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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.
This table summarizes the differences between CLV and CAV:
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.
Next: Loading Mechanism