# What is a cyclic redundancy check? **URL:** \ **Category:** Factual Questions\ **Created:** [August 16, 2005, 9:28am UTC](https://boards.straightdope.com/t/what-is-a-cyclic-redundancy-check/317244 "2005-08-16T09:28:47Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Thin\_Lizzy](https://avatars.discourse-cdn.com/v4/letter/t/e9a140/32.png) [@Thin\_Lizzy](https://boards.straightdope.com/u/Thin_Lizzy)\ **Post date:** [August 16, 2005, 9:28am UTC](https://boards.straightdope.com/t/what-is-a-cyclic-redundancy-check/317244/1 "2005-08-16T09:28:47Z")
Sometimes when copying cds to my HD I get this message and the cds won’t copy. what does it mean? ---
**Author:** ![Harmonix](https://avatars.discourse-cdn.com/v4/letter/h/3ec8ea/32.png) [@Harmonix](https://boards.straightdope.com/u/Harmonix)\ **Post date:** [August 16, 2005, 9:40am UTC](https://boards.straightdope.com/t/what-is-a-cyclic-redundancy-check/317244/2 "2005-08-16T09:40:39Z")
CRC is used to verify if the data you write to your HD is truely the same as on the cd. When you run into a CRC error it’s because it copied wrong. ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [August 16, 2005, 9:42am UTC](https://boards.straightdope.com/t/what-is-a-cyclic-redundancy-check/317244/3 "2005-08-16T09:42:57Z")
A CRC is a form of error-correction used on networks and storage devices. A CRC checksum is easy to calculate for a large set of data. If you send the CRC with the data, the receiving machine or device can re-compute the CRC and make sure it matches with the one that was sent. If they don’t match, then there was an error in transmission and the data is incorrect. If you want the gory details about how they’re calculated you can read [this](http://en.wikipedia.org/wiki/Cyclic_redundancy_check). ---
**Author:** ![SentientMeat](https://avatars.discourse-cdn.com/v4/letter/s/43a26b/32.png) [@SentientMeat](https://boards.straightdope.com/u/SentientMeat)\ **Post date:** [August 16, 2005, 9:44am UTC](https://boards.straightdope.com/t/what-is-a-cyclic-redundancy-check/317244/4 "2005-08-16T09:44:50Z")
It is an [error check](http://en.wikipedia.org/wiki/Cyclic_redundancy_check) attached to a given block of 1’s and 0’s to make sure it has been received correctly. If your data is, say, 111000111 and the particular operation on this gives, say, a 0, then that result is appended to the data to give 1110001110. Then, when that is received somewhere else, the receiver performs the same operation on the 111000111 data and, if the answer is 0, no error is assumed to have arisen in transmission. However, if there _was_ an error somewhere (_eg._ 1 **0** 1000111 is received), then when the operation is carried out it will give a **1** , not the 0 which was appended upon transmission. An error is therefore assumed to have taken place. ---
**Author:** ![Thin\_Lizzy](https://avatars.discourse-cdn.com/v4/letter/t/e9a140/32.png) [@Thin\_Lizzy](https://boards.straightdope.com/u/Thin_Lizzy)\ **Post date:** [August 16, 2005, 9:56am UTC](https://boards.straightdope.com/t/what-is-a-cyclic-redundancy-check/317244/5 "2005-08-16T09:56:07Z")
it keeps happening with the same disc. is there a way to prevent it, or skip the error check? using XP btw ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [August 16, 2005, 10:42am UTC](https://boards.straightdope.com/t/what-is-a-cyclic-redundancy-check/317244/6 "2005-08-16T10:42:43Z")
I presume you’re copying data files or executables, not photos or music? If so, you don’t want to skip the error check - the file will likely be useless anyway. There’s a possibility it’s just a badly scratched or dirty CD. If that doesn’t work out, you need to re-source the CD. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [August 16, 2005, 1:08pm UTC](https://boards.straightdope.com/t/what-is-a-cyclic-redundancy-check/317244/7 "2005-08-16T13:08:52Z")
There are also copy protection schemes that deliberately record the data with CRC errors precisely so PCs can’t be used to copy them. The simpler audio players will still play the music just fine, but copying to a computer will fail every time.