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[ The PC Guide | Systems and Components Reference Guide | CD-ROM Drives | CD-ROM Performance and Reliability ]

Rated "X" Speed, CLV and CAV

It seems that every computer component has a commonly-used metric that is supposed to provide a universal way of allowing you to compare them, but which in fact can be very misleading if you don't fully explore what it means. With processors it is the MHz clock speed; with memory the DRAM chip speed; with monitors it is often the nominal screen size; and with CD-ROM drives it is the "X" speed.

The "X" speed of the drive refers to its nominal spin rate. A 1X drive spins that the speed of a standard audio CD; a 2X drive spins at twice this speed, etc. Since regular CD-ROM drives use constant linear velocity (CLV), this means that the actual spin speed of the disk changes from about 210 to 539 RPM depending on whether the inside or outside of the disk is being read. A 2X drive would double this to a range of 420 to 1078 RPM, etc.

Most drives 12X and down use CLV. Newer and faster drives changed to a fixed spindle speed, which is a system called constant angular velocity or CAV. Here, the spindle speed remains the same and the transfer rate changes depending on where you are on the disk. All of this is explained in more detail in the section describing the spindle motor. I personally find these drives enjoyable to use, because they make much less noise when seeking around the surface of the disk, since they don't have to change speeds.

Some people think that the performance of a drive scales linearly with the "X" number of the drive. So they think that a 12X drive will transfer data at 12 times the speed of a 1X drive, will have 1/12th the access time, etc. Even more, they think that if it takes 15 minutes to install Windows 95 on a 4X drive, that it will take only 5 minutes on a 12 X drive. Of course, it is really never this simple.

With a standard CLV drive, the "X" number refers to the spin rate and therefore the theoretical maximum transfer rate. So a 12X drive will have a theoretical transfer rate of 12*150KB/s = 1.8MB/s. This is only theoretical however, and is affected by command overhead, the interface speed, etc.

The access time of the drive, which refers to how quickly it can move around on the disk when not reading sequentially, gets better with faster drives but definitely not linearly. No 12X drive I know of has an access time 1/12th that of a standard 1X CD player. Most real-world uses of the drive--such as installing software--require the read head to do random accesses over various parts of the surface of the disk. You will only partially get the benefits of the 12 times speed of the drive when doing sequential reads of substantial length.

With newer CAV drives, you can't even use the "X" to figure out the transfer rate! Since the drive spins at the same speed but there is less data in the middle of the disk, the highest transfer rate (the one that they put in the specifications and which corresponds to the "X" speed) only applies to reading data from the outermost part of the disk. A 24X drive is only 24X at the edge of the CD; in the middle it will be significantly slower, in fact as much as 60% slower. To make matters worse, on a CD information is recorded on the inside first, and then moves to the outside. If the disk is half-full then, none of the data will be read back at anything even near 24X on such a drive. See this section on transfer rates for more. (On the other hand, these CAV drives also have the advantage of not having to change speeds, which can make them perform more smoothly even if their raw transfer rate is below that implied by "the big number").

It is also important to realize that not all drives with the same "X" rating are created equal. They may all spin at the same speed, but differences in control circuitry, the ability to speed up or slow down the motor, and other quality factors, can mean that one 8X drive performs a lot better than another one. Here, as with most things, spending less money usually means you will get less performance. A $200 8X drive is, all else being equal, going to provide better performance (reliability, etc.) than a $100 8X drive, or nobody would buy the $200 model.

Note: There are now on the market so-called "100X" CD-ROM drives. Be aware that these are not a 100X CD-ROM drive at all--not even close. When you read the specifications up close you find out what is really going on: this is a plain 12X CD-ROM drive--not even close to the fastest available--that uses the hard disk to buffer the contents of the CD and therefore dramatically speed up the CD-ROM. Well, this is far from being anything new. You can do the same thing with any CD-ROM drive if you have 650 MB of free disk space, and there are many different CD-ROM caching programs around that work similarly. Don't be fooled by marketing people doing this sort of thing, which is in my opinion blatant false advertising.

Next: Speed Change Time

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