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Q. How reliable are CDs as a backup medium?

By Hugh Robjohns

CD-Rs are a cheap, reliable backup medium, but there are better methods of keeping your data safe.CD-Rs are a cheap, reliable backup medium, but there are better methods of keeping your data safe.

I'm wondering how reliable CDs are as a backup medium. Which are the best to go for and how do they differ? Also, how does the burn speed affect the quality of the CD? And what about using DVDs?

Enrique Fdez de Velazco Bernal

Technical Editor Hugh Robjohns replies: Well, there are quite a few issues to tackle here. Let's start with the first one — reliability. As far as I'm concerned, audio CDs are very reliable and they seem to have a relatively long and stable life. The format has been around since 1982 and the discs I bought over 20 years ago still play perfectly today, which is more than I can say for some old DAT tapes! However, audio CDs have relatively weak error protection, and if you are looking to archive audio masters, I would suggest storing them as audio data files on CD-R, rather than on their 'playable' audio CD equivalent. Besides being more robust, this would also allow tracks with word lengths above 16-bit and sample rates other than 44.1kHz. For archiving, make sure you have more than one copy, ideally stored in different places, and try to keep them at a constant (cool) temperature away from light — especially strong sunlight.

In response to your other points, the physical structure of CD-Rs is more or less identical, regardless of the recommended burn speed. The difference lies in the formulation of the dye layer — faster burn speeds require more reactive dye layers. Obviously, if you want to burn audio in real time you need a disc that can accommodate 1x speed, and slower burn speeds (below about 12x) generally produce more accurate discs with less data jitter, which work better as audio CDs. On the down side, the error protection is weaker in this format, so any help it gets from a sharper recording has to be good. Discs that can accommodate burn speeds between 12x and 52x are generally best used for data-only applications, where the stronger error correction can help counteract any deficiencies in burning precision.

Again, there is no physical difference, other than the chemical composition of the dye layer, but there are some CD-Rs specifically branded as audio CD-Rs and intended for use in consumer audio CD recorders. These differ from standard CD-Rs in that a code is stamped into the guide track on the disc. Some consumer CD recorders won't record unless they see this special code, which prevents the use of standard discs. The idea is that an additional levy would be charged on these stamped audio CD-Rs to raise funds to compensate record companies for lost revenue due to CD copying, but that's a topic to discuss on another day!

As for DVDs, the same issues affect them as CD-Rs, except that there are several different recordable DVD formats and not all machines are compatible with all types. Secondly, because a DVD can hold a lot more data, a corrupted disc will mean the loss of considerably more data or material, making it even more important to spread the risk by making multiple copies and storing them in different places. To be absolutely sure, I would use different types of disc and different batches, so that, should one format or batch prove unreliable, you won't lose all your safety backup copies as well!

RAID arrays, like this one from Adaptec, offer ideal long-term backup solutions. But they come at a price. RAID arrays, like this one from Adaptec, offer ideal long-term backup solutions. But they come at a price. Personally, I don't favour the use of CD-R or DVD-R for archiving and backups at all. The danger is that you will make an archive copy, put it on the shelf and never look at it again until the day you need it, by which time irreparable damage may have occurred. Also, it is possible that suitable CD or DVD drives might become rare in 20 or 30 years time. Try finding a working Philips N2000, multitrack Minidisc recorder or Betamax video machine these days!

Instead, I have taken to storing everything on hard discs in a RAID array with automatic backups. Hard drives are relatively cheap and reliable — Terabyte arrays are quite affordable, and copying data between drives is trivial and can be easily automated. Access to archive material is instant and can be done via networks or over the Internet if required. It is easy to copy material on to on-line storage facilities as well, for added security of external backups. And while hard drive types, sizes and interfaces will change — SCSI, SCSI II, IDE, SATA, for example — there will always be a sufficient overlap to allow data to be copied from the outgoing format to a new one. Data doesn't degrade through the process and so this approach is about as future-proof as it can be. Of course, how practical this is depends on how much material you need to archive, but it certainly works well for me and I know of many others who have taken the same approach. 

Published September 2006