Re: Is the cake bad ? (How much pressure can a DVD take before it loses it's data ?)

May 12, 2008 3 Replies


Hello,


>
>Sony for example sells dvd's which come in a "cake" package.
>
>This means multiple dvd discs are stacked on top of each other without any
>further protection except the cake around them.
>
>The manual of a dvd recorder says:
>
>"Don't stack discs".
>
>No further explanation is giving.
>
>I start to wonder: Why ?
>
>I google and some simple explanatations pop up:
>
>"Don't shuffle them like a deck of cards".
>
>But I wonder, is this the true reason or is there more too it ?
>
>My initial thought was:
>
>It must be the gravity. Stacking DVD discs must be bad. The puts/dents which
>are made in the DVD by the laser could bend back because of gravity and the
>weight, resulting in data loss.
>
>The special dvd jewel cases/enclosures could be ment to prevent that from
>happening. Without the special dvd jewel cases the dvd's would be exposed to
>too much pressure ?!?
>
>So it's time to ask the following question:
>
>How much pressure/weight can a DVD take before it loses it's data ?
>
>Bye,
> Skybuck.
>

Best not to stack disks at the beach on a windy day and then sit on them.. :P


D from BC British Columbia Canada


The problem happens after the blanks are burned. The chemical layer tends to migrate out of the disc. The process is now known as "CD ROT". Stacking the burned discs back in the cake pack will contribute to the issue. The easiest way to to simply lay the cake box on it's side when it has burned discs in it. Better protection can be provided with a jewel case for each disc. If you use a case for each disc after they are burned then the issue is somewhat halted. We won't know the actual life expectancy of burned discs until the have aged considerably more, but the general estimate is that it is going to be much less than the earlier quoted 70 years.

- Tim -

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I thought the major contributor was scratches to the metalized side of the disk (the side opposite of where the LED/laser diode scans.)

Perturbations here cause reflected light to scatter (and/or lose intensity) eventually to the point where the read cannot take place. The same happens to the scan side of the disk, but only as relates to scattering, not total luminosity. Presumably, if you took the reflective surface off altogether, you could not read the disk, even though the scanned surface side could be in pristine condition.

Do you have a reference for the "chemical" argument you gave above? I would be interested in reviewing that sometime. Thanks!

-mpm

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I got that info when the first realization was made that burned cds go bad. Do a search for "CD Rot" and you should find the info. It was on the same site as the originator of the problem. This guy had thousands of cds, and had about 25% go bad due to the problem. It was a major loss for his pirated music collection.

- Tim -

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