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Are DVDs Archival?

Aug 25, 2005 12:05 PM, D.W. Leitner

A perennial personal initiative I never quite get around to is to convert, once and for all, my crowded shelves of Beta, U-Matic, Hi-8 and VHS samples of my work I’ve accumulated through the years to a stable, archival digital video medium, namely DVD.

After all, shelf space in Manhattan is precious real estate. A trim, compact row of DVDs would do wonders for my overflowing studio apartment.

No more dubbing hassles or costs either. I could readily burn DVD copies as needed using my own computer. I could dump those legacy VTRs and VCRs and free up even more precious real estate.

But each time I glance at those packed shelves, my spirits sag. Instead of neatly organized cassette cases, I see instead a tangle of flaking oxides, brittle binders, missing lubricants, and evaporating plasticizers that promise nasty head clogs, tape jams, and cassettes that won’t play. Where to find a Hi-8 deck with a TBC these days, anyway? No wonder I put the whole thing off.

And then there’s this disturbing thought: what if I go through the considerable effort to convert my precious oeuvre to the ultimate in digital archiving, DVDs, then discard all my analog tape masters only to encounter someday a DVD that unexplainedly won’t play?

Could this happen? How archival a medium is DVD, anyway?

Consider this: videotapes, both analog and digital, store video in a linear, frame-based manner, in which a discrete frame occupies a fixed length of tape. This means that if the beginning of the tape is damaged, for example, the remainder of the tape usually remains playable.

DVDs, however, don’t record video. They’re file-based. They store compressed MPEG2 files, which upon uncompression reconstruct a passable facsimile of the original video signal. In a DVD’s MPEG2 bitstream, the bit-rate per frame is widely variable, contingent upon degree of compression, with most of the original frames tossed out during compression and replaced with virtual frames (called P for predicted and B for bidirectional) that no longer exist by themselves.

Guess what happens when a file’s header is damaged? That hour of precious video, now a single MPEG2 file, goes poof. Completely. You can’t rewind, repack, or clean heads. Nothing to “try” in desperation, to attempt a save.

But I digress. The question remains: as media, how secure are DVDs we presently use? Are they indestructible, or do they physically degrade over time, like film, videotape, and all things plastic?

Surprisingly--or perhaps not surprisingly--the answers to this basic question are elusive and hard to track down.

Most, even those in media production, don’t realize that CDs and DVDs are structurally different from one another in a fundamental way that affects durability.

In a CD, the reflective metal layer that contains data exists in the top surface of the CD, the label side. Only a thin coat of lacquer protects it from damage. Yes, if you bear down with a ballpoint pen, you’ll likely penetrate the lacquer and tear the metal reflective layer. Don’t believe me? Scratch the label side of a CD with a sharp object. You can see clear through the plastic polycarbonate base of the CD. Now try to play it.

I recently struggled mightily to read a critical JPEG file on a CD-R disc, only to discover on close inspection a pin prick on the label side. Who knows how it got there? For all I know, it flaked off on its own, a defective disc coating. There was nothing I could do to rescue it.

My question is, why do people mistakenly think the clear plastic side of a CD is the side more vulnerable to scratches?

Likewise, the structure and vulnerabilities of DVDs is poorly understood. Unlike a CD, the metal reflective layer of a DVD is not on the surface--thank goodness--but instead sandwiched between two tough layers of polycarbonate.

This however creates false confidence that DVD data is safer.

Did you know that polycarbonate can absorb moisture, which oxidizes the aluminum used as a reflective layer in pressed DVD-ROM discs like those sold by Hollywood? When aluminum oxidizes it becomes dull, like an aluminum pot. Disc readability flat-lines.

More bad news. Oxygen can also suffuse polycarbonate with sulfur dioxide, a common air pollutant that corrodes the silver sometimes used as a reflective layer.

To write data, popular write-once discs like CD-R and DVD-R use photosensitive organic dyes. Sound familiar? Remember those fading Kodak color motion picture prints Martin Scorcese campaigned 20 years ago to improve? Susceptible to color loss over time, with accelerated fading from UV, high temperatures, and humidity?

The same story with CD-Rs and DVD-Rs, although I guess the fact we don’t shoot high-intensity xenon beams through them prolongs their life compared to film prints. But still.

Recently, on a shoot interviewing KKK in North Carolina, I bought a big spindle of Sony DVD-R blanks on sale at a Circuit City. I left them in the car while we filmed under the broiling Southern sun. When I got back to Manhattan, the first several discs wouldn’t burn due to assorted disc errors. Sony discs have always been 100% reliable for me. Then the answer dawned on me.

I’ve only scratched the surface, so to speak, of the topic of DVD archival merit. The best, most succinct discussion of this important issue is a 2003 document entitled “Care and Handling of CDs and DVDs--A Guide for Librarians and Archivists,” by Fred R. Byers, which can be found at: www.itl.nist.gov/div895/carefordisc/CDandDVDCareandHandlingGuide.pdf

It’s co-published by NIST, the National Institute of Standards and Technology of the U.S. Dept. of Commerce, and the Council on Library and Information Resources, a Washington D. C. nonprofit created in 1997 to “foster new approaches to the management of digital and nondigital information resources so that they will be available in the future.” Notably, over 170 major American universities along with the Library of Congress, the Bill & Melinda Gates Foundation, and the Andrew W. Mellon Foundation fund the CLIR.

NIST reports a consensus among manufacturers that under recommended storage conditions [italics mine], CD-R, DVD-R, and DVD+R discs “should” function 100 to 200 years; that CD-RW, DVD-RW, DVD+RW, and DVD-RAM discs “should” function 25 years, and that while “little information is available for CD-ROM and DVD-ROM discs… expectations vary from 20 to 100 years.” They also note that “few, if any, life expectancy reports for these discs have been published by independent laboratories [italics mine]. An accelerated aging study at NIST estimated the life expectancy of one type of DVD-R for authoring disc to be 30 years if stored at 25 degrees C (77 degrees F) and 50% relative humidity.”

What if stored under less than these optimum conditions?

While not immortal, DVDs appear to outlast videotape. So I still intend to convert those shelves of personal tapes. Only I think I’ll make multiple DVD masters, anticipating a future in which one or two unexpectedly turn into coasters.

I’ll probably fill up those shelves again with my multiple sets of duplicate DVD masters.

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