Upgrading & Repairing PCs: Optical Storage

This chapter is from the book 

DVD

DVD in simplest terms is a high-capacity CD. In fact, every DVD-ROM drive is a CD-ROM drive; that is, it can read CDs as well as DVDs. (Some older 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-sized discs. A CD 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 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 disc—a disc that is the same overall diameter and thickness of a current CD. With Moving Picture Experts Group standard 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 film—including 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.

The initial application for DVDs was as an upgrade for CDs as well as a replacement for prerecorded videotapes. As with CDs, which initially were designed only for music, DVDs have since developed into a wider range of uses, including video rental, computer data storage, and high-quality audio.

DVD History

DVD had a somewhat rocky start. During 1995, two competing standards for high-capacity CD 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 copy protection and other items were agreed on, 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, DC) 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. 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.dvdservices.org. They have since introduced the DVD+RW format, which is the fastest, most flexible and backward-compatible recordable DVD format. DVD-R/RW and DVD+R/RW are not just for computer uses either: You can purchase DVD set-top recorders from many vendors (some of which also contain VCRs to enable you to dub non-copy-protected VCR tapes to DVD).

DVD Construction and Technology

DVD technology is similar to CD technology. Both use the same size discs (120mm diameter, 1.2mm thick, with a 15mm hole in the center) 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. Each layer is separately stamped, and the layers are 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. These 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. 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 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:

• A 2.25 times smaller pit length (0.9–0.4 microns)
• A 2.16 times reduced track pitch (1.6–0.74 microns)
• A slightly larger data area on the disc (8,605–8,759 square millimeters)
• About 1.06 times more efficient channel bit modulation
• About 1.32 times more efficient error-correction code
• About 1.06 times less sector overhead (2,048/2,352–2,048/2,064 bytes)

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 11.9 shows how the grooved tracks with pits and lands are just over four times as dense on a DVD as compared to a CD.

DVD drives use a shorter wavelength laser (650nm) 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 semi-reflective 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 Sectors

The 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 composed of sectors, with each sector containing 2,048 bytes of data. The disc is divided into four main areas:

• Hub clamping area—The hub clamp area is just that: a part of the disc where the hub mechanism in the drive can grip the disc. No data or information is stored in that area.
• Lead-in zone—The lead-in zone contains buffer zones, reference code, and mainly a control data zone with information about the disc. The control data zone consists of 16 sectors of information repeated 192 times, for a total of 3,072 sectors. Contained in the 16 (repeated) sectors is information about the disc, including disc category and version number, disc size and maximum transfer rate, disc structure, recording density, and data zone allocation. The entire lead-in zone takes up to 196,607 (2FFFFh) sectors on the disc. Unlike CDs, the basic structure of all sectors on a DVD is the same. The buffer zone sectors in the lead-in zone have all 00h (zero hex) recorded for data.
• Data zone—The data zone contains the video, audio, or other data on the disc and starts at sector number 196,608 (30000h). The total number of sectors in the data zone can be up to 2,292,897 per layer for single-layer discs.
• Lead-out (or middle) zone—The lead-out zone marks the end of the data zone. All the sectors in the lead-out zone contain zero (00h) for data. This is called the middle zone if the disc is dual-layer and is recorded in opposite track path (OPT) mode, in which the second layer starts from the outside of the disc and is read in the opposite direction from the first layer.

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 11.10 shows these zones in actual relative scale as they appear on a DVD.

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 the outer part of the track is being read, 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 11.8 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.

Table 11.8. DVD Technical Parameters

DVD Type:	Single-Layer	Dual-Layer
1x read speed (m/sec)	3.49	3.84
Laser wavelength (nm)	650	650
Numerical aperture (lens)	0.60	0.60
Media refractive index	1.55	1.55
Track (turn) spacing (um)	0.74	0.74
Turns per mm	1,351	1,351
Turns per inch	34,324	34,324
Total track length (m)	11,836	11,836
Total track length (feet)	38,832	38,832
Total track length (miles)	7.35	7.35
Media bit cell length (nm)	133.3	146.7
Media byte length (um)	1.07	1.17
Media sector length (mm)	5.16	5.68
Pit width (um)	0.40	0.40
Pit depth (um)	0.105	0.105
Min. nominal pit length (um)	0.40	0.44
Max. nominal pit length (um)	1.87	2.05
Lead-in inner radius (mm)	22	22
Data zone inner radius (mm)	24	24
Data zone outer radius (mm)	58	58
Lead-out outer radius (mm)	58.5	58.5
Data zone width (mm)	34	34
Data zone area (mm2)	8,759	8,759
Total track area width (mm)	36.5	36.5
Max. rotating speed 1x CLV (rpm)	1,515	1,667
Min. rotating speed 1x CLV (rpm)	570	627
Track revolutions (data zone)	45,946	45,946
Track revolutions (total)	49,324	49,324
Data zone sectors per layer per side	2,292,897	2,083,909
Sectors per second	676	676
Media data rate (Mbps)	26.15625	26.15625
Media bits per sector	38,688	38,688
Media bytes per sector	4,836	4,836
Interface data rate (Mbps)	11.08	11.08
Interface data bits per sector	16,384	16,384
Interface data bytes per sector	2,048	2,048
DVD Type:	Single-Layer	Dual-Layer
Track time per layer (minutes)	56.52	51.37
Track time per side (minutes)	56.52	102.74
MPEG-2 video per layer (minutes)	133	121
MPEG-2 video per side (minutes)	133	242
B = Byte (8 bits) KB = Kilobyte (1,000 bytes) KiB = Kibibyte (1,024 bytes) MB = Megabyte (1,000,000 bytes) MiB = Mebibyte (1,048,576 bytes) GB = Gigabyte (1,000,000,000 bytes) GiB = Gibibyte (1,073,741,824 bytes) Mbps = Megabits per second m = Meters mm = Millimeters (thousandths of a meter) mm2 = Square millimeters um = Micrometers = Microns (millionths of a meter) nm = Nanometers (billionths of a meter) rpm = Revolutions per minute CLV = Constant linear velocity

As you can see from the information in Table 11.8, 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 two or more than ten 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 are added. After the entire ECC frame is converted into a physical sector, 4,836 total bytes are stored.

Table 11.9 shows the sector, frame, and audio data calculations.

Table 11.9. DVD Data Frame, ECC Frame, and Physical Sector Layout and Information

DVD Data Frame:
ID bytes	4
IED bytes	2
CI	6
Data bytes	2,048
Error detection code (EDC)	4
Data frame total bytes	2,064
DVD ECC Frame:
Data frame total bytes	2,064
PO bytes	182
Parity inner (PI) bytes	120
ECC frame total bytes	2,366
DVD Media Physical Sectors:
ECC frame bytes	2,366
8-to-16 modulation bits	37,856
Synchronization bits	832
Total encoded media bits/sector	38,688
Total encoded media bytes/sector	4,836
Original data bits/sector	16,384
Original data bytes/sector	2,048
Ratio of original to media data	2.36
ID = Identification Data IED = ID Error Detection code CI = Copyright Info EDC = Error Detection Code PO = Parity Outer GB = Gigabyte (1,000,000,000 bytes) GiB = Gibibyte (1,073,741,824 bytes) Mbps = Megabits per second m = Meters mm = Millimeters (thousandths of a meter) mm2 = Square millimeters um = Micrometers = Microns (millionths of a meter) nm = Nanometers (billionths of a meter) rpm = Revolutions per minute CLV = Constant linear velocity

Unlike CDs, DVDs do not use subcodes. Instead, they use the ID bytes in each data frame to store the sector number and information about the sectors.

Handling DVD Errors

DVDs 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 yet 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 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 here:

              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 one PI bit is added for each row, using odd parity. This means you count up the 1 bits: In the first row there are four, 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 this 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 1 1 0  |  1
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:

• DVD-5 (4.7GB single-side, single-layer)—A DVD-5 is constructed from two substrates bonded with adhesive. One is stamped with a recorded layer (called Layer 0), and the other is blank. An aluminum coating typically is applied to the single recorded layer.
• DVD-9 (8.5GB single-side, dual-layer)—A DVD-9 is constructed of two stamped substrates bonded together to form two recorded layers for one side of the disc, along with a blank substrate for the other side. The outer stamped layer (0) is coated with a semitransparent gold coating to both reflect light if the laser is focused on it and pass light if the laser is focused on the layer below. A single laser is used to read both layers; only the focus of the laser is changed.
• DVD-10 (9.4GB double-side, single-layer)—A DVD-10 is constructed of two stamped substrates bonded together back to back. The recorded layer (Layer 0 on each side) usually is coated with aluminum. Note that these discs are double-sided; however, drives have a read laser only on the bottom, which means the disc must be removed and flipped to read the other side.
• DVD-18 (17.1GB double-side, dual-layer)—A DVD-18 combines both double layers and double sides. Two stamped layers form each side, and the substrate pairs are bonded back to back. The outer layers (Layer 0 on each side) are coated with semitransparent gold, whereas the inner layers (Layer 1 on each side) are coated with aluminum. The reflectivity of a single-layer disc is 45%–85%, and for a dual-layer disc the reflectivity is 18%–30%. The automatic gain control (AGC) circuitry in the drive compensates for the different reflective properties.

Figure 11.11 shows the construction of each of the DVD disc types.

Note that although Figure 11.11 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 opposite track path (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 location—it 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. 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. OTP recording is also used by dual-layer (DL) DVD rewritable drives.

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 11.12 shows the differences between PTP and OTP on a DVD.

DVDs store up to 17.1GB, depending on the type. Table 11.10 shows the precise capacities of the various types of DVDs.

Table 11.10. DVD Capacity

	Single-Layer	Dual-Layer
DVD Designation	DVD-5	DVD-9
B	4,695,853,056	8,535,691,264
KiB	4,585,794	8,335,636
KB	4,695,853	8,535,691
MiB	4,478	8,140
MB	4,696	8,536
GiB	4.4	7.9
	Single-Layer	Dual-Layer
DVD Designation	DVD-5	DVD-9
GB	4.7	8.5
MPEG-2 video (approx. minutes)	133	242
MPEG-2 video (hours:minutes)	2:13	4:02
	Single-Layer Double-Sided	Dual-Layer Double-Sided
DVD Designation	DVD-10	DVD-18
B	9,391,706,112	17,071,382,528
KiB	9,171,588	16,671,272
KB	9,391,706	17,071,383
MiB	8,957	16,281
MB	9,392	17,071
GiB	8.7	15.9
GB	9.4	17.1
MPEG-2 video (approx. minutes)	266	484
MPEG-2 video (hours:minutes)	4:26	8:04
B = Byte (8 bits) KB = Kilobyte (1,000 bytes) KiB = Kibibyte (1,024 bytes) MB = Megabyte (1,000,000 bytes) MiB = Mebibyte (1,048,576 bytes) GB = Gigabyte (1,000,000,000 bytes) GiB = Gibibyte (1,073,741,824 bytes)

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 accomplish 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 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 discs—and the fact that they must be ejected and flipped to play both sides—most 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.

Data Encoding on the DVD Disc

As 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 eight to sixteen modulation, which is a modified version of the eight to fourteen modulation (EFM) used on CDs. Because of this, eight to sixteen 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 two or more than ten adjacent 0 bits (resulting in no fewer than three or no more than eleven time intervals between 1s). EFM+ is a form of RLL encoding called RLL 2,10 (RLL x,y, where x equals the minimum and y equals 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 (due to the added merge and sync bits), EFM+ is slightly more efficient because only slightly more than 16 bits are generated for each byte encoded.

Note that although no more than ten 0s are allowed in the modulation generated by EFM+, the sync bits added when physical sectors are written can have up to thirteen 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.

Recordable DVD Standards

The history of recordable DVD drives has been a troubled one. It dates back to April 1997, when the DVD Forum announced specifications for rewritable and recordable DVD: DVD-RAM, and DVD-R. Later, it added DVD-RW to the mix. Dissatisfied with these standards, the industry leaders in optical recording and drives formed their own group called the DVD+RW Alliance and created another standard—DVD+R and DVD+RW. For several years, drives based on one family of standards could not freely interchange media with drives using the other family of standards.

Fortunately, all recent drives support both DVD-R/RW and DVD+R/RW media, including dual-layer (DL) DVD+R media, and most also support DVD-RAM. Thus, by using a modern drive that supports all of these types of media, you can choose the right media for a particular task. For example, use DVD-RAM for easy drag-and-drop file backups and DVD-R for creating video DVDs compatible with older DVD set-top boxes.

Table 11.11 compares the competing recordable DVD standards, and Table 11.12 breaks down the compatibilities between the drives and media.

Table 11.11. Recordable DVD Standards

Format	Introduced	Capacity	Compatibility
DVD-RAM	July 1997	Up to 4.7GB per side	Compatible with SuperMulti and Super AllWrite drives. Incompatible with older DVD drives that do not support the MultiRead2 standard.
DVD-R/RW	July 1997; Nov. 1999	4.7GB per side	Compatible with DVD-R/RW, SuperMulti and Super AllWrite DVD recorders/drives. Compatible with most DVD set-top boxes.
DVD+R/RW	Mar. 2001; May 2001	4.7GB per side	Compatible with DVD+R/RW, SuperMulti and Super AllWrite DVD recorders/drives. Compatible with most recent DVD set-top boxes.
DVD+R DL	Oct. 2003	8.5GB	Older DVD drives may require firmware updates to read DL media. Some older SuperMulti and Super AllWrite drives do not support DL media.
DVD-R DL	Feb. 2005	8.5GB	For compatibility with older DVD drives, use the Layer Jump Recording method. Older DVD drives may also require firmware updates to read DL media. Some older SuperMulti and Super AllWrite drives do not support DL media.

Table 11.12. DVD Drive and Media Compatibility

	Media (Discs)
Drives	CD-ROM	CD-R	CD-RW	DVD Drive	DVD-ROM	DVD-R	DVD-RAM	DVD-RW	DVD+RW	DVD+R
DVD-Video Player	R	?	?	R	—	R	?	R	R	R
DVD-ROM Drive	R	R	R	R	R	R	?	R	R1	R
DVD-R Drive	R	R/W	R/W	R	R	R/W	—	R	R	R
DVD-RAM Drive	R	R	R	R	R	R6	R/W	R	R1	R
DVD-RW Drive	R	R/W	R/W	R	R	R/W	—	R/W	R	R
DVD+R/RW Drive	R	R/W	R/W	R	R	R	R3	R	R/W	R/W2
DVD-Multi Drive4	R	R/W	R/W	R	R	R	R/W	R/W	R1	R
DVD±R/RW Drive	R	R/W	R/W	R	R	R/W	R5	R/W	R/W	R/W
DVD Super Multi Drive7	R	R/W	R/W	R	R	R/W8	R/W	R/W	R/W	R/W9
R = Read. W = Write. —= Will not read or write. ? = MultiRead/MultiPlay drives will read. 1 = Might require media's compatibility bit be changed to alternate (Type 2). 2 = Some first-generation DVD+RW drives will not write DVD+R discs; see your drive manufacturer for an update or trade-in. 3 = Read compatibility with DVD-RAM varies by drive; check documentation for details. 4 = DVD Forum specification for drives that are compatible with all DVD Forum standards. (DVD+R/RW is not a DVD Forum standard.) 5 = Some of these drives can also write to DVD-RAM media. 6 = Some of these drives can also write to DVD-R media. 7 = Identifies drives that work with DVD+R/RW, DVD-R/RW, DVD+R DL, and DVD-RAM media. 8 = Some of these drives also work with dual-layer (DL) media. 9 = Also supports dual-layer (DL) media.

DVD+R/RW offers low drive and media prices, provides the highest compatibility with existing formats, and has features that make it the most ideal for both video recording and data storage in PCs. However, with most recent drives, you can now select the best media for the job.

DVD-RAM

DVD-RAM is the rewritable DVD standard endorsed by Panasonic, Hitachi, and Toshiba; it is part of the DVD Forum's list of supported standards. DVD-RAM uses a phase-change technology similar to that of CD-RW. Unfortunately, DVD-RAM discs can't be read by most standard DVD-ROM drives because of differences in both reflectivity of the media and the data format. (DVD-R, by comparison, is backward-compatible with DVD-ROM.)

DVD-ROM drives that can read DVD-RAM discs began to come on the market in early 1999 and follow the MultiRead2 specification. DVD-ROM drives and DVD-Video players labeled as "MultiRead2 compliant" are capable of reading DVD-RAM discs. See the section "MultiRead Specifications," earlier in this chapter, for more information. Although the MultiRead2 logo is not used on current products, some recent and current DVD-ROM drives can read DVD-RAM media; check the specification sheet for a particular drive to verify compatibility.

The first DVD-RAM drives were introduced in spring 1998 and had a capacity of 2.6GB (single-sided) or 5.2GB (double-sided). DVD-RAM Version 2 discs with 4.7GB arrived in late 1999, and double-sided 9.4GB discs arrived in 2000. DVD-RAM drives typically read DVD-Video, DVD-ROM, and CD media. Although DVD-ROM drives, older DVD+R/RW and DVD-R/RW drives, and DVD-Video players can't read DVD-RAM media, DVD Multi and DVD Super Multi drives can read/write DVD-RAM.

DVD-RAM uses what is called the wobbled land and groove recording method, which records signals on both the lands (the areas between grooves) and inside the grooves that are preformed on the disc. The tracks wobble, which provides clock data for the drive. Special sector header pits are prepressed into the disc during the manufacturing process as well. See Figure 11.13, which shows the wobbled tracks (lands and grooves) with data recorded both on the lands and in the grooves. This is unlike CD-R or CD-RW, in which data is recorded on the groove only.

The disc is recorded using phase-change recording, in which data is written by selectively heating spots in the grooves or on the lands with a high-powered laser. The DVD-RAM drive write laser transforms the film from a crystalline to an amorphous state by heating a spot, which is then rendered less reflective than the remaining crystalline portions. The signal is read as the difference of the laser reflection rate between the crystalline and amorphous states. The modulation and error-correction codes are the same as for DVD-Video and DVD-ROM, ensuring compatibility with other DVD formats. For rewriting, a lower-powered laser reheats the spot to a lower temperature, where it recrystallizes.

Disc cartridges or caddies originally were required for both single- and double-sided discs but have now been made optional for single-sided discs and are seldom used today. Double-sided discs must remain inside the caddy at all times for protection; however, single-sided discs can be taken out of the cartridge if necessary.

DVD-RAM specifications are shown in Table 11.13.

Table 11.13. DVD-RAM Specifications

Storage capacity	2.6GB single-sided; 5.2GB double-sided
Disc diameter	80mm–120mm
Disc thickness	1.2mm (0.6mmx2: bonded structure)
Recording method	Phase change
Laser wavelength	650nm
Data bit length	0.41–0.43 microns
Recording track pitch	0.74 microns
Track format	Wobbled land and groove

In the past, I have been opposed to DVD-RAM because of a lack of compatibility with other drive types. However, if you use drives supporting the DVD Super Multi standard, you can read and write DVD-RAM as well as other rewritable DVD formats. With the ability to read, write, and erase data without the need to use UDF packet-writing software, DVD-RAM can be a useful alternative to other types of rewritable DVD—assuming all your drives can use it.

DVD-R

DVD-R is a write-once medium similar to CD-R, which was originally created by Pioneer and released by the DVD Forum in July 1997. You can play DVD-R discs on standard DVD-ROM drives. Some DVD-RAM drives can also write to DVD-R media.

DVD-R has a single-sided storage capacity of 4.7GB—about seven times that of a CD-R-and double that for a double-sided disc. These discs use an organic dye recording layer that allows for a low material cost, similar to CD-R.

To enable positioning accuracy, DVD-R uses a wobbled groove recording, in which special grooved tracks are preengraved on the disc during the manufacturing process. Data is recorded within the grooves only. The grooved tracks wobble slightly right and left, and the frequency of the wobble contains clock data for the drive to read, as well as clock data for the drive. The grooves are spaced more closely together than with DVD-RAM, but data is recorded only in the grooves and not on the lands (see Figure 11.14).

Table 11.14 has the basic specifications for DVD-R drives.

Table 11.14. DVD-R Specifications

Storage capacity	4.7GB single-sided; 9.4GB double-sided
Disc diameter	80mm–120mm
Disc thickness	1.2mm (0.6mmx2: bonded structure)
Recording method	Organic dye layer recording method
Laser wavelength	635nm (recording); 635nm/650nm (playback)
Data bit length	0.293 microns
Recording track pitch	0.80 microns
Track format	Wobbled groove

DVD-R media is currently available in speeds up to 16x, although some drives feature faster burn speeds. Some vendors are now producing double-sided single-layer DVD-R media with capacities of 9.4GB. This media is designed primarily for DVD jukeboxes, although it can be used by standard DVD rewritable drives.

DVD-R DL

DVD-R DL was introduced in February 2005 and is sometimes known as DVD-R for Dual Layer or DVD-R9. DVD-R DL is essentially a dual-layer version of the DVD-R disc, using the same recording method, laser wavelength, and other specifications. However, DVD-R DL discs have two recording layers, with the reflective surface of the top layer being semi-transparent to permit recording on the second layer. Because of the lower reflectivity of the top layer, some DVD-ROM drives cannot read DVD-R DL media.

DVD-R DL media is currently available in 4x speed from a relatively small number of suppliers, although some rewritable DVD drives support faster write speeds.

DVD-RW

The DVD Forum introduced DVD-RW in November 1999. Created and endorsed originally by Pioneer, DVD-RW is basically an extension to DVD-R, just as CD-RW is an extension to CD-R. DVD-RW uses a phase-change technology and is somewhat more compatible with standard DVD drives than DVD-RAM. Drives based on this technology began shipping in late 1999, but early models achieved only moderate popularity because Pioneer was the only source for the drives and because of limitations in their performance.

The most common types of DVD-RW media support 2x speeds, although 4x and 6x media is also available. Drives supporting 2x/4x and faster media have several advantages over original 1x/2x DVD-RW drives, including these:

• Quick formatting—1x/2x drives require that the entire DVD-RW disc be formatted before the media can be used, a process that can take about an hour. 2x/4x and faster drives can use DVD-RW media in a few seconds after insertion, formatting the media in the background as necessary. This is similar to the way in which DVD+RW drives work.
• Quick finalizing—2x/4x DVD-RW drives close media containing small amounts of data (under 1GB) more quickly than 1x/2x drives.
• Quick grow—Instead of erasing the media to add files, as with 1x/2x DVD-RW drives, 2x/4x and faster DVD-RW drives can unfinalize the media and add more files without deleting existing files.

However, most DVD-RW drives still don't support lossless linking, Mount Rainier, or selective deletion of files—all of which are major features of DVD+RW.

DVD+RW and DVD+R

DVD+RW, also called DVD Phase Change Rewritable, has been the premier DVD recordable standard because it is the least expensive, easiest to use, fastest, and most compatible with existing formats. It was developed and is supported by Philips, Sony, Hewlett-Packard, Mitsubishi Chemical (MCC/Verbatim), Ricoh, Yamaha, and Thomson, who are all part of an industry standard group called the DVD+RW Alliance (www.dvdservices.org). Microsoft joined the alliance in February 2003. DVD+RW is also supported by major DVD/CD-creation software vendors and many drive vendors, including HP, Philips, Ricoh, and many remarketers of OEM drive mechanisms. Although DVD-RW has increased in popularity with the advent of faster burning times and easier operation, DVD+RW is the most popular rewritable DVD format.

Table 11.15 lists the basic specifications for DVD+RW drives.

Table 11.15. DVD+RW Specifications

Storage capacity	4.7GB single-sided; 9.4GB double-sided
Disc diameter	120mm
Disc thickness	1.2mm (0.6mmx2: bonded structure)
Recording method	Phase change
Laser wavelength	650nm (recording/playback)
Data bit length	0.4 microns
Recording track pitch	0.74 microns
Track format	Wobbled groove

Note that DVD+R, the recordable version of DVD+RW, was actually introduced after DVD+RW. This is the opposite of DVD-RW, which grew out of DVD-R. One of the major reasons for the development of DVD+R was to provide a lower-cost method for permanent data archiving with DVD+RW drives, and another was because of compatibility issues with DVD-ROM and DVD video players being incapable of reading media created with DVD+RW drives. However, most standard DVD-ROM drives or DVD players can read both DVD+R and DVD+RW media without problems.

The basic structure of a DVD+RW or DVD+R disc resembles that of a DVD-R disc with data written in the grooves only (refer to Figure 11.14), but the groove is wobbled at a frequency different from that used by DVD-R/RW or DVD-RAM. The DVD+R/RW groove also contains positioning information. These differences mean that DVD+R/RW media offers more accurate positioning for lossless linking, but drives made only for DVD+R/RW media can't write to other types of DVD rewritable or recordable media.

Although some first-generation DVD+RW drives worked only with rewritable media, all current and future DVD+RW drives are designed to work with both DVD+R (writable) and DVD+RW (rewritable) media. The +R discs can be written only once and are less expensive than the +RW discs.

Some of the features of DVD+RW include the following:

• Single-sided discs (4.7GB).
• Double-sided discs (9.4GB).
• Up to 4 hours video recording (single-sided discs).
• Up to 8 hours video recording (double-sided discs).
• Bare discs—no caddy required.
• 650nm laser (same as DVD-Video).
• Constant linear data density.
• CLV and CAV recording.
• Write speeds 1x–4x and higher (depending on the drive).
• DVD-Video data rates.
• UDF (Universal Disc Format) file system.
• Defect management integral to the drive.
• Quick formatting.
• Uses same 8-to-16 modulation and error-correcting codes as DVD-ROM.
• Sequential and random recording.
• Lossless linking. (Multiple recording sessions don't waste space.)
• Spiral groove with radial wobble.
• After recording, all physical parameters comply with the DVD-ROM specification.

DVD+RW technology is similar to CD-RW, and DVD+RW drives can read DVD-ROMs and all CD formats, including CD-R and CD-RW.

With DVD+RW, the writing process can be suspended and continued without a loss of space linking the recording sessions together. This increases efficiency in random writing and video applications. This "lossless linking" also enables the selective replacement of any individual 32KB block of data (the minimum recording unit) with a new block, accurately positioning with a space of 1 micron. To enable this high accuracy for placement of data on the track, the pre-groove is wobbled at a higher frequency. The timing and addressing information read from the groove is very accurate.

The quick formatting feature means you can pop a DVD+R or DVD+RW blank into the drive and almost instantly begin writing to it. The actual formatting is carried out in the background ahead of where any writing will occur.

DVD+R/RW is the format I prefer and recommend, and it has been the format most users prefer for data recording. However, today's multiformat drives support both DVD+R/RW and DVD-R/RW (and Super Multi Drives support DVD-RAM), so you can choose the right media for a particular task.

When DVD+RW drives were introduced in 2001, some users of DVD-ROM and standalone DVD players were unable to read DVD+RW media, even though others were able to do so. The first drives to support DVD+R (writable) media (which works with a wider range of older drives) was not introduced until mid-2002, so this was a significant problem.

The most common reason for this problem turned out to be the contents of the Book Type Field located in the lead-in section of every DVD disc. Some drives require that this field indicate that the media is a DVD-ROM before they can read it. However, by default, DVD+RW drives write DVD+RW as the type into this field when DVD+RW media is used.

The following are three possible solutions:

• Upgrade the firmware in the DVD+RW recorder so it writes compatible information into the Book Type Field automatically.
• Change the Book Type Field during the creation of a disc with a DVD mastering program.
• Use a compatibility utility to change the contents of the Book Type Field for a particular DVD+RW disc as necessary. These utilities may be provided by the drive manufacturer (sometimes a firmware upgrade is also necessary) or by a third-party utility.

Changing the Book Type Field is known as bitsetting.

See "Updating the Firmware in an Optical Drive," p. 599 (this chapter).

DVD+R DL

DVD+R DL, also known as DVD-R9, is a dual-layer version of the DVD+R standard that was introduced in October 2003. DVD+R DL is essentially a dual-layer version of the DVD+R disc, using the same recording method, laser wavelength, and other specifications. However, DVD+R DL discs have two recording layers, with the reflective surface of the top layer being semi-transparent to permit recording on the second layer. Because of the lower reflectivity of the top layer, some DVD-ROM drives cannot read DVD+R DL media. DVD+RW DL media is typically rated at 8x recording speeds.

Multiformat Rewritable DVD Drives

The DVD Multi specification from the DVD Forum was developed for drives and players that are compatible with all DVD Forum standards, including DVD-R/RW, DVD-RAM, DVD-ROM, DVD-Video, and eventually DVD Audio (DVD+R/RW are not DVD Forum specifications and are not supported). The original version of DVD Multi was published in February 2001; the current version, version 1.01, was approved by the DVD Forum and published in December 2001. The first DVD Multi products for computers reached the market in early 2003.

To provide support for different types of DVD media in a single drive, all rewritable DVD drive vendors now sell drives compatible with both DVD+R/RW and DVD-R/RW discs. These drives are commonly known as DVD'R/RW. LG's Super Multi Drive series was the first to also add compatibility with DVD-RAM, and most current DVD'R/RW drives from other makers are also compatible with DVD-RAM. Many (but not all) current drives also support DVD-R DL, so you can now buy a single drive that supports all common formats supported by both the DVD Forum and the DVD+RW Alliance. Lite-On uses the term Super AllWrite to refer to drives that support all these media types.