones in actual relative scale as they appear on a DVD. Figure 11.7. Areas on a DVD (side view). Officially, the spiral track of a standard DVD starts with the lead-in zone and ends at the finish of the lead-out zone. This single spiral track is about 11.84 kilometers or 7.35 miles long. An interesting fact is that in a 20x CAV drive, when reading the outer part of the track, the data moves at an actual speed of 156 miles per hour (251km/h) past the laser. What is more amazing is that even when the data is traveling at that speed, the laser pickup can accurately read bits (pit/land transitions) spaced as little as only 0.4 microns or 15.75 millionths of an inch apart! DVDs come in both single- and dual-layer as well as single- and double-sided versions. The double-sided discs are essentially the same as two single-sided discs glued together back to back, but subtle differences do exist between the single- and dual-layer discs. Table 11.6 shows some of the basic information about DVD technology, including single- and dual-layer DVDs. The dual-layer versions are recorded with slightly longer pits, resulting in slightly less information being stored in each layer.
As you can see from the information in Table 11.6, the spiral track is divided into sectors that are stored at the rate of 676 sectors per second. Each sector contains 2,048 bytes of data. When being written, the sectors are first formatted into data frames of 2,064 bytes: 2,048 are data, 4 bytes contain ID information, 2 bytes contain ID error detection (IED) codes, 6 bytes contain copyright information, and 4 bytes contain EDC for the frame. The data frames then have ECC information added to convert them into ECC frames. Each ECC frame contains the 2,064-byte data frame plus 182 parity outer (PO) bytes and 120 parity inner (PI) bytes, for a total of 2,366 bytes for each ECC frame. Finally, the ECC frames are converted into physical sectors on the disc. This is done by taking 91 bytes at a time from the ECC frame and converting them into recorded bits via 8 to 16 modulation. This is where each byte (8 bits) is converted into a special 16-bit value, which is selected from a table. These values are designed using an RLL 2,10 scheme, which is designed so that the encoded information never has a run of fewer than 2 or more than 10 0 bits in a row. After each group of 91 bytes is converted via the 8 to 16 modulation, 32 bits (4 bytes) of synchronization information is added. After the entire ECC frame is converted into a physical sector, 4,836 total bytes are stored. Table 11.7 shows the sector, frame, and audio data calculations.
Unlike CDs, DVDs do not use subcodes and instead use the ID bytes in each data frame to store the sector number and information about the sectors. Handling ErrorsDVDs use more powerful error correcting codes than were first devised for CDs. Unlike CDs, which have different levels of error correction depending on whether audio/video or data is being stored, DVDs treat all information equally and apply the full error correction to all sectors. The main error correcting in DVDs takes place in the ECC frame. Parity outer (column) and parity inner (row) bits are added to detect and correct errors. The scheme is simple and yet very effective. The information from the data frames is first broken up into 192 rows of 172 bytes each. Then, a polynomial equation is used to calculate and add 10 PI bytes to each row, making the rows now 182 bytes each. Finally, another polynomial equation is used to calculate 16 PO (Parity Outer) bytes for each column, resulting in 16 bytes (rows) being added to each column. What started out as 192 rows of 172 bytes becomes 208 rows of 182 bytes with the PI and PO information added. The function of the PI and PO bytes can be explained with a simple example using simple parity. In this example, 2 bytes are stored (01001110 = N, 01001111 = O). To add the error correcting information, they are organized in rows, as shown in the following: Data bits 1 2 3 4 5 6 7 8 ------------------------------- Byte 1 0 1 0 0 1 1 1 0 Byte 2 0 1 0 0 1 1 1 1 ------------------------------- Then, 1 PI bit is added for each row, using odd parity. This means you count up the 1 bits: In the first row there are 4, so the parity bit is created as a 1, making the sum an odd number. In the second row, the parity bit is a 0 because the sum of the 1s was already an odd number. The result is as follows: Data bits | 1 2 3 4 5 6 7 8 | PI -----------------------------|--------- Byte 1 0 1 0 0 1 1 1 0 | 1 Byte 2 0 1 0 0 1 1 1 1 | 0 -----------------------------|--------- Next, the parity bits for each column are added and calculated the same as before. In other words, the parity bit will be such that the sum of the 1s in each column is an odd number. The result is as follows: Data bits | 1 2 3 4 5 6 7 8 | PI ---------------------------------|--------- Byte 1 0 1 0 0 1 1 1 0 | 1 Byte 2 0 1 0 0 1 1 1 1 | 0 ---------------------------------|--------- PO 1 1 1 1 1 1 1 0 | 1 Now the code is complete, and the extra bits are stored along with the data. So, instead of just the 2 bytes being stored, 11 additional bits are stored for error correction. When the data is read back, the error correction bit calculations are repeated and they're checked to see whether they are the same as before. To see how it works, let's change one of the data bits (due to a read error) and recalculate the error correcting bits as follows: Data bits | 1 2 3 4 5 6 7 8 | PI ---------------------------------|--------- Byte 1 0 1 0 0 1 0 1 0 | 0 Byte 2 0 1 0 0 1 1 1 1 | 0 ---------------------------------|--------- PO 1 1 1 1 1 0 1 0 | 1 Now, when you compare the PI and PO bits you calculated after reading the data to what was originally stored, you see a change in the PI bit for byte (row) 1 and in the PO bit for bit (column) 6. This identifies the precise row and column where the error was, which is at byte 1 (row 1), bit 6 (column 6). That bit was read as a 0, and you now know it is wrong, so it must have been a 1. The error correction circuitry then simply changes it back to a 1 before passing it back to the system. As you can see, with some extra information added to each row and column, error correction codes can indeed detect and correct errors on-the-fly. Besides the ECC frames, DVDs also scramble the data in the frames using a bit-shift technique and also interleave parts of the ECC frames when they are actually recorded on the disc. These schemes serve to store the data somewhat out of sequence, preventing a scratch from corrupting consecutive pieces of data. DVD Capacity (Sides and Layers)Four main types of DVDs are available, categorized by whether they are single- or double-sided, and single- or dual-layered. They are designated as follows:
Figure 11.8 shows the construction of each of the DVD disc types. Figure 11.8. DVD disk types and construction. Note that although Figure 11.8 shows two lasers reading the bottom of the dual-layer discs, in actual practice only one laser is used. Only the focus is changed to read the different layers. Dual-layer discs can have the layers recorded in two ways: either OTP or parallel track path (PTP). OTP minimizes the time needed to switch from one layer to the other when reading the disc. When reaching the inside of the disc (end of Layer 0), the laser pickup remains in the same locationit merely moves toward the disc slightly to focus on Layer 1. When written in OTP mode, the lead-out zone toward the outer part of the disc is called a middle zone instead. Discs written in PTP have both spiral layers written (and read) from the inside out. When changing from Layer 0 to Layer 1, PTP discs require the laser pickup to move from the outside (end of the first layer) back to the inside (start of the second layer), as well as for the focus of the laser to change. Virtually all discs are written in OTP mode to make the layer change quicker. To allow the layers to be read more easily even though they are on top of one another, discs written in PTP mode have the spiral direction changed from one layer to the other. Layer 0 has a spiral winding clockwise (which is read counterclockwise), whereas Layer 1 has a spiral winding counterclockwise. This typically requires that the drive spin the disc in the opposite direction to read that layer, but with OTP the spiral is read from the outside in on the second layer. So Layer 0 spirals from the inside out, and Layer 1 spirals from the outside in. Figure 11.9 shows the differences between PTP and OTP on a DVD. Figure 11.9. PTP versus OTP. DVDs store up to 4.7GB17.1 GB, depending on the type. Table 11.8 shows the precise capacities of the various types of DVDs.
As you might notice, the capacity of dual-layer discs is slightly less than twice of single-layer discs, even though the layers take up the same space on the discs (the spiral tracks are the same length). This was done intentionally to improve the readability of both layers in a dual-layer configuration. To do this, the bit cell spacing was slightly increased, which increases the length of each pit and land. When reading a dual-layer disc, the drive spins slightly faster to compensate, resulting in the same data rate. However, because the distance on the track is covered more quickly, less overall data can be stored. Besides the standard four capacities listed here, a double-sided disc with one layer on one side and two layers on the other can also be produced. This would be called a DVD-14 and have a capacity of 13.2GB, or about 6 hours and 15 minutes of MPEG-2 video. Additionally, 80mm discs, which store less data in each configuration than the standard 120mm discs, can be produced. Because of the manufacturing difficulties and the extra expense of double-sided discsand the fact that they must be ejected and flipped to play both sidesmost DVDs are configured as either a DVD-5 (single-sided, single-layer) or a DVD-9 (single-sided, dual-layer), which allows up to 8.5GB of data or 242 minutes of uninterrupted MPEG-2 video to be played. The 133-minute capacity of DVD-5 video discs accommodates 95% or more of the movies ever made. Note When you view a dual-layer DVD movie, you will see a momentary pause onscreen when the player starts to play the second layer. This is normal, and it takes so little time that if you blink, you might miss it. Data Encoding on the DiscAs with CDs, the pits and lands themselves do not determine the bits; instead, the transitions (changes in reflectivity) from pit to land and land to pit determine the actual bits on the disc. The disc track is divided into bit cells or time intervals (T), and a pit or land used to represent data is required to be a minimum of 3T or a maximum of 11T intervals (cells) long. A 3T long pit or land represents a 1001, and a 11T long pit or land represents a 100000000001. Data is stored using 8 to 16 modulation, which is a modified version of the eight to fourteen modulation (EFM) used on CDs. Because of this, 8 to 16 modulation is sometimes called EFM+. This modulation takes each byte (8 bits) and converts it into a 16-bit value for storage. The 16-bit conversion codes are designed so that there are never fewer than 2 or more than 10 adjacent 0 bits (resulting in no fewer than 3 or no more than 11 time intervals between 1s). EFM+ is a form of RLL encoding called RLL 2,10 (RLL x, y where x = the minimum and y = the maximum run of 0s). This is designed to prevent long strings of 0s, which could more easily be misread due to clocks becoming out of sync, as well as to limit the minimum and maximum frequency of transitions actually placed on the recording media. Unlike CDs, no merge bits exist between codes. The 16-bit modulation codes are designed so that they will not violate the RLL 2,10 form without needing merge bits. Because the EFM used on CDs really requires more than 17 bits for each byte (because of the added merge and sync bits), EFM+ is slightly more efficient since only slightly more than 16 bits are generated for each byte encoded. Note that although no more than 10 0s are allowed in the modulation generated by EFM+, the sync bits added when physical sectors are written can have up to 13 0s, meaning a time period of up to 14T between 1s written on the disc and pits or lands up to 14T intervals or bit cells in length. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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