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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.

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

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.

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.

Article C1944 - May 20, 2004 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

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22 Comments
Paul
May 27, 2004 10:54 PM

this info helps
but id like to know how many KB/sec each multiple is for dvd if i remember correctly 1x cd-rom is 1500 Kb/sec and im pretty sure each dvd multiple is faster
thanks

Dude
July 10, 2004 5:23 PM

1x CD is 150kb/sec
so 52x is 7.8Mb/sec and 48x is 7.2Mb/sec

Can someone PLEASE post what 1x DVD speed is?

Thanks

Leo
July 10, 2004 5:32 PM

The problem is that it varies. I've read between 1400 and 580 RPM depending on whenre on the disk you are. 2x and 4x may not be strict multiples either, since manufacturers will play games with things like "effective" and "burst" data rates. Try this site: http://www.osta.org/technology/dvdqa/dvdqa4.htm

winston
October 9, 2004 1:22 AM

i thought 150kb/s for cd-rom was only an estimation, what is the exact speed??

ReKleSS
October 20, 2004 1:19 AM

1x for DVD is approximately 1400kb/s, I think... this is according to readcd.
-ReK

Mike
February 3, 2005 7:49 AM

I can never remember the what the order of the
notation on the oudside of a cd or dvd drive.
For example I have a dvd/cdrw and on the front
it says "52x 24x 52x 16x" I am guessing that
that means it can read a cd at 52x and a dvd
at 24x then it can write a cd at 52x and write
a dvd at 16x. Is that correct?

Leo
February 3, 2005 6:11 PM

I believe it varies, but I would assume what you've assumed as well.

zia ur rehman
June 2, 2005 4:48 AM

plz tell me the maximum rps speed of a 52x cd rom? and can cd rom attain rps speed more than 1000

tech-helper
September 9, 2005 7:51 PM

hello,

in the info. provided above ...its given that

"A 1X CD-ROM spins 500 times per second"

and the next line 48x means 24000 rotations per MINUTE.
but 48 x 500(rps) = 24000 (rps)
and its given 24000 rpm (units are different)

can u plz tell is it rps or rpm?

bye,
tech-helper

Leo
September 11, 2005 9:11 PM

Typo - should be RPM.

Anne
January 2, 2006 5:22 PM

May I use this article for reference in my physics experiment? Oh and where else do you publish your work?

shashi
January 4, 2006 12:18 PM

Thanx for providing the detailed info, till date I was nt aware about the basic things about x in 48x.

Renjith
January 6, 2006 7:08 AM

Hello Sir,

Really a good article. I searched for the same in different sites, but not a bit from any other. At last I got this site, and I got what I want from here. Thank you

thunderdome
January 13, 2006 10:38 PM

hi,
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. X=150 kilobytes (KB) per second.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.For more goto "http://www.pcguide.com/ref/cd/perfRated-c.html".

AMIT UPADHYAY
May 31, 2006 3:34 AM

thanks...i was searching for this 'X' for about half an hour untill i came across this article of yours.
good article.

David
August 10, 2007 7:00 AM

On a 12x8x32x DVD-RW drive, what does the 8x stand for?

subnullgom
January 20, 2008 7:52 AM

12x8x32 > 12 = max writespeed on a normal recordable cd, 8 = max writespeed on a rewritable cd and 32 is the max readingspeed of a cd.

Kathy
April 17, 2008 5:50 PM

Does your comment about increased speed pertain to data as well as audio. Would fast CDs with higher speed function well with data instead of audio, or do the same problems apply? Thanks.
Kathy

Tan Jian Wei
July 9, 2008 6:24 AM

There are some cases that the cd in the cd drive explodes in the cd-rom drive ---youtube link---
http://www.youtube.com/watch?v=v0QT4Y1Y3ug

Monina Lao
December 6, 2008 6:49 AM

oh thanks finally able to understand the what the speed X really means

PD Brannan
May 11, 2009 2:43 PM

I just recently added a cd/dvd reader-writer to my tower, and have been asking MANY questions about the "X" factor.
Your explanation is the only one I have understood, lol! Thank-you!

Bob
August 12, 2010 1:32 AM

Does the X speed improve quality?

Nope. It's all about saving time. If anything, slower can often be better quality and you'll often see readers and burners slow down if they're having problems.
Leo
14-Aug-2010

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