It's said CD / DVD / VCD is read by laser , and never touch the disc physcially, and so it would
never scratch the disc and cause any data rot to the disc, is that true?
My discs are more expensive than the DVD writer...
Yes. The head is servo-controlled to float at a precise distance from the disc....
Optical disc drive - WikipediaTwo main servomechanisms are used, the first to maintain the proper distance between lens and disc, to ensure the laser beam is focused as a small laser spot on the disc. The second servo moves the pickup head along the disc's radius, keeping the beam on the track, a continuous spiral data path.
You're more likely to damage the disk by handling it.
Radio stations that played audio CDs used to keep the disks in caddies, so there was no need to handle them. I vaguely recall having a CD drive that used caddies, maybe 30 years ago. I don't miss caddies.
The only "data rot" I have ever suffered was an official MS Windows Vista upgrade disk, which de-laminated and became completely unreadable. (That had nothing to do with a drive; it happened in its jewel box.) I made a backup copy before it failed, so there was no inconvenience. Some might say that losing a Vista upgrade disk was a good thing.
Whilst the lens in optical pickups never touches the disc in normal operation, a faulty drive or even a lens that loses tracking or focus on a damaged disc can and does 'hit' the rotating disc surface with a loud 'clack' sound. If you look at the objective lens you will see an outer rim that is fractionally higher than the centre of the lens and the purpose of this is to prevent the lens getting scuffed in that critical area.
Although I've encountered this issue many times (as a service tech) I've not known it damage the discs at all.
Unfortunately they do, some more, some less, I did not have any player, that did not. I recall when I bought the brand new DVD, put it into DVD player and after taking it out, I cried, it was scratched like it went through hell. DVD 16x runs at up to 25600 rpm, at that speed even the smallest vibration could cause damage.
There is theory and reality when it comes to Optical Discs.
Theory: It is read/written by a laser diode and optics which never touch the disc.
Reality:
People damage/scratch discs by trapping discs in trays whilst closing, or touching stuff particularly on slimline Laptop type drives where the optics comes out on the tray/caddy.
Scratch discs by bad storage, throwing around disc on a dirty desk, not using protective cases.
Putting labels on/writing on discs unbalancing them and in the case of writing print through on CDs in particular can damage reflective/dye layers.
In a CD the active layer is close to the label side 0.1mm, in a DVD it is in the centre 0.6mm from each face.
Radial cracks can develop from the centre hole.
Dyes can deteriorate in time on cheap quality discs. They need to be checked say every 5 years, at the slightest sign of damage re-burn on new discs.
Commercial CDs with printed on labels have destroyed the reflective layer where the data is, easily seen by holding up to the light, and confirmed by disc scanning.
I have been using CDs for 20 years and have examples of all the above. That is why I never label CDs/DVDs only the protective box.
"so never touch and never scratch the discs right? "
Indeed, but there is all the above possibilities...
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