Re[2]: [AV Media Matters] The Functions and Science of 'CD

[Graeme Jaye <gjaye@retemail.es>]

Hi Ed

e> Another set of tests done with water vapor, carbon dioxide and
e> oxygen demonstrate the phenomenon as well. Water vapor as a
e> penetrant at approx. 40 degrees C and 90% RH produced a vapor
e> transmission rate of 170.5 g.25 micron/m2.day.

Well... I'm no expert on this subject, but it doesn't seem to me this
is a particularly high rate of permeability when set against the
presence of water, on a disc being repaired, for a maximum  period of
about ten minutes - and often much less.

e> That is one of several reasons why warnings are printed on many optical
e> discs not to clean them with water! Test measurements of the warp caused
to
e> optical discs just by UV drying of various types and thicknesses of
printing
e> inks, show that considerable risks are involved in wetting and subsequent
e> drying of polycarbonate.

Interesting - you're way ahead of me in this area, of course :-).  But
then my criteria are somewhat less stringent than yours would be.  I
don't manufacture discs, or even archive them, I just fix them when
they stop working.

e> Graeme states: The question of destroying the reflective coating
e> through damage to the lacquer integrity is a more serious issue, I
e> grant - but I have seen no evidence of this so far. Obviously more
e> work needs to be done in this area - certainly from the point of
e> view of archival use.

e> Response: Well such work has been going on in Paris, France for a number
of
e> years and at the Joint Technical Symposium there last January I have seen
e> what damage is done when the lacquer is penetrated in a relatively short
e> time. A number of discs could not be played back as the reflective layer
had
e> disappeared. The manufacturers have also reported the premature oxidation
of
e> reflective layers. In fact, I have initiated a dialogue with the
e> manufacturers as to the possibility of molecule hopping mechanisms that
e> might cause this.

I have seen this sort of damage on more than one occasion.  Usually
the disc has been sent to us for 'repair' - but a quick visual
examination shows the problem is not what the owner thought it was.

I was actually given one audio disc during an exhibition, where we
were running an 'on the spot' service.  The reflective layer was
crazed all over, just like a garden path, and the disc was obviously
only suitable for the waste bin.  I asked the owner about it and he
told me that the disc had been purchased in India about a year before
(I can't recall the content, a jazz recording, I think).

About three weeks later, *another* disc turned up at a completely
different show.  This had exactly the same problem and, once more, I
had the rare opportunity to ask the owner about it.  This (classical
recording) had also been purchased in India!!

Both owners said the 'crazed' look of the disc had been present from
purchase and they had though it was just part of the label design.

Be that as it may - these were obviously faulty product.  On the other
hand, I have good quality, commercial recordings, of my own, which
have been deliberately damaged (and then repaired) several times over
a period of three years.  None of these show any visible sign of
reflective breakdown and they all play perfectly as far as my ears are
concerned.

It would seem the quality of the original manufacture is as much a
factor as anything we might do to it afterwards.

e> Graeme states:
e>> The standards of successful treatment to recover music from a CD
e>> is not the same as that of a disc used to preserve valuable
e>> heritage content.True - but, as far I am aware, no manufacturer of
e>> such devices has made a pitch at the archival market.  Possibly,
e>> this is because the requirements which would be placed on the
e>> machine in question would raise its cost to a point where no one
e>> could afford it anyway :-).

e> Response: The cost of such devices (if they were able to provide
e> reasonable success rates) may not be justifiable for an individual
e> archive. But commercial facilities that serve archives are willing
e> to buy technology that helps solve customer problems.

This is true.  There would be organisations willing to purchase such
equipment - no matter what it cost and provided it did the job - and
supply a service to those that needed it.

I think what I was really saying was that the cost of R&D for a
machine with such a limited market would be prohibitively expensive
and the small market which the archival business represents is not
something one can take to the bank manager as the basis of a business
plan.

Consequently the manufacturers look for a wider market than
this. Hence the great efforts made to sell them (and price them) into
the domestic market sector, rather than the archival one.

e> Graeme states: Perhaps someone out there is already working on a
e> 'wet gate' CD player - we can but hope :-)

e> Response: That is not an unreasonable assumption, it obviously has solved
e> the problem for film with base scratches.

I knew about the film problem and it's solution - but, in truth, I was
being a little tongue in cheek when I said that.  I realise that it is
not an easy project and there are a great many problems that would
need to be overcome before a practical device could be made available.

e> As a direct result of the discussion which has been taking place
e> here, I am seriously considering doing this for my own interest.
e> Obviously, I can not do this on behalf of the manufacturers I
e> represent (although I expect they would be interested in the
e> results, particularly if favourable).

e> However, although I am sure there *is* someone here with the necessary
e> equipment and a bit of spare time (not to mention an acceptable rate
e> for the job) I have yet to find them.

e> Response: I appreciate your approach, after all it will help you to
qualify
e> your systems and services for a wider range of users!

I'm sure you will have seem Jim's generous offer re their CD tester. I
have written to him about this and hope we will be able to do
something in the not too distant future.

Believe me, I personally am as interested in knowing the answers as
much as anyone else here.  However, that is a personal thing and I
also have to wear my 'importers hat' and try and sell whatever it is
we are given by the manufacturers using whatever I have available -
even if it is only subjective in nature.

Graeme Jaye

gjaye@retemail.es

Audio Restoration and CD Repair

http://www.personal-cd.com