Re: CD Quality and Longevity

From: Tim Au Yeung (ytau@UCALGARY.CA)
Date: Sun May 19 2002 - 18:06:28 CDT 

Message-Id: <200205192308.g4JN8Xm14068@sitelicense.arizona.edu>
Date:         Sun, 19 May 2002 17:06:28 -0600
From: Tim Au Yeung <mailto:ytau@UCALGARY.CA>
Subject:      Re: CD Quality and Longevity
To: mailto:IMAGELIB@listserv.arizona.edu

<pre>
> We've had a number of questions regarding what types of CDs to purchase
and
> concerns over their longevity as a stable storage medium.
>
> I've seen a few technical documents but being neither chemist nor engineer
I
> didn't get any clear answers. Are they any good general documents arguing
> for one type of CD over others?
>
> We've been recommending the Mitsui Gold CDs and I'm aware that many other
> digital projects also recommend them. If anyone is willing to share how
> they arrived at their recommendation it would be appreciated.
>
> Thanks,
>
> Richard Urban
> Operations Coordinator
> Colorado Digitization Project

Hi Richard,

There are a number of online articles that do a decent job of explaining CD-Rs and particularly the theoretical longetivity of a CD-R:

http://www.medialinenews.com/issues/2001/news/0314/0314.1.shtml http://computers.cnet.com/hardware/0-1091-8-8020643-5.html http://www.osta.org/technology/cdqa.htm http://www.emediapro.net/EM1998/starrett10.html

Here's the advice I generally give:

For the most part, the longetivity of a CD-R has less to do with the type of disc you buy and more to do with having good procedures in place for the orderly migration of the data.

To give a brief backgrounder, CD-Rs generally use one of three kinds of dye: cyanine, phthalocyanine and azo. There is a perception that phthalocyanine is the most stable dye and therefore, the reason why people recommend Mitsui -- they developed the phthalocyanine dye technique. However, most of the lab tests which may (or may not) prove this are based on artificial aging processes that may not simulate real life accurately; CD-Rs haven't been in the field long enough for the type of longitudinal data necessary to say conclusively the superiority of one over another.

While Mitsui is a good recommendation, you also have to remember that Mitsui also manufactures for other brands and so may be available under a number of names. For archival purposes, I can't recommend the Mitsui bulk on a spindle variety though. Remember that the plastic part of the CD-R (the bottom side) which everyone spends a great deal of effort to protect is actually the less important side -- it's the top layer which actually holds the data and therefore is the more critical of the two layers (technically it's the middle layer but since the top layer is so thin and bonded to the middle layer, you can consider the two together). The bulk Mitsui CD-Rs typically have no coating over the top layer exposing the metallic layer and the dye layer to damage by scratching. Even a hole as small as a pinprick can completely corrupt your data depending on the data. Brands like Kodak which use the phthalocyanine dye also add a protective coating over the dye/metallic layer and provide better protection for your CD-Rs. Another option is to use printable CD-Rs (which add a layer for inkjet/thermal printing) which also have an additional layer. You can get discs which are intended for long term archival use (like from Ricoh) but there's no clear indication that they really provide superior performance over the long term -- if the decision is between writing 1 copy on the "archival" grade CD-Rs and 2 copies on a normal CD-R, choose writing 2 copies.

The primary warning with phthalocyanine is to avoid writing at 1x -- generally the advice that writing a CD-R at a slower speed ensures better data transfer stops at 2x as phthalocyanine discs have significant problems at 1x.

Other advice for CD-R longevity:

1. Always make 2 copies and store 1 copy offsite 2. Periodically check both copies using a metric like a cyclic redundancy checksum for data integrity 3. Run the verification process after a write using your CD-writing software but don't trust it alone. Use something like a CRC afterwards. I've had discs that reported being good writes only to do a CRC check afterwards with significant errors. 4. Store data with recovery information like a parity volume (see parchive.sourceforge.net) -- even if the data corrupts, it can be recovered. 5. If you can, create a rosetta disc every so often -- that is, a disc holding the versions of applications you're currently using on your files. Don't throw out old versions as you never know when compatibility becomes an issue.

Tim
---------------------------- Tim Au Yeung Manager, Digitization Initiatives Information Resources University of Calgary

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