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As its name implies, CD-Read Only Memory is fundamentally an adaptation of the Compact Disc to storing digital information-rock and roll comes to computer storage. Contrary to the implications of the name, however, you can write to CD ROM discs with your PC, providing you buy the right (which means expensive) equipment. For most applications, however, the CD ROM is true to its designation-it delivers data from elsewhere into your PC. Once a CD ROM disk is pressed, the data it holds cannot be altered. Its pits are present for eternity.

In the beginning, CD ROM was an entity unto itself, a storage medium that mimicked other mass storage devices. It used its own storage format. The kind of data that the CD ROM lent itself to was unlike that of other storage systems, however. The CD ROM supplied an excellent means for distributing sounds and images for multimedia systems; consequently, engineers adapted its storage format to better suit a mixture of data types. The original CD ROM format was extended to cover these additional kinds of data with its Extended Architecture. The result was the Yellow Book standard.

Format

The Yellow Book describes how to put information on a CD ROM disk. It does not, however, define how to organize that data into files. In the DOS world, two file standards have been popular. The first was called High Sierra format. Later this format was upgraded to the current standard, the ISO 9660 specification.

The only practical difference between these two standards is that the driver software supplied with some CD ROM players, particularly older ones, meant for use with High Sierra formatted disks may not recognize ISO 9660 disks. You're likely to get an error message that says something like "Disc not High Sierra." The problem is that the old version of the Microsoft CD ROM extensions-the driver that adapts your CD ROM player to work with DOS-cannot recognize ISO 9660 disks.

To meld CD ROM technology with DOS, Microsoft Corporation created a standard bit of operating code to add onto DOS to make the players work. These are called the DOS CD ROM extensions, and several versions have been written. The CD ROM extensions before Version 2.0 exhibit the incompatibility problem between High Sierra and ISO 9660 noted earlier. The solution is to buy a software upgrade to the CD ROM extensions that came with your CD ROM player from the vendor who sold you the equipment. A better solution is to avoid the problem and ensure any CD ROM player you purchase comes with Version 2.0 or later of the Microsoft CD ROM extensions.

ISO 9660 embraces all forms of data you're likely to use with your PC. Compatible disks can hold files for data as well as audio and video information.

For Window 95, Microsoft created another set of extensions to ISO 9660. Called the Joliet CD ROM Recording Specification, these extensions add support for longer file names-but to 128 characters instead of the 255-character maximum of Windows 95-as well as nesting of directories beyond eight levels, allowing directory names to use extensions, and broadening the character set. To maintain compatibility with ISO 9660, the extra Joliet data must fit in a 240-character limit, foreclosing on the possibility of encoding all Windows 95 directory data.

Players

In that a computer CD ROM player has the same basic job as a CD-Digital Audio machine in your home stereo, you'd expect the technology inside each to be about the same. In fact, all have similar mechanisms.

CD ROM players tend to be more expensive than stereo models because retrieving computer data is more demanding. A tiny musical flaw that might pass unnoticed even by trained ears could have disastrous consequences in a data stream. Misreading a decimal point as a number, even zero, can result in error laden calculations. To minimize, if not eliminate, such problems, computer CD ROM players require different error correction circuitry than is built into stereo equipment that uses much more powerful algorithms. CD-DA errors are corrected at the small frame level, 24 bytes at a time. CD ROM data errors are corrected at the large frame level, 2048 more bytes at a time.

CD ROM players also require more intimate control and faster access times. The toughest job a digital audio player faces is moving from track to track when you press a button. A CD ROM player must skate between tracks as quickly as possible-in milliseconds if your human expectations are to be fulfilled.

Even the link between your PC and CD ROM player complicates the drive and makes it more expensive. By itself a CD ROM player does nothing but spin its disk. Your computer must tell the player what information to look for and read out. And your computer is needed to display-visually and aurally-the information the CD ROM player finds, be it text, a graphic image, or a musical selection. Sending those commands requires an interface of some kind, in most cases either SCSI or ATAPI. Neither is needed in a digital audio player.

Transfer Rate

Unlike music and video systems, which require real time playback of their data (unless you prefer to watch the recorded world race by as if overdosed on adrenaline), digital data is not ordinarily locked to a specific time frame. In fact, most people would rather have information shipped as quickly as possible from disc to memory.

For real time playback, the original CD-Digital Audio system required a 150-kilobyte-per-second data transfer rate. In the data domain, however, that's almighty slow-one-quarter to one-tenth the throughput of a modern hard disk even after you account for all the overhead. Raw hard disk transfer rates exceed 50 times the CD-DA rate.

The transfer rate of a Compact Disc system is a direct function of the speed at which the disc itself spins. Increasing the data transfer rate requires higher rotation speeds. Consequently, today's CD ROM players operate at multiples of the standard CD DA spin rate. The speeds are usually expressed as a multiple of the spin rate of the original audio CD speed-for example 1x, 2x, 4x, or even 20x.

Because of the different sizes of blocks and error-correction methods used by different formats, the exact transfer rate at a given spin rate varies with the type of CD. Tablebelow lists the transfer rate of various common CD formats at different speed ratings.

Actual Transfer Rates in Bytes per Second in Various CD Modes

In common parlance, the Mode 1 rate is taken as the basis for measuring transfer speed, and it is usually rounded to 150 KB/sec. Consequently double-speed (2x) drives spin twice as fast to deliver 300 KB/sec transfer rates; quadruple-speed (4x) drives, 600 KB/sec; and so on.

Faster is, as always, better. But speed ratings can be misleading. Today's fast drives (rated up to 20x) operate with constant angular velocity recording-they spin at a constant rotation rate so their actual data transfer rate varies, increasing as the head travels from the inside edge of the spiral track to the outside edge. These CD drives almost universally are rated at the highest speed they achieve, the potential peak transfer rate. In actual operation, the transfer rate varies from this rated speed (achieved only at the outer edge of the disc) down to one-half that rate (at the inner edge of the recorded area). Because the data begins at the inside of the disc and progresses outward, these high speed drives may actually never run up to their rated rate on actual discs. Moreover, if a basic original 1x drive spun its disc at a constant rate, it would, by this rating system, be called a 2x drive.

As a practical matter, the rated speeds give you a means for comparing drives even though the ratings don't reflect actual performance. The minimum speed you should expect from a new playback-only CD drive is 8x. Most software works adequately with drives as slow as 4x (the latest releases may be more demanding). CD recorders operate at lower rates but are catching up.

High speed drives can retain their compatibility with Red Book audio by buffering. They read the audio data at their higher rate and pump it into their buffer. Then they unload the buffer at the real time rate.

Access Time

Compared to magnetic hard disks, CD ROM players are laggardly beasts. Mass is the reason. The optical head of the CD system is substantially more massive than the flyweight mechanisms of hard disks. Instead of a delicate read/write head, the CD ROM player has a large optical assembly that typically moves on a track. The assembly has more mass to move, which translates into a longer wait for the head to settle into place.

Besides the mass of the head, the constant linear velocity recording system of CD ROMS slows the access speed. Because the spin rate of the disk platter varies depending on how far the read/write head is located from the center of the disk, as the head moves from track to track, the spin rate of the disk changes. With music, which is normally played sequentially, that's no problem. The speed difference between tracks is tiny, and the drive can quickly adjust for it. Make the CD system into a random-access mechanism, and suddenly speed changes become a big issue. The drive might have to move its head from the innermost to outermost track, requiring a drastic speed change. The inertia of the disk spin guarantees a wait while the disk spins up or down.

Some old CD ROM players required nearly a second to find and read a given large frame of data. Modern designs cut that time to 100 to 200 milliseconds, still about ten times longer than the typical hard disk drive.

Mechanism

Nearly all CD ROM players fit a standard half-height 5.25-inch drive bay for one very practical reason: A 4.6-inch disc simply won't fit into a 3.5-inch drive slot. Drives can be internal or external, the latter usually including a power supply. The best choice is what works for you-internal for lower cost if you have expansion space inside your PC, external if you don't (for example, if you have a notebook computer).

Disk handling and how you get a disc into a CD ROM player is an important aspect of drive design. Some mechanisms incorporate a sliding drawer much like that on most audio CD systems using either a micromotor or a spring to slide the drawer out. You simply drop the disc in and slide the drawer closed. The only important drawback to this design is that drives must be mounted horizontally lest the discs fall out.

Some CD ROM players make the job even easier-and safer for your discs. You load your discs into a special carrier called a caddy which resembles the plastic jewel box case that most commercial music CDs come in. When you want to load a disk into the CD ROM player, you slide the whole carrier into a waiting slot. Most people buy a carrier for each CD ROM disk they have because of this convenience and the extra protection the carrier affords the disk-no scratches and no fingerprints, guaranteed! If your CD system gets heavy use by younger folk or uncaring office personnel, use of a caddy will extend the life of your investment in CD media. Most drives that use caddies will operate either in horizontal or vertical orientation.

At one time three forms of caddy were used, but now the industry has settled on one. The most common was initially used by Denon, Hitachi, some Matsushita, Sony, Toshiba, and newer NEC drives. It is the survivor. Its design resembles a 3.5-inch floppy disk with a single metal shutter that slides back to let the drive optics see the disk. You open the caddy by squeezing tabs at the end opposite the shutter to drop in a disc.

The other two caddy designs have essentially disappeared from the market. The Philips-style caddy was clear smoked plastic and opened by pressing two tabs. The CD slides inside between large white plastic pincers. The other style was that used by the old NEC CDR-77/88 drive.

Changers

As CD collections grow, the old idea from the stereo system-the disc or record changer-becomes increasingly compelling. You can load up several discs and have them at ready access. No more shuffling through stacks of discs and jewel cases. You can keep your favorite discs just a keystroke away.

The first CD ROM changers were, in fact, derived from those used in stereo systems. Pioneer adapted its six-CD cartridge to computer use to create the first changer-only natural in that Pioneer has patented that changer design. More recently, other manufacturers have developed changers that don't need cartridges. As with single-disc CD drives, the choice between cartridge and free-disc system is one of preference. Either style of drive works.

Similarly, CD changers operate at the same speeds as single-CD units, although the fastest changers lag behind the fastest single-disc drives. Today 6X changers are commonplace. Unfortunately, software has not tracked the developments in CD changers. Both applications and operating systems have problems with the multi-disc drives.

Most driver software for CD changers assign separate drive letters to each disc (or disk position) in the changer. A four-disc changer would thus get four drive letters. To access the disc in a particular changer slot, you only need to use the appropriate drive letter.

Life isn't so simple, however. Most CD-based applications don't expect being loaded into a changer and react badly. Although some applications will run no matter where you load them, some will force an icon to pop on your screen and ask in which slot to find the disc. Some are worse-they will refuse to run except in a favored slot with the favored drive letter. Worse, the error message you get won't help you find the problem. It may tell you that the program can't find its drive, even if you've installed the program for that exact drive. The only solution is to find the slot the program favors and always use that slot for that particular program.

Dumb old DOS doesn't mind changers particularly. It just accepts whatever drive letters you give it. Advanced Windows 95, however, outsmarts itself by testing and sensing all the drives that are connected to it. As a result, when Windows boots up, it will chunk through each slot in your CD changer and wait for it to come up to speed. In that spinning up and down each disc may take 20 seconds, cycling through the whole changer can add minutes to the already long boot-up interval. In addition, some changer drivers do not interface well with Windows and may not properly inform Windows when you change discs. Change discs without closing a program, and the system may hang until you slide the program's disc back into the right slot and close it properly.

Certainly better drivers and applications that are more aware will come onto the market. But you should beware that, as with any new technology, CD changers are not perfect.

Controls

The MPC specification requires a volume control on the front panel of any CD drive you have in your multimedia PC. This control is useful if you decide to use your drive for playing back music while you work. You can plug headphones into the jack, also on the front of the drive, and use the volume control to adjust the loudness of the playback independent of the CD control software you run on your PC.

Other than the volume control, CD drives need no physical controls. All of their functions are operated by the software you run on your PC.