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

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.

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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.

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22 comments on “What does the 'X' in a 48X speed CD-ROM mean?”

  1. 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

    Reply
  2. 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?

    Reply
  3. 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

    Reply
  4. 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

    Reply
  5. 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”.

    Reply
  6. 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.

    Reply
  7. 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

    Reply
  8. 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!

    Reply
  9. 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

    Reply

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