Summary ✦ Seek and access times, and sustained data transfer rate, are the most important determiners of disk performance in your system. ✦ RAID systems can give you greatly increased performance and reliability. ✦ Be sure to get enough disk space. Your requirements will always increase over time. 128 Part IV ✦ Storage 9 9 CHAPTER CD and DVD O ptical drives originated from the need for a high- capacity, removable medium for multimedia and software distribution. Inexpensive, high-capacity digital technologies for distributing music and video from the consumer electronics industry, including both CD and DVD, met the requirement well. Other optical formats, including magneto-optical drives, were popular for a while before writable CD and DVD technologies matured, but all of the others have essentially died out in the face of the overwhelming manufacturing volume pushing CD and DVD forward. CDs provide a small, inexpensive, and rugged way to hold about 650MB of data; DVD extends the CD technol- ogy to provide far more capacity and greater data trans- fer rates. This chapter covers how CD-ROMs and DVDs work, how you can use CD-ROM and DVD drives in your computer system, and how you can make your own CD-ROMs. We’ll look at the newest technology and dis- cuss what you can expect from a quality CD-ROM or DVD drive. Chapter 17 covers how you can use writable DVD and video hardware to make your own DVDs. What Is a CD-ROM? A CD-ROM most resembles the vinyl long-play records still dear to the hearts of audio fanatics. On a record, a single spiral in the vinyl winds from the outside to the inside, with the analog signal encoded in the deflections along the course of the groove. On a CD-ROM, a single spiral encased in plastic winds from the inside to the outside, with the digital data encoded by the presence or absence of tiny optical pits. A record stores sound as analog levels, which can degrade over time as the vinyl wears or becomes dirty. A CD-ROM stores its data as numbers, which never degrade unless the disk becomes unreadable. Figure 9-1 shows what’s inside CD media. In cross-sec- tion, a CD is a layer of reflective aluminum with lacquer on top and protective plastic underneath (recordable CDs use a similar structure, but use an organic dye ✦✦✦✦ In This Chapter Acquiring inexpensive, rugged, high-capacity storage Understanding CD and DVD technology ✦✦✦✦ instead of an aluminum layer). The zeroes and ones (transformed in a way that makes the recording more reliable) get turned into flats and pits on the surface of the reflective layer when the CD is mastered. The layers include: ✦ Spiral data track — The information on the CD-ROM is recorded in a continuous spiral starting at the inside edge of the recorded area and continuing to the outer edge of the disk. ✦ Top surface — The top of the CD-ROM is lacquer over the aluminum layer. The CD-ROM label is painted on top of the lacquer. ✦ Reflective aluminum — A reflective aluminum surface carries the flats and pits that physically encode the information. The flats and pits, by reflecting light differently, enable the CD-ROM drive to read back the information. ✦ Plastic coating — The back side of the disk is covered by a plastic coating that protects the aluminum layer. Figure 9-1: The data spiral on a CD-ROM is nearly 3 miles long. The CD mastering process is similar to the vinyl record-making process. A mir- ror image of the disk, with all the pits and flats, is used to stamp out the plastic bottom of the disk with an accurate impression of the entire spiral. Aluminum is then deposited on the plastic and covered with lacquer, resulting in the fin- ished disk. The accuracy required in the process is far greater than vinyl records needed: Adjacent turns of the spiral along the disk are only 1.6 micrometers apart, which means that there are nearly 16,000 of them every inch. Pit Flat Top surface with label Reflective aluminum surface CD-ROM cross-section Plastic coating over reflective aluminum End of continuous spiral track Start of continuous spiral track 130 Part IV ✦ Storage Mastering a CD is straightforward, if not easy. You feed data to the laser head at a constant rate, turn the master disk at a constant rate, and sweep the laser head from the inside to the outside at a constant rate. The laser burns pits into the master as required. The end result is a precise, even spiral of pits and flats. Reading a CD from beginning to end — without pauses or other interruptions — is straightforward, too. The read laser head sweeps the same way the record head did when making the master, allowing light to reflect back off the aluminum surface plated onto the CD, as in Figure 9-2. Light reflected Chapter 9 ✦ CD and DVD 131 Coding Data onto a CD-ROM The flats and pits on a CD-ROM do not directly correspond to the ones and zeroes that eventually make it into your computer. Instead, the data stream is coded in a particular way before recording, with the coding reversed when you read back the disk to recover the original data pattern. The conversion from your data to what’s recorded changes every 8 bits of your data into 14 bits that get recorded, allowing the drive to compensate for limitations of the physical device. The following table shows how that transformation works for some of the 256 possible 8-bit values. For example, if a byte value of 3 needs to be recorded, the 8-bit value would be 00000011. After that byte gets remapped for recording, the result is 10001000100000. Value 8-bit Representation 14-bit Representation 0 00000000 01001000100000 1 00000001 10000100000000 2 00000010 10010000100000 3 00000011 10001000100000 4 00000100 01000100000000 The pattern of pits and flats used on the CD to record the bits is interesting. A 1 is indicated by a change from a pit to a flat (or a flat to a pit). The length of the subsequent pit or flat (after the transition) indicates how many 0 bits follow the 1 bit before the next 1 bit occurs. If you look back at the table in this sidebar, you’ll see that there is never a pattern in the 14-bit representation where two ones occur together, which is necessary since you can’t put two transitions back- to-back. The actual 14-bit code is more restrictive yet; a 1 will always be followed by at least two 0s. This pattern limits the minimum size of the pits and flats and in turn allows designers to make decisions about the wavelength of the laser in the drive and about the lenses used with the laser. When the 14-bit codes are read back from the CD, the drive converts back to the 8-bit code the computer expects to see and (after passing the data through some powerful error-correcting circuits) sends the data out onto the I/O bus. from a flat on the CD bounces back cleanly, sending most of the light back to a photodetector in the drive. Light reflected from a pit on the CD is scattered by the shape of the pit, sending most of the light away from the photodetector. A sensor converts the change in intensity of the reflected laser beam as the beam sweeps from a pit to a flat into the pattern of 1s and 0s for the 14-bit representation. Figure 9-2: The presence and absence of reflected light, and the transition between them, indicate the pattern recorded on the CD to the drive. As with magnetic disks, there’s more room per revolution of the disk to pack in data as you go farther out from the center. In the same way that magnetic disks pack in more sectors at the outside, so do CDs pack in more pits and flats, and therefore more data. The similarities between CD and magnetic disk don’t stop there. If you look at the pattern 00000010001000000001001000010000001001000100000 you can’t necessarily tell where one bit pattern stops and the next one starts. If, instead, we give you the same information, like this, 00000 01000100000000 10010000100000 01001000100000 you can tell that the sequence starts with the last part of a bit, followed by the bits for the sequence 420. Pit Scattered diffuse reflection Clean high-intensity reflection Flat 132 Part IV ✦ Storage The information that gives you the bit boundaries, and that divides the infor- mation on a disk into sectors, is called framing. The framing information on a CD doesn’t address the data as cylinder-head-sector (as on a magnetic disk) because the spiral arrangement of the data means that there aren’t distinct cylinders and because there’s only one head. The lack of those features makes sector placement on a CD a little more complicated than on a regular disk. The smallest unit above the byte is called a frame, containing 24 bytes. Frames are grouped into blocks, which contain 98 frames (2,353 bytes). A CD-ROM actually carries only 2,048 data bytes per frame — the remainder goes to added error correction, synchronization, and addressing bytes. When your computer asks the CD-ROM to read a specific frame, the sequence of events is similar to that for a read from a magnetic disk. The head has to move to the right place and pick off the data. Because the data is arranged in a spiral, though, this is a difficult process. The controller in the CD-ROM uses the following sequence: 1. Position the head as close as possible to where the frame should be. 2. Wait for the CD to turn enough for pits and flats to spiral under the laser beam, and start tracking outwards along the spiral. 3. Wait for synchronization with a frame, and read the frame address. 4. Adjust position based on how far the frame the head found is ahead or behind the one you wanted. Unlike CD-ROM drives, CD audio drives rarely change position to a specific place — they do it only when you say to go to a specific track, which is almost never on the computer time scale of billions of operations every second. In an application like that, a few tenths of a second longer to find the right place is far less important than making the drive reliable and inexpensive. Because computer CD-ROM drives were originally built from the CD audio technology, it was inevitable that the first generation drives were slow to seek from one place to another. The sustained transfer rate off a first-generation CD-ROM drive was also driven by the capabilities of the CD audio equipment, which meant that the drive transfers about 1.2 megabits per second (153.6K per second). Video compres- sion technology gets pretty lousy below about 1.5 megabits per second, which drove the industry to quickly develop the second, “double speed” generation of CD-ROMs. Since then, CD-ROM manufacturers boosted performance to 4, 8, 16, 24, 32, and now 48 times the basic audio CD rate by spinning the disk faster and building better drive mechanisms to hold or reduce the seek times. Table 9-1 shows what that means. Manufacturers have boosted the data transfer rates up to 7.2 megabytes per second from the 150 kilobytes per second of the original CD-ROM drives. The sustained transfer rates from CD-ROMs are now as high as some economy hard disk drives. Chapter 9 ✦ CD and DVD 133 Table 9-1 Increases in CD-ROM Speed Increase the Data Transfer Rate Speed Multiplier Data Transfer Rate (Per Second) 1150K 2 300K 4 600K 6 900K 8 1.2MB 12 1.8MB 16 2.4MB 24 3.6MB 32 4.8MB 40 6.0MB 48 7.2MB Although they’re essentially a commodity item, CD-ROM drives do have differ- ences besides transfer rate. Unless you’re simply buying a cheap drive you can use to load software, look at the stated reliability of the drive. We’ve found all the optical drives — both CD and DVD — to be some of the least reliable com- ponents in computers, perhaps only exceeded in failure rate by fans and power supplies. Even ignoring copy protection schemes that embed errors into sectors on a CD-ROM, you may encounter problems reading disks on some CD-ROM drives. The major component of the problem is vibration from slightly out of balance disks spun at high speeds, creating vibration in the laser that affects the signal read off the disk. The problem started to be reported with 8X or faster CD-ROM drives and has continued with faster units. The vibration problem is more severe even than issues of tolerance in the manufacture of the CD-ROMs. The vibration dampening built into the disk carrier and drive mechanism is crucial — better-designed drives isolate the laser and pickup from vibration, producing a cleaner and more reliable signal (all the more reason to buy from quality manufacturers). The importance of the speed rating of a CD-ROM drive is overhyped. If your current CD-ROM works reliably, you’ll probably not benefit from a faster CD-ROM drive. Multimedia files don’t require the fastest drives, and soft- ware loads are something you do once and forget. If your 8X or faster CD-ROM is working reliably, consider keeping it. 134 Part IV ✦ Storage Hooking a CD-ROM onto the same ATA port as a hard disk is going to reduce hard disk performance. If you can, keep the CD-ROM on the sec- ondary port away from the hard disks. CD-ROM drivers for ATA drives are built into Windows 9x, but if the drive is on the secondary port, you may have to set up the driver manually. You can do this with the Add New Hardware Wizard in the Control Panel. Windows 2000 and Windows XP should recognize drives on secondary ports auto- matically. Bootable CD-ROM Until early 1995, you had no way to boot your computer from the CD-ROM drive, which meant that if you built a new machine (or replaced the disk in an old one), you had to boot from a floppy, install drivers, and build up the disk contents from there. The El Torito Bootable CD-ROM Format Specification — standardized in January of 1995 — changed that. (Legend has it that the name El Torito is from the El Torito Mexican restaurant where the specification was initially worked Chapter 9 ✦ CD and DVD 135 Keeping the Data Flowing at a Constant Rate Older CD-ROM drives used the constant linear velocity (CLV) approach to read- ing the disk, in which the rotation rate varies based on the distance of the head from the center in a way that keeps the rate of travel along the data track con- stant. Because the length of one rotation’s worth of the data spiral gets longer as the head moves from the inside to the outside of the disk, the distance trav- eled along the spiral per rotation gets longer. The size and spacing of the pits and flats remain constant, however, so more pits and flats occur per rotation towards the outside of the disk. If the rotation rate (in RPM) stayed constant, the increased data content towards the outside would mean that less data flowed at the inside of the disk and more at the outside. Slowing the rotation rate at the outside of the disk keeps the data rate constant. Newer CD-ROM drive designs abandoned CLV for constant angular velocity (CAV), in which the rotation rate is independent of the head position. CAV is the same approach used in hard disk drives. CD-ROMs do still vary the rotation rate, however, to help adapt to the transfer rate required by the computer. The reason for the rate changes is that seeking to the correct block is time-consuming for a CD-ROM, and if the computer can’t take data at full rate, the drive spinning at full rate will move past the point the computer is reading. When that happens, the drive has to seek back to the current read point — a slow operation. By slow- ing the rotation rate to match the computer, the drive avoids the seek and gives better performance. The drive avoids having to reposition the head backwards, saving tens of milliseconds. out.) Essentially, all systems now have BIOS support for El Torito, so if you have a bootable CD-ROM, you can load the drive, start the machine, and have it come up from the operating system on the CD-ROM. If you’re building a machine up from an empty hard disk, the bootable CD-ROMs you get for Windows, FreeBSD, and Linux let you start the install without shuffling floppies or worrying about drivers. Bootable CD-ROMs are so successful and prevalent that many manufacturers are dropping floppy disk drives from their systems, relying on CD-ROM if an emergency boot is ever required. 136 Part IV ✦ Storage What Are All Those CD-ROM Disk Formats, Anyhow? The more you look into how computers are built, the more specifications you find. That’s because manufacturers need precise definitions of what to expect to build products that work with each other. A large pile of standards exists just for CD-ROM alone. Here are some of the more important: ✦ Red Book — The Red Book defines the physical format of audio CDs. This is also called CD-Digital Audio, or CD-DA. ✦ Yellow Book — The Yellow Book defines the physical format for data CDs, so its purpose is similar to that of the Red Book. It’s possible to mix audio and data on the same CD. ✦ Green Book — The Green Book defines the physical format for CD- Interactive, or CD-I, a format used in a game player from Philips. However, having a CD-I compatible drive doesn’t mean you can do any- thing with a CD-I disk on your PC. In general, you can’t without some added hardware and software in the computer. ✦ Orange Book — The Orange Book defines the physical format for record- able CDs. There are two kinds — magneto-optical and write-once. The CD-R is a write-once device. (Magneto-optical drives have remained expensive and are not widespread.) ✦ CD-ROM/XA — This stands for CD-ROM/eXtended Architecture and is a combination of Yellow Book and Green Book. CD-ROM/XA has generally superseded the Yellow Book. ✦ CD Plus — Also called CD Extra, this is a specific combination of audio and data on the CD. ✦ ISO 9660 — Once called the High Sierra format, ISO 9660 defines the file and directory layouts on a CD. Extensions such as Joliet and Romeo have been defined to handle Windows 95 and NT long file names. Some of the other standards you’ll see referenced include single and multises- sion Kodak Photo CD and Video CD. The only time you’ll really need to worry about any CD standards is when new ones emerge because the product you’re looking at may or may not support the newer standard. Otherwise, the drive and software manufacturers tend to sup- port them all to avoid being at a competitive disadvantage. Recordable CD-ROMs Recordable CD-ROMs (CD-R) are the least expensive option for offline data storage. If you need archival copies of files — say, of work you’ve done and can’t afford to lose, of critical audit data, or of original customer material — CD-R is for you. CD-R works on the same pit and flat principles as CD-ROM. The difference is that a CD-R disk uses a different material for the reflective surface that can be burned by a laser to form pits, and a CD-R drive includes a more powerful laser to burn the disk. Making your own bootable CD-ROMs lets you store both diagnostic soft- ware and an archive of data to load onto an empty computer. Creating a bootable archive CD-ROM means you can create and test a disaster recov- ery disk, ensuring that it has everything you need to reconstruct your oper- ation. It used to be that CD-R was very picky about delays while burning, with any interruption in the data flow to the drive likely to ruin the disk you were recording. The enormous increases in system speed since the advent of CD-R, combined with improved interfaces into the CD-R drives, eliminated that prob- lem, making CD-R creation so nearly foolproof that CD-R is even suitable for writing out files stored on network drives. Not all CD-R media is rated for the fastest rates recorders can operate at. The faster the drive spins while burning, the less time the laser has on a pit. Drives can compensate by boosting the laser power, but you should be sure the blank disks you get are rated for the burn speed your drive is capa- ble of. If the disks are rated for lower speeds, reduce the burn speed in your software. CD-Rewritable (CD-RW) is a variant of CD-R in which you can erase the content of the disk and reuse the media. The CD-RW is a read/write optical disk, a remov- able-media device holding 660MB. Not all CD-ROMs can read CD-RW disks, how- ever, and not all software writes CD-RW in a format compatible across many machines. The most reliable use of CD-RW is for file archiving on a single machine; go beyond that boundary and you’ll risk compatibility problems. DVD With even small hard disks now in the tens of gigabytes, and with people filling those disks with great abandon, it’s not surprising that interest developed in creating a higher-capacity version of CD-ROM. The same DVD format that replaced VHS videotapes provides that technology. (DVD used to stand for Digital Versatile Disk and some other variants, but the “official” name is now simply DVD.) Video drove the development of DVD — consider that the DVD of the movie The Lord of the Rings — The Two Towers has 7.29GB of files! Table 9-2 shows why the new format was required: You just can’t fit a lot of high- quality video on a CD-ROM with MPEG 1 or MPEG 2 compression (MPEG 4 didn’t exist at the time), and if you add a high-quality stereo sound track, the Chapter 9 ✦ CD and DVD 137 [...]... data rates of between 4 and 10 megabits per second, much faster than MPEG 1 Table 9 -4 shows that all sizes of DVD hold over 2 hours of video (plus the audio tracks) at 4 Mbps, and that two-sided DVDs hold over 2 hours at 10 Mbps Table 9 -4 DVD Video Capacity in Hours versus MPEG 2 Data Rate Single-Sided Capacity Data Rate (Mbps) Double-Sided Capacity 4. 7 8 .4 9 .4 17.0 4 2.67 Hours 4. 78 Hours 5.35 Hours... the Sony DRU510a handles DVD-R/RW, DVD+R/RW, and CD-R/RW Chapter 9 ✦ CD and DVD 141 Table 9-5 DVD Format Comparison Format Write Speed Random Write Defect Management DVD-R 2X 4X No No DVD-RW 1X 2X No No DVD+R 2.4X 4X No No DVD+RW 2.4X 4X Optional Yes DVD-RAM 2X Yes Yes We’ve shown two write speeds in Table 9-5 for all but DVD-RAM, reflecting both the original speeds of the formats and the faster speeds... estimates as recently as 2001, when U.S broadband usage was a mere 11 percent, surpassed only by South Korea where a whopping 57 percent had broadband access That change accounts for the differences between this chapter and the version in the previous edition of the PC Upgrade and Repair Bible, in which we focused almost to exclusion on dial-up analog modems and Integrated Services Digital Network (ISDN)... upper is less than 4 KHz Figure 11-1 shows a typical telephone line frequency response The Chapter 11 ✦ Modems 159 3750 345 0 3150 2850 2550 2250 1950 1650 1350 1050 750 45 0 -27 -29 -31 -33 -35 -37 -39 -41 -43 -45 -47 -49 -51 -53 -55 -57 -59 150 Signal Level response changes somewhat from call to call because you’re likely to get a different connection through the switch each time, and the condition... the ability to disconnect it from the PC means failures and errors in the PC won’t damage the data stored in the disconnected disk The Seagate external drive in Figure 10 -4 can connect via either USB 2.0 or IEEE 13 94, and holds 160GB Backup IEEE 13 94 USB 2.0 On/Off Power Figure 10 -4: USB-connected external hard disk drive ©20 04 Barry Press & Marcia Press ✦ External floppy drives — Although floppy drives... drives and set-top video players You can get both 4. 7GB and 9.4GB media In practice, there’s not a lot of difference between the DVD+R/RW and DVD-R/ RW formats — it seems mostly like the unfortunate result of large companies failing to cooperate in the consumer’s best interest Many companies now ship DVD writers able to handle both, making your life easier The Sony DRU510a we’ve designed into the desktop. .. what a modem does Understanding dial-up, DSL, cable, and wireless Internet access Choosing Internet access Choosing a modem ✦ ✦ ✦ ✦ 158 Part V ✦ Networks and Communications Signals and Very Long Wires Modems are the underlying magic in computer communications and networks What makes them interesting (and difficult to design) is that doing what a modem does — pushing tens of thousands to millions of bits... disk drives are no longer sold with all current-generation PCs, having fallen victim to what’s now itself a commodity item, the CD-ROM, when the price of CD-ROM writers and blank media became as low as those for floppy drives and disks At the end of the floppy’s lifetime, the 3.5-inch, 1 .44 MB floppy disk drive was a universal standard, with 5.25-inch and the older 8-inch floppies only distant memories Higher... Two-wire switched 1,200 V.22bis Full duplex Two-wire switched 2 ,40 0 V.27 Full duplex Four-wire leased 4, 800 Half duplex Two-wire switched 4, 800 V.29 (includes Group 3 fax) Full duplex Half duplex Four-wire leased Two-wire switched 9,600 9,600 V.32bis Full duplex Two-wire switched 14, 400 V.FC (non-standard) Full duplex Two-wire switched 28,800 V. 34 Full duplex Two-wire switched 33,600 V.90 Full duplex Two-wire... configuration and setup was difficult and because there just weren’t enough total ports Even years ago, it was reasonable to want to connect a modem or two and a mouse, along with a joystick and a printer or two Today, you can add MP3 players, digital still and video cameras, personal digital assistants, home automation controllers, GPS receivers, scanners, game pads, video capture interfaces, and more to . CD-ROM recording format standards define how files and file names (and audio tracks) are stored, no standards for how video and other special- ized files are compressed and stored on CD-ROM exist hours at 10 Mbps. Table 9 -4 DVD Video Capacity in Hours versus MPEG 2 Data Rate Single-Sided Capacity Double-Sided Capacity Data Rate (Mbps) 4. 7 8 .4 9 .4 17.0 4 2.67 Hours 4. 78 Hours 5.35 Hours 9.67. 9-5 DVD Format Comparison Format Write Speed Random Write Defect Management DVD-R 2X No No 4X DVD-RW 1X No No 2X DVD+R 2.4X No No 4X DVD+RW 2.4X Optional Yes 4X DVD-RAM 2X Yes Yes We’ve shown two write