At 09:03 AM 8/19/2004 +0000, Don Cox wrote:
>The claim is that the difference is audible. I said "reports" because I
>haven't tried them.
>However, there is an analogy with photographic film. The sharpest films
>have anti-halation dye behind the emulsion, dye in the substrate, and
>some dye in the emulsion itself. All this is to reduce scattered light.
>As burning a CD is a photographic process, it is not unreasonable that
>similar advantages would occur.
>If the dye also absorbs the light (probably UV) that causes fading of
>the written disk, it could increase the life of the disk.
>So I think the physics is plausible, and the reports are worth
I overstated my criticism and I apologize. However, the fact is that the
claims have been tested. The process is optical, so there might be some
effect, but it has proved to be unmeasurable and listening tests have been
inconsistent. In other words, color around the edge of the disc, black dye
and other such steps have not proved out.
The black pigment (or blue or red or amber - all have been available) is in
the otherwise clear layer below the dye layer. Unlike anti-halation, its
absorption occurs before as well as after the light strikes the sensitive
dye. The pigment has proved to be of esthetic value only. Fortunately, it
does not do any measurable harm except for a negligible increase in the
power setting effected in calibration.
Fading of dye in strong sunlight (and loss of information, of course) is
known to occur but the mechanism is not clear. Since fading is reliably
reported on discs behind a car's windshield (windscreen in UK), it is
unlikely to be due to UV; window glass is virtually opaque to UV. It may be
due to IR, which is present at moderate level in sunlight, or to some
heating effect. In my own test, an azo dye preserved the shape of a paper
or metal pattern blocking the light. I did not test with glass nor did I
try to quantify what I was seeing.
Decay of the information in strong light also occurs on erasable discs -
indeed, I have been able to erase one with exposure to summer sun. Since no
dye is involved, it seems certain to be a thermal effect. Erasure has also
been reliably reported by using a UV PROM burner. Quantitative studies are
lacking and those tests date back to the era of single-speed (2x) media.
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