Re: [AV Media Matters] The Functions and Science of 'CD Polishing'.

[lanyap@inlink.com]

Hi Ho everyone,

While there is not much to disagree with in Mr. Zwaneveld's discussion I
would like to weight in from, perhaps, a different perspective.  It's fair
to say that from the archivist's perspective present below, the
construction of the current digital disc format is a poor one as the
optical surface is readily susceptible damage and the recorded surface can
be breached & thus oxidized.  Then again the original manufacturers did not
make these discs for the purpose of long term storage of information,
regardless of type.  Issues of playback compatibility as technology
advances is itself a serious drawback.

Furthermore these comments and others recently made to the group along with
remarks made in conversations with several conservation/preservation
professional points out a serious technical problem.  Specifically,
interpretation of test data.  The areas in question include but may not be
limited to the hygroscopic nature of the polycarbonate surface, the ability
of moisture to penetrate to the recorded surface and the relationships
between error measurements and redundancy of information on audio, video &
data-only recordings.

To date, it does not appear that data exists to support a claim that even
repeated short term exposure (</= 1 min) to water or mild modern
surfactants will harm the polycarbonate surface or penetrate to the data
layer.  There is, in fact, no need to wet the edge of these discs in order
to cleaning the optical surface.

Manufacturing & playback are imperfect processes at present and some degree
of error in bit reading is always present, thus the need & use of
redundancy.  While not an excuse for permitting or supporting the use of
products for cleaning & polishing that significantly increase reading
errors, care must be taken in extrapolating the meaning of such measurement
to the ability to adequate & confident use of the digital source.

For our part, we will have our CD cleaning & polishing materials evaluated
by those who can do so independently & while I can try to offer some
financial support to them, the nature of these products in the current
market place does not justify a $5-20,000 investment.  The preservationist
may desire the products but their own financial limitations would never
support the cost of merchandise properly priced to reflect such costs in
time, labor & risk from expensive & lengthy investigations.  Please respect
the fact that institutions, in general, to not support such commercial
products by open recommendation, do not purchase large amounts of the
materials and thus the burden of bringing products developed for their use
to the public marketplace is still left to the manufacturer.

We look forward to reporting the test results for our materials once
available, regards,

At 06:39 AM 8/15/00 -0700, e.ha.zwaneveld@nfb.ca wrote:
>Greetings,
>
>1. The problem: Compact Disc media in all their recorded and replicated
>forms, are subject to reading surface damage, particularly scratches,
gouges
>and pits. Disc reading surface damage is a serious nuisance to owners of CD
>or DVD  recordings. It is especially a problem when such discs are rented
or
>loaned as with libraries and in 'dynamic manual archives' when the original
>recordings become degraded. The focused laser beam used to read back the
>pit-shaped signals below the polycarbonate disc surface is deflected as a
>result of scratches, gouges and pits and may be unable to read back the
>information. Although it is possible that cleaning of such abrasions could
>help remove any accumulated particulate deposits, such cleaning does not
>resolve the essential problem which is to cause the reason for light
>scattering to be taken away. Another point about the harm done by water
>cleaning of CD media is not that it harms the polycarbonate support. But
the
>unintended penetration of water through the porous polycarbonate and
>protective lacquer layer is then able to attack and cause the recording/
>reflective layer to be oxidized and lose its ability to hold the signal or
>to reflect it to enable playback. The standards of successful treatment to
>recover music from a CD is not the same as that of a disc used to preserve
>valuable heritage content. That is why the error correction circuitry in
the
>playback systems has wider tolerances for Audio CD then the for other
>applications of optical disc media. The issue is not whether the repair
>resolves the problem sufficiently for the eye or ear to consider it
>subjectively acceptable. The issue is whether a focused laser beam can do
>its job without impairments, dictated and specified for that application.
>Potential users of any process may not wish to test some 'solutions'. The
>information they have may indicate these 'solutions' to be inadequate.
>Although references are made in passing to chemical know-how invested in
>these products, we have not yet read anything that explains which
scientific
>principles are used, not an encouragement to explore such products! The
>vendor may also demonstrate disregard for the problems the potential
>customer is trying to solve. Just sending samples is no longer good enough!
>A little while back I received a film cleaning solvent 'solution' from a
>supplier, but without chemical specifications that would enable us to
>consider the science behind it or to know whether its use would damage
film.
>We took it to a chemical waste recycling plant, without even applying it to
>any film. A vendor who belittles the use of objective measurements because
>the user's objectives are more scientific than most of his customers, has
>not understood that he is in the problem-solving business and not in the
>business of selling snake-oil. Perhaps we can raise the level of our
>discussion by looking at the science of the problem:
>
>2. The scattering of light: J.W. Rayleigh and Lord Strutt, in Phil. Mag.
(4)
>41 in 1871 on pp. 107, 224 and 447, first demonstrated that the scattering
>of light is caused by optical inhomogeneities. The molecules at a damaged
>surface act as secondary sources of light, capable of preventing all of the
>focused light source to get to the reading surface and by missing its
>targets, fail to read the recorded or embedded signal.
>
>3. Light refraction or light-ray-bending power of any light transmitting
>medium: The first basic law of optical refraction was formulated by
Snellius
>(1618) and also independently by Descartes (1637). It required another two
>centuries before a correlation was established between the refractive index
>and the chemical structure. Gladstone and Dale (1858) found that for
organic
>liquids, the ratio (n-1)/p, if measured for a standard wavelength, is,
>(nearly independent of the temperature), a characteristic "constant" of the
>substance considered. This was named "specific refraction". Therefore, the
>ratio between the sine of the angle at which the light approaches the
medium
>(the incident angle) and that which passes through the medium (the angle of
>refraction), both angles measured with respect to the normal or optical
axis
>at the point of entry. It may be written thus: refractive index = sine i :
>sine r.
>
>4. The refractive index for polycarbonate (poly[1,1,-ethane bis (4-phenyl)]
>carbonate) is 1.585. If we wish to neutralize the undesirable light
>scattering behavior of the damaged areas (particularly the 'crater or cliff
>edges'), we could then apply a liquid, and if it has a matching refractive
>index, the scratch, gouge or pit will become invisible. It now remains to
>identify a liquid that can be applied to a damaged CD reading surface, that
>has in its dried state the matching refractive index. From the reports we
>have, it would appear that  the existing solutions that have been
formulated
>have their limits.
>
>5. Examples of known similar uses in our industry: Film base scratches are
>unacceptable because they are enlarged and annoying when viewed in
>projection. When they occur in negative film base they turn white in
>projection and are far more annoying then when black as will occur with
>print film base scratches. So the traditional approach was to rejuvenate or
>'polish' the film base. Using highly polished glass rolls and chemical film
>base softeners, the film was wrapped in its soft state around such glass
>wheels. If the proper degree of softening was created, any scratches would
>be flattened and would disappear again. If they were very bad, the surface
>was first roughened by wrapping the film in its softened state around a
>glass wheel with a matte surface, thereafter around a highly polished glass
>wheel. Another common approach was to regenerate the scratched film
surface,
>not by grinding, but by coating it with a proprietary 'film coating',
>consisting of some form of lacquer. The disadvantage with such lacquer
>applications was that sooner or later the film needed to be cleaned,
usually
>in a solvent, and it degraded the lacquered surface, requiring ultimately
>its removal. That was a very laborious and seldom satisfactory process. I
>have managed motion picture laboratories where we were not only equipped to
>polish film base and to rejuvenate the surfaces. If a customer had been
>persuaded to have some outfit apply lacquer to their original negative,
>there was little we could do to save their valuable film content. This
might
>explain my impatience with unverifiable formulas and results that are not
>measured! The refractive index for triacetate film base is 1.478, but the
>refractive index of gelatine in the emulsion is 1.520. Therefore it is not
>possible to exactly match both requirements in a single solution. But there
>are several solutions or mixtures thereof that offer a reasonable
>compromise. Percholoroethylene has a refractive index of 1.504 and it does
>not contain water. Thus it has been applied in immersion and application
wet
>printing and even projection, to render the scratches invisible when the
>(base) scratched film is being printed or projected, as long as the surface
>remains wet during the reading. This wet state content transfer in printing
>or telecine transfer has become the most desirable solution for dealing
with
>scratched film base surfaces.
>
>6. Other 'solutions'` applied to film which also must maintain an optically
>pure light transmission surface have included various lacquers, not unlike
>the so-called 'protective lacquer layer' that is routinely applied to the
>label side of optical CD media. In view of the above, it should be obvious
>that the lacquer approach for film scratch concealment is hardly used
today.
>But we do not have the luxury with optical discs to play them back in a wet
>state, we cannot diffuse the light source and polycarbonate is not the same
>as cellulose triacetate film base material.
>
>7. Coming back to the question of scratched, gouged or pitted polycarbonate
>CD or DVD disc media, we note from the previous discussion that there are
>several approaches. The most radical one is to use a more or less abrasive,
>over a variable period of time. This method essentially grinds away the
>majority of the perfectly good and smooth optical disc surface surrounding
>the scratches, gouges and pits, until its surface has reached the thickness
>that equals the depth of the damage. Whether this disc surface
>'rejuvenation' happens while the polycarbonate is in a softened state is
>doubtful, nor is it advisable because softening would disturb the sandwich
>structure of the bonded disc surfaces. It should be kept in mind though
that
>the thickness of the polycarbonate substrate should not be made less than
>the specification tolerances provided. Therefore the sharp cliff or crater
>edges, if it is done well, become smooth again and perhaps arriving at a
>disc surface that will be optically true enough not to deflect the laser
>beam anymore. The abrasive used for the tests at the Canadian Conservation
>Institute was apparently a 'light abrasive'. Remarks made earlier about the
>experience factor are probably also quite relevant. Another approach, not
>applicable in the case of optical disc media, is to increase light source
>diffusion, it has the effect of making the hard edges of scratches softer
>and therefore less visible. Another approach that has also been used with
>film base materials, was to apply clear shoe polish or wax to the scratches
>and hopefully also smoothen the sharp edges! The other solution apparently
>used by the gentleman who cannot stand insistent and admittedly rather
>impatient questions, is based on known techniques for the elimination of
>scratches on optically critical surfaces in which if I understand his
>argument correctly the 'cleaning' of the scratches is emphasised.
>
>8. Several times the argument has been used that to measure the improvement
>of the treatments is too expensive, assuming that the user would have to
buy
>such testing equipment. Understandably, many users would find the cost and
>technical complexity of such testing methods prohibitively expensive. That
>is not the only solution though.
>
>9. In all cases, users who have damaged discs that are causing drop-outs,
>are advised to use the services of commercial testing services to benchmark
>and do a quality evaluation of their disc before choosing the 'polishing'
or
>any other 'rejuvenation' method of their choice. Thus they will have a
>passport of the conditions before treatment. Then, after treatment, send it
>back for another test report. One example of such a professional service is
>Media Sciences, Inc. (Dr. Jerome Hartke) in Marlborough, MA, USA. (See
>http://www.mscience.com or jhartke@mscience.com) This company is equipped
>with a wide variety of test equipment and measured many relevant criteria.
>If I am not mistaken, VidiPax (Jim Lindner)in NY, NY also provides some CD
>testing services (See: http://www.panix.com or vidipax@panix.com/~vidipax).
>
>10. And if you want to be really nice, share the results with the
>manufacturers of these disc rejuvenation and cleaning products. Because of
>their market focus, the present discussion partners assume that all
>customers (and potential customers if they show some sensitivity to their
>applications) are supposed to evaluate their work subjectively. Many of us
>who have extended-term life expectancy of content in mind, do not subscribe
>to that approach, because neither the eye nor the ear is a reliable
>measuring instrument. In addition, please do not forget to share your
>results with us on this listserv. Most archivists are not housed in ivory
>towers but in basements. Their collections suffer neglect not because they
>do not care, but because the agenda of many media and recording system
>manufacturers is focused on the short-term playback of the content. Those
>priorities dictate serious investments in copying content, usually when the
>storage medium is in its worst condition. Sharing measured and repeatable
>quality measurements will facilitate our ability to save our audio-visual
>works. Many discs suffer abuse. Because the people whose services or
>products are focused on consumers, have not provided us with reliable
>information to go by that will enable us to predict whether a solution will
>solve a particular problem or not. Why we set our standards so high, is
that
>we somehow will have to establish when damage approaches a level where we
>cannot without loss of content integrity copy the recording anymore.
Because
>optical disc playback technology does not enable error rate indication, we
>cannot determine without measurement when the critical point approaches. If
>there is any conflict at all, it is between a short-term solution focus vs
a
>long-term content preservation focus.
>
>Thank you for an enlightening discussion,
>
>Ed H. Zwaneveld,
>Technological Research and Development
>National Film Board of Canada,
>and Chair AMIA Preservation Committee
>August 14, 2000
>
>
>
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