Question about CRC errors

[Hulsie]

This question is more of a general computer question than just an 8-Bit question, but since I've experienced it the most there, I thought I would ask it here.

 

Do CRC errors always equal damaged/corrupted/unusable data?

 

I've been imaging a lot of old disks I've accumulated and got a lot of CRC errors in ProSystem. The disks weren't kept very well. All the games on the disk still seem to work though, and I don't get any load errors, and everything works on the image file too. I can also write the image back to a disk without a problem.

 

I only vaguely understand what CRC is used for (other than the fact that it is a method of checking for errors) but it isn't clear to me if CRC errors always mean bad data.

 

Also, does copying/imaging disks actually copy the bad CRC properties?

[Rybags]

CRC formulas are used as an advancement over other earlier ones like parity and checksums.

 

Parity on it 's own has the weakness in that it can potentially miss 50% of bit inversions. Checksums have the weakness in that they won't pick up errors where bytes in a record are moved around (not that it's a typical source of error in data storage or transmission).

 

CRC tends to be more robust in that the CRC formula will generate different values if bytes are moved around or single bit errors occur in 1-<few> bytes in a record.

 

Copying in the "innocent" sense generally won't replicate CRC errors. CRC, checksum, parity aren't considered part of the "payload" ie the data that the end user or program needs.

Copying in the sense of duplicating commercial software e.g. nybblers or hardware mods that copy stuff will usually try as best as possible to replicate the original data and that includes bad checksums, CRCs etc.

 

Bad CRC might or might not mean unrecoverable data. There are other mechanisms these days to allow rebuilding of data records when there's only a few single-bit errors present. CDs and DVDs use these, it's worth researching to find how they work.

Generally they increase the overall amount of data by a reasonably significant amount, but increase the overall reliability of the media in use.

 

Of course all these systems can fail if the CRC itself is the source of corruption, but some systems even have redundancies for that.

Edited June 1, 2013 by Rybags

[Hulsie]

Fascinating. Thank you.