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MillimeterMPEG-2, a Smart Archival Choice?
Feb 23, 2006 10:54 AM, D.W. Leitner
In past columns we’ve asked, “How archival a medium is DVD, anyway?”
We’ve learned that the polycarbonate that makes up the disc can absorb moisture, that the aluminum or silver reflective layer sandwiched between a DVD’s two .6mm polycarbonate discs can tarnish and lose shininess. That to write data, DVDs use photosensitive organic dyes which, like those of color film, are susceptible to fading over time, particularly when exposed to UV, high temperatures, or humidity.
Not to be alarmist, I cited an accelerated aging study by the National Institute of Standards and Technology (U.S. Dept. of Commerce) that estimated the life expectancy of one type of DVD-R to be “30 years if stored at 25 degrees C (77 degrees F) and 50% relative humidity.”
We next discussed archival-quality writable DVDs like Mitsui Advanced Media’s Gold Archive Grade DVD-R with a 24-karat gold reflective layer — gold doesn’t tarnish — and their patented Phthalocyanine organic dye for which MAM claims a shelf life of “over 100 years.”
For this column, however, let’s pretend that writable DVDs are, like diamonds, forever.
Here’s an entirely different concern regarding DVD archival suitability: is “lossy” interframe MPEG-2 a smart archival choice?
Most people agree that well-made DVDs look terrific. Much cleaner than analog tape snaking around spinning head drums. No stretching, flaking, clogging, crinkles, dirty tape ends. No tearing, jitter, color bleed, drop-outs, unstable sync, poor head contact, timing or tension or azimuth or tracking errors. (VHS and ¾-inch Umatic can easily suffer all the above, all at once.)
That well-made DVDs look terrific is also a testament to the efficiency of long-GOP (group of pictures) MPEG-2, which, much like Rodney Dangerfield used to complain, doesn’t get enough respect these days. MPEG-2, the basis of our 1996 ATSC digital broadcasting standard, is compression invented in the early ‘90s to shoehorn six 480i standard-definition digital channels, each about 3 Mbps, into 19 Mbps of ATSC throughput in a conventional 6 MHz TV channel. (Or, alternately, fit in two 1080i HD channels.)
MPEG-2 contemporaneously found a second home in the DVD-ROM, which also arrived in 1996. (The 4.7 GB DVD-R we burn today was introduced the following year.) Key to the DVD’s rapid success was the reliability of playback, which uses track buffering to ensure seamless playback of variable-rate MPEG-2 streams which can range, according to spec, from 1.0 Mbps to 10.8 Mbps. However since rates above 6 Mbps result in little noticeable picture improvement, today’s DVD video data rates are usually between 3.5 and 6 Mbps.
Which brings us to the subject of this column.
Consider that project you produced in 1990 which was onlined to a 1-inch Type C broadcast master. You haven’t seen a 1-inch Type C reel-to-reel deck, once the industry’s workhorse, in years. By all rights you should archive your aging analog master tape to a “lossless” digital format like D-1 (270 Mbps, uncompressed, although half of chroma detail discarded in 4:2:2 sampling) or DigiBeta (90 Mbps, 2:1 compression, also 4:2:2).
Both of these routes are expensive and would require future access to D-1 or DigiBeta decks for playback. Hence the appeal of archiving to an inexpensive, compact, readily accessible format like DVD. Especially if you have a large collection or library and severely limited funds.
What is lost in translation encoding to a thin 6 Mbps MPEG-2 bitstream typical of high-quality DVDs? Did I mention that DVDs use 4:2:0 sampling, discarding half of the half of chroma that 4:2:2 left behind? (We won’t get into the ontological implications of long-GOP I, B, and P frames, where frames of the original no longer exist except as predictions or “hints.”)
That lossy 6 Mbps looks as good as it does is a genuine miracle. But the original question posed by this series remains, “Suppose we want to convert aging 1-inch, Betacam, 3/4-inch, 8mm, even VHS tapes to digital form in order to replace our analog masters with digital masters. Is conventional DVD good enough for this?”
Setting aside the key question of robustness over time, I think the answer is not yet.
Let’s acknowledge, however, that the archival advantages of optical discs are too numerous to ignore: cheap, compact, can be burned and played without need for proprietary formats or decks, widespread use assures viability far into the future. And, not least, they’re random access. So what enhancements, going forward, can we expect to bolster our case for eventual archival use of optical discs?
We’re 10 years into the MiniDV era — a consumer digital tape format with mild 5:1 intraframe compression – and 10 years into long-GOP MPEG era, as described above. Nearly 10 years old, as well, is the 4.7 GB DVD-R, whose video capacity at 6 Mbps (audio 226 Kbps) remains a mere 1 hr, 45 min.
The coming decade, starting now, will bring the improved efficiencies of scalable (many devices) MPEG-4, designed to achieve MPEG-2 quality at half the bit rate. (An MPEG-4 codec called H.264 can already stream impressive HD off a conventional DVD at 6-7 Mbps.)
The recent introduction of MPEG-2 and MPEG-4 acquisition (HDV or XDCAM HD, and HDCAM SR, respectively) and editing of long-GOP MPEG-2 (editing HDV with Final Cut Pro) will spread and democratize its use. To a video archivist, compression that’s twice as efficient means retaining twice the quality of the original at the same bit-rate or storage costs.
Then there’s the new wavelet-based, intraframe JPEG2000 compression, recently adopted by Hollywood’s Digital Cinema Initiative for theatrical distribution. Intraframe means that every frame of the original is reproduced as a discrete frame.
One of its innovations is that drop-outs cause an affected area to turn soft in focus instead of macro-blocky. Archivists might note that JPEG2000 makes unnecessary the managing of a separate collection of preview thumb-nails; intrinsic to each JPEG2000 frame is a scaled down version. (Grass Valley’s new Infinity camcorder, which debuts at NAB, introduces acquisition in JPEG2000.)
And when the dust settles in the Blu-ray vs. HD-DVD battle, we’ll have a new optical disc that can store between 15-25 GB on a single layer.
So in the meantime, maybe the economical, if interim, answer to serious archiving remains tape. Perhaps a light 3.3:1, near-lossless, intra-frame encoding like DVCPRO 50. In others words, I wouldn’t throw out those analog masters just yet.
Continue the discussion on Crosstalk the Millimeter Forum.