Re: saving digital photographs
"H.S." <hs.samix@gmail.com> writes:
> Russell L. Harris wrote:
>> Both the CD and the DVD are quite susceptible to mechanical damage,
>> particularly on the back side, on which is the reflective layer, just
>> under the paint or label. Moreover, the reflective layer is prone to
>> flaking and peeling with time and with variation in temperature and
>> humidity. And the reflective layer, unless it is gold, is susceptible
>> to dulling with oxidation, for the sandwich construction does not
>> provide a hermetic seal for the reflective layer. There even have been
>> reports of a strain of bacteria, said to be found in tropical regions of
>> the earth, which feeds upon aluminum, and which has been known to eat
>> away the reflective layer of CDs. Finally, any dye-based system such as
>> user-burnable CD or DVD is prone to chemical changes of the dye with
>> time, temperature, and humidity. Consequently, CD and DVD are not
>> worthy candidates for archival storage.
>
> If I were to save important data on an optical disc, I realize that it
> should be recopied to another one within five years.
Five years is appropriate for data saved to flash media or a
conventional disc drive. But for optical media, five years is wildly
optimistic; one year is much more realistic if the data has lasting
value.
And at least two other factors need to be considered:
(1) It is essential that the integrity of data be verified once it is
archived to optical media. Most computer users fail to take into
account the fact that the applications -- music and video -- for which
CD and DVD commonly are used do not demand perfect data integrity. CD
and DVD players do not grind to a halt every time a corrupted byte is
encountered; indeed, such players do not check for data integrity.
Consequently, manufacturers of CD and DVD media have little technical
incentive to strive for perfection: apart from a time-consuming data
integrity check made on a computer, the consumer typically cannot
distinguish between a perfect media and flawed media, until the number
of defects on a disk becomes quite large.
(2) The commodity nature of CD and DVD media militates against
perfection: the cost of attaining perfection becomes prohibitive when
the per-disc end-user price is running at twenty-five cents to a dollar.
The same phenomenon was seen as the end-user price of 3.5-inch floppy
media plummeted; media quality became so poor that every other diskette
was unusable, and diskettes which initially were usable soon developed
defects. This factor accelerated the demise of the 3.5 inch
floppy system.
A Google search for terms such as "cd dvd quality" is in order. Web
sites devoted to the matter of CD and DVD quality paint a very dismal
picture regarding the suitability of consumer optical media for archival
storage of integrity-critical data. Here's the URL of one web site
which addresses the matter:
http://cdmediaworld.com/hardware/cdrom/cd_quality.shtml
The fact is inescapable: while flash media and conventional fixed disk
drives are designed to maintain the integrity of data, optical storage
systems are not.
RLH
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