cd rom

CD-ROM Drive Construction and Operation
In terms of construction and basic components, CD-ROMs are rather similar in most regards to other storage devices that use circular, spinning media, which isn't that much of a surprise. The big difference of course is the way the information is recorded on the media, and the way that it is read from the media as well. This section takes a look at the basics of how CD-ROM drives are constructed and how they work.



Optical "Head" Assembly
The middle two letters in "CD-ROM" stand for "read only", so it shouldn't be any surprise that standard CD-ROM drives are read only devices, and cannot be written to. (Newer variants of CD-ROMs, CD-R and CD-RW drives, break this long-standing rule of this type of device.) Most people know this anyway because CD-ROMs use the same basic technologies that CD audio players have for many years, and everyone knows that these devices can only play back, not record.

The reason that the word "head" is in quotes is that CD-ROM drives do not use a read head in the conventional sense the way a floppy disk or hard disk does. It isn't just that the head cannot record, it really isn't a single solid head that moves over the surface of CD-ROM media, reading it. The head is a lens--sometimes called a pickup-- that moves from the inside to the outside of the surface of the CD-ROM disk, accessing different parts of the disk as it spins. This is just like how a hard disk or floppy disk head works, but the CD-ROM lens is only one part of an assembly of components that together, read the information off the surface of the disk.

Here's how the CD-ROM works, in a nutshell (I'm not going to go into the gory details of how the laser beams are manipulated within the drive because that can get complicated--there are in fact several slightly different ways that the internals work):

1. A beam of light energy is emitted from an infrared laser diode and aimed toward a reflecting mirror. The mirror is part of the head assembly, which moves linearly along the surface of the disk.
2. The light reflects off the mirror and through a focusing lens, and shines onto a specific point on the disk.
3. A certain amount of light is reflected back from the disk. The amount of light reflected depends on which part of the disk the beam strikes: each position on the disk is encoded as a one or a zero based on the presence or absence of "pits" in the surface of the disk. This is discussed in more detail in the section on CD-ROM media.
4. A series of collectors, mirrors and lenses accumulates and focuses the reflected light from the surface of the disk and sends it toward a photodetector.
5. The photodetector transforms the light energy into electrical energy. The strength of the signal is dependent on how much light was reflected from the disk.


Most of these components are fixed in place; only the head assembly containing the mirror and read lens moves. This makes for a relatively simplified design. CD-ROMs are of course single-sided media, and the drive therefore has only one "head" to go with this single data surface. Details on how data is encoded onto and decoded from the CD-ROM disk are included in the section on CD-ROM media.

Since the read head on a CD-ROM is optical, it avoids many of the problems associated with magnetic heads. There is no contact with the media as with floppy disks so there is no wear or dirt buildup problem. There is no intricate close-to-contact flying height as with a hard disk so there is no concern about head crashes and the like. However, since the mechanism uses light, it is important that the path used by the laser beam be unobstructed. Dirt on the media can cause problems for CD-ROMs, and over time dust can also accumulate on the focus lens of the read head, causing errors as well.

Head Actuator Mechanism
Most people don't think of a CD-ROM drive as having a head actuator, in the sense that a hard disk or floppy disk drive does. In fact, however, the lens assembly does move across the CD-ROM media in a similar way to how the heads on a hard disk or floppy disk drive do.

As described in the section on the read head, only part of the whole mechanism used to read the CD-ROM actually moves. This is the lens and mirror assembly that focuses the laser energy onto the surface of the disk. The technology used to move the read head on a CD-ROM drive is in some ways a combination of those used for floppy disk drives and for hard disk drives.

Mechanically, the head moves in and out on a set of rails, much as the head of a floppy disk drive does. At one end of its travel the head is positioned on the outermost edge of the disk, and on the other end it is near the hub of the CD. However, due to the dense way the information is recorded on the CD, CD-ROM drives cannot use the simple stepper motor positioning of a floppy disk. CD-ROM media actually use a tighter density of tracks than even hard disks do!

Instead, the positioning of the head is controlled by an integrated microcontroller and servo system. This is similar to the way the actuator on a hard disk is positioned. This means that the alignment problems found on floppy drives (and much older hard disks) are not generally a concern for CD-ROM drives, and there is some tolerance for a CD that is slightly off center (but not a lot).

Like a floppy disk, the head actuator on a CD-ROM is relatively slow. The amount of time taken to move the heads from the innermost to the outermost tracks--called a full-stroke seek--is about an order of magnitude higher than it is for hard disks.
Spindle Motor, Constant Linear Velocity (CLV) and Constant Angular Velocity (CAV)
Like all spinning-disk media, the CD-ROM drive includes a spindle motor that turns the media containing the data to be read. The spindle motor of a standard CD-ROM is very different from that of a hard disk or floppy drive in one very important way: it does not spin at a constant speed. Rather, the speed of the drive varies depending on what part of the disk (inside vs. outside) is being read.

Standard hard disks and floppy disks spin the disk at a constant speed. Regardless of where the heads are, the same speed is used to turn the media. This is called constant angular velocity (CAV) because it takes the same amount of time for a turn of the 360 degrees of the disk at all times. Since the tracks on the inside of the disk are much smaller than those on the outside of the disk, this constant speed means that when the heads are on the outside of the disk they will traverse a much longer linear path than they do when on the inside. Hence, the linear velocity is not constant. Newer hard disks take advantage of this fact by storing more information on the outer tracks of the disk than they do on the inner tracks, a process called zoned bit recording. They also have higher transfer rates when reading data on the outside of the disk, since more of it spins past the head in each unit of time.

CD-ROMs take a different approach. They adjust the speed of the motor so that the linear velocity of the disk is always constant. When the head is on the outside of the disk, the motor runs slower, and when it is on the inside, it runs faster. This is done to ensure that the same amount of data always goes past the read head in a given period of time. This is called constant linear velocity or CLV.

The reason that CD-ROMs work this way is based on their heritage of being derived from audio CDs. Early CD players did not have the necessary smarts or buffer memory to allow them to deal with bits arriving at a different rate depending on what part of the disk they were using. Therefore, the CD standard was designed around CLV to ensure that the same amount of data would be read from the disk each second no matter what part of it was being accessed. CD-ROMs were designed to follow this methodology.

The speed of the spindle motor is controlled by the microcontroller, tied to the positioning of the head actuator. The data signals coming from the disk are used to synchronize the speed of the motor and make sure that the disk is turning at the correct rate.

The first CD-ROMs operated at the same speed as standard audio CD players: roughly 210 to 539 RPM, depending on the location of the heads. This results in a standard transfer rate of 150 KB/s. It was realized fairly quickly that by increasing the speed of the spindle motor, and using sufficiently powerful electronics, it would be possible to increase the transfer rate substantially. There's no advantage to reading a music CD at double the normal speed, but there definitely is for data CDs. Thus the double-speed, or 2X CD-ROM was born. It followed in short order with 3X, 4X and even faster drives. This is discussed more in the performance section.

Virtually all of these drives up to about 12X or so still vary the motor speed to maintain constant linear velocity. As the speed of the drives has increased, many newer drives have come out that actually revert back to the CAV method used for hard disks. In this case, their transfer rate will vary depending on where on the disk they are working, again, just like it does for a hard disk. The "X" rating can be somewhat specious for these drives, since they achieve it only--at best--at the outer edge of the disk. No CAV drive claiming to be 24X actually transfers at that rate over the whole disk. Of course, hard disk drives are the same way and nobody seems to complain about their claims. Some drives actually use a partial CLV or mixed CLV/CAV implementation where the speed of the disk is varied but not as much as in a true CLV drive.

The change back to CAV as the drives get faster and faster is being done due to the tremendous difficulty in changing the speed of the motor when it is going so fast. It is one thing to change a disk spinning at 210 RPM to 539 and back again, but quite another to change it from 5,040 to 12,936 and then back to 5,040! This spin-up and spin-down action is actually one factor contributing to the slow performance of CD-ROMs especially on random accesses.

There are in fact some drives that use a mixture of CLV and CAV. This is a compromise design that uses CAV when reading the outside of the disk, but then speeds up the spin rate of the disk while reading the inside of the disk. This is done to improve the transfer rates at the inside edge of the disk, which can be 60% lower than the rates at the outside of the disk in a regular CAV drive.

Loading Mechanism
The loading mechanism refers to the mechanical components that are responsible for loading CDs into the CD-ROM drive. There are two different ways that CD-ROM media are normally loaded into the CD-ROM drive.
The most popular loading mechanism used today is the tray. With this system, a plastic tray, driven by gears, holds the CD. When the eject button is pressed the tray slides out of the drive, and the CD is placed upon it. The tray is then loaded back into the drive when the eject button is pressed a second time. Most drives will also respond to a slight "push" on the drive tray by activating the mechanism and retracting the tray.
Many older CD-ROM drives, and many higher-end drives even today, use caddies. These are small carriers made of plastic. A hinge on one side opens up to let you put a disk within the caddy, and a metal cover on the bottom slides out of the way to allow access to the CD by the drive. The caddy is inserted into the CD-ROM drive as a sort of "virtual cartridge". In fact, the CD inside the caddy is pretty similar to the way a 3.5" floppy disk works within its jacket--a media disk inside a plastic protective carrier with a sliding metal access panel. Of course the CD is still removable. Also, the CD caddies are much more solidly built.
Interestingly, some of the other loading mechanisms found on audio CD players seem to be rarely used for CD-ROMs. For example, many car stereos use direct front-loading of the CD; I've never seen this used for CD-ROMs, but I am told that the first Compaq Deskpro 2000 models used this technique. I've also never seen a top-loading CD-ROM drive (but the reason for that is pretty obvious).
Of the two mechanisms, the tray is far more common because it makes for a cheaper drive and also for cheaper use of the media. Most consumer-grade drives use trays for this reason. There are problems with these tray drives however:

1. Fragile Mechanism: One problem is that the mechanism for moving the tray in and out of the drive is really not hard to break if it is mishandled. The CD, when placed in the tray, just sort of "sits there" loose, and if you put it in the tray off-center it is possible for the disk to get stuck in the tray when it retracts, potentially damaging both disk and drive. This problem makes these drives harder for children to use, as they may misalign the disk and cause it to jam in the drive.
2. Increased Handling: Trays mean each disk must be handled a fair bit, which can increase the chances of wear, dirt accumulation and scratches on the media. Caddies eliminate virtually all handling of the individual disk.
3. No Vertical Orientation: These drives cannot be side-mounted, as the CD would fall right out of the tray. This isn't a concern for most people but it is for some. There are in fact some CD-ROMs that have four tabs around the perimeter of the tray for holding the disk in place. This might work mounted vertically, but I personally still would not do it.
Caddies are used on many high-end drives and are a much better mechanism, if you can afford to use them properly. This means that you basically need a caddy for each CD you use on a regular basis. Unfortunately, this can be an expensive proposition, because the stupid things still cost a lot of money--like $5 or more a piece! Frankly, it's kind of ridiculous that they cost this much considering that similar cartridges cost a fraction of this price. It must be just due to the fact that they are sold in such low volume. At any rate, if you have twenty CDs that you use on a regular basis then you are looking at an extra $100 for caddies, which turns most people off to these types of drives in rather short order.
Another possibility is to simply use one or two caddies and then switch the CDs into them as you need them. This is of course an option, but in doing this you lose one of the biggest advantages of the caddy: the reduced handling. You also make using your drive a pain in the rear end, because each time you use a CD you have the added step of swapping the disk into the caddy. (In fact, you have more media handling than you would with a tray drive.) If you can't afford the caddies you are usually better off with a tray drive, unless you especially need the ability to mount the drive vertically.
Logic Board
Every CD-ROM drive contains a logic board. As with the hard disk drive, the job of this board is to both control the drive and interface to the PC either using IDE/ATAPI or SCSI (in most cases).