What does the 'X' in a 48X speed CD-ROM mean?

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Summary: CD-ROMs and now DVDs have become common components in computer equipment. But what does it mean when someone says the speed is 48X? We'll explain.

The 48 part is easy. We know it's some kind of measure of speed. 48 is faster than 16, and 52 is faster than 48.

The X? It's really just shorthand for "times" - as in multiplication. That 48X drive is 48 times faster.

48 times faster than what?

48 times faster than a 1X drive of course.

OK, that's not very helpful. Let's examine what a 1X drive is and whether or not we're going to see numbers much higher than 52 any time soon.

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By now you know that the basic technology used to create CD-ROMs is fundamentally the same as that used to create the audio CDs you might play on your stereo. In fact, the audio CDs came first.

Audio CDs are by definition "1X". The very first CD-ROMS were built using the same mechanisms that audio CDs were and were limited to the same speed that an audio CD required. An audio disk can hold roughly 70 minutes of music and a 1X CD-ROM takes about 70 minutes to be read completely.

70 minutes might be the right amount of time for a symphony or two but that's ages if all you're trying to do is copy 650 megabytes - the typical capacity of a CD-ROM. So manufacturers started making the disks spin faster so that the electronics could read the data off the disk faster. Soon 2X drives appeared - only 35 minutes for a full disk. Then 4X and we were down to around 18 minutes.

Today 48X is a very common speed for a CD-ROM reader, and means that the disk is spinning 48 times faster than your audio disk. In theory that means your CD-ROM should be able to be completely read in just under a minute and a half.

In theory.

A couple of things work against that theoretical ideal. 48X is very fast. A 1X CD-ROM spins 500 times, or revolutions, per minute (RPM), at its fastest (it spins quickly when the inner tracks are being read, and slows down to around 350 RPM as the outer tracks are read.) That means that a 48X drive might attempt to spin the disk at up to 24,000 RPM. At that speed even minor imperfections or dust can get in the way. When that happens the drive will compensate by slowing down (you might actually hear the drive change speed), or attempting to re-read the data from the disk over and over until it works. The net result is that it takes longer to get the data off the disk.

650meg in a minute and a half is also very fast. So fast that many computers or their data paths to the CD-ROM are too slow or too busy to accept data at that rate. Hence, the data is transferred at less than top-speed.

Finally, many manufacturers will claim something like 48X when in fact 48X is only a maximum "burst" speed; meaning that the drive or its electronics are not able to sustain that speed long enough to read an entire disk.

Will we see 100X drives? 1000X? At this point it's unlikely. Even at current speeds we're starting to push the envelope on the ability of the plastic of the CD's themselves. Spin them much faster, and the CDs stand a real chance of flying apart.

As a side note, DVD readers are also starting to use the "X" notation. But this time it's against the basic speed of a video DVD. DVDs spin faster to begin with and are packed with more data per inch. So a 1X DVD is already faster and denser than a 1X CD. Thus a 1X DVD player will deliver data much faster than a 1X CD player.

Related:

• How Stuff Works - How CDs Work

Article C1944 - May 20, 2004