[AV Media Matters] The Permeability of Polycarbonate (PC) used in optical compac

[e.ha.zwaneveld@nfb.ca]

Greetings,

One of our CD polishing discussion partners last week, dismissed the
permeability issue of polycarbonate substrates that are used for optical
compact disc media, implying that a 10 minute inundation with water will do
no harm because he has never seen it to do harm. While respecting his point
of view, I would like to quote Dr. Martin Dubs, Optical Storage R&D Manager
of Balzers Process Systems (BPS), serendipitously writing in Medialine of
June 2000, p.38, in an article entitled: "Sputtering Configurations for
Super Audio CD (SACD) Hybrid." I just happened to receive the publication
yesterday. In his article, Dr. Dubs reports on the production process for
this newest of CD-based product and the manufacturing decisions that have
been made by Philips ODTC (the primary patent holder of CD) and its partners
Sonopress and Balzers Process System (BPS). BPS has been Philips sputtering
partner throughout the development of the SACD. The COMET metallizing system
is used for the production of both the reflective and semi-reflective layers
of the SACD.  It is BPS's DVD-9 metallizing platform (state-of-the-art), and
their CUBE UNO is used to (please note) "sputter the functional humidity
barrier layer". Actually, three metallizers are applied, each metallizer has
a specific function. The SWIVEL is used for the silver, reflective layer;
CUBE LITE is used for the silicon-based material of the semi-reflective HD
layer; CUBE UNO is used for the silicon-based compound for the "humidity
protection layer". Explaining the purpose for this CUBE UNO layer, he
states: CUBE UNO provides a layer of silicon-based compound, different from
that of the semi-reflective layer, which prevents post-production warping
caused by water absorption. Precisely defined, deformation of the Hybrid
disc occurs where there are uneven rates of absorption and evaporation of
ambient humidity within the disc substrate. If the water uptake and release
are kept in balance, then deformation does not occur. The water absorption
characteristics are determined by the substrate material. Sonopress produces
on a base of polycarbonate which is sensitive to water absorption. The
problem emerges because the backside is protected by a silver layer plus a
lacquer layer while the front-side is left unprotected. Thus the substrate
experiences an uneven absorption and evaporation rate between the front-side
and the back-side. Front-side coating is necessary to stabilize the
substrate against fluctuations in humidity. The protective layer is
reactively sputtered at a 10-15 micron thickness"..."Without CUBE UNO,
warping occurs every time", asserts Sonopress' Schaper. (and they should
know.)

The question might be asked as to why this newest CD product has such a
humidity barrier layer in it, while this is not the case with any other CD
products.

The answer lies in the purpose for which this disc has been designed, which
includes high on the list, the archiving of audio.
In their technical specifications of the SACD, entitled: "Super Audio
Compact Disc-A Technical Proposal", by Philips and Sony, dated April 1999,
under the heading "Digital Stream Digital (DSD) in Archiving", Philips/Sony
list Archiving first under their requirements for the next generation of CD:
Music companies count as their assets the musical heritage of over 100 years
of recording. These include fragile, ancient acetates and lacquers plus
hundreds of thousands of reels of audio tape. All these media have a finite
archival life. For example, tape manufacturers typically specify 30 years
life. This suggests that the master tapes from the 50's and 60's require
immediate transfer onto some newer, more durable medium. Because these
precious masters may not be able to tolerate more than a single playback,
today's archival copy must capture "all of the original recording, down to
the merest hint of harmonics, buried in noise. The technology behind the
Suepr Audio Compact Disc must support such ultra-high quality archiving."

The original Compact Disc Design engineers calculated that it might take as
long as 30 years for humidity and other pollutants to penetrate through the
polycarbonate substrate in temperate climates. This implies that in tropical
climates optical CD life expectancy (LE) will be a fraction of that, and
when the disc is exposed to water or solvents that also will reduce the time
needed to permeate and harm the information/reflective layers. In addition,
as stated before, warping will occur 'every time'. Therefore, immediate harm
will perhaps not be seen, but disc Life Expectancy (LE) and playability
reduced is costly. Unless we are misreading it, the humidity barrier is
located at the reading side of the SACD, and such a layer is not located
under the lacquer to protect the reflective metallic layer from the label
side. The label and the lacquer could hardly be considered to have the
beneficial effect of the silicon-based compound applied by the CUBE UNO. But
that is an issue that should be investigated, i.e: From which side of the
SACD will degradation due to water absorption occur?

Philips/Sony further state: "DSD is a superlative solution for music
companies desperate to transfer archival recordings before they disintegrate
forever. The unprecedented frequency response and dynamic range means that
DSD will capture every nuance of the original, down to the noise floor and
below. The Hybrid configuration, that includes a two-layer approach, one
normal CD and an additional semi-transmissive high density (HD) layer,
enables a single disc to play compatibly on both standard CD and Super Audio
Compact Disc players."

That is why this latest optical disc design also includes Digital
Watermarking using visible and invisible approaches to offer a more secure
means of protecting the recordings against piracy.

The day when SACD writers will become available could be the time that audio
archivists have been waiting for, to gain protection for their audio
collections beyond the 30 year Life Expectancy barrier of regular CD media.
Yes, this implies that I do not accept the ridiculous and misleading claims
of life expectancy of 100 or 200 years, based on very short-term simulations
and very limited evaluation criteria that are not published, that are
misleading when applied to applications demanding long signal and content
life. I highly recommend the evaluation of the SACD systems that are now
available and thorough validation testing of the benefits of the humidity
barrier(s) included in SACD media.

Without measuring the state of a CD or DVD or SACD when new, to benchmark
its ability to not only record but also to maintain the signals, it is easy
for wishful thinking to influence the design of processes that the medium is
subjected to. Post-treatrment evaluation is a valid means to verify whether
such treatments cause immediate and short-term harm or degradation to the
signals. If measurable degradation is not evident in a reasonable number of
samples, then the method has been validated as a substrate reconditioning
method. Meanwhile we should not assume that we can expose optical disc media
that are vulnerable when exposed to water, high humidity and solvents. I
would also like to commend Jim Lindner of VidiPax in New York, NY., for his
offer to test 100 discs free of charge. I look forward to reading the
resultrs of the evaluations and commit to consider them as we develop
recommended practices for extended-term life expectancy of optical disc
media.

Best regards,

Ed H. Zwaneveld,
Technological Research and Development,
National Film Board of Canada, and
Chair AMIA Preservation Committee
August 24, 2000