Mayayana wrote:
"Char Jackson" wrote

| Mayayana mentioned grooves in plastic within the context of backing up
| data to DVD. Obviously, there are no grooves... ;-)
|

My understanding is that the writer cuts grooves
at various depths underneath the surface as a way
to record data. Is that wrong? In any case, it cuts
some kind of marks in plastic. It's not just magnetic
storage.

There is one web site, a really old one, with an
authoritative description of all of the technical details
on optical media formats.

You shouldn't get your ideas from us without
reading that first.

The disc has a spiral pattern on it. It has two possible
encodings. The optical head uses the spiral pattern with
encoding, for "servo", to move the arm in such a way that
the player can track a spiral path. Of the two encodings,
one is technically superior. Although both DVD formats
are still for sale at the store, and few people seem
to know the difference. (Even I don't remember which
is the "good one" :-) I have to look it up.)

https://www.pctechguide.com/dvd/dvd-rw

(picture of wobble groove - schematic, not actual)

On DVDRAM discs, the pattern is concentric circles. And
a DVDRAM works like a hard drive in a sense.

Originally, optical devices needed long lead-in and
lead-out sequences when writing. But a variety of
modern things with names like "burn-proof" make it possible
to write, even when the HDD source is not delivering data
in time for the write head. That means there is some
way to splice the write, without ruining it. Many years
ago, we used to make "coasters", before there was
Burn-proof, because any little data burp (failure to
deliver data in time) would cause the disc to get
a bad spot. That's fixed now.

Optical media uses pits. Although there's going to be
a difference between pressed and burned media. It's possible
these are pressed, but I don't know that for sure.

https://mybroadband.co.za/news/wp-co...en-Krasnow.jpg

And you can find FAQs.

http://www.cdrfaq.org/faq02.html#S2-38

recorded CD

The organic dye or phase-change film changes state
in a way that affects how light is reflected

pressed CD

The depth of the pit is chosen to cause a 1/2 phase difference
in the reflected light. If the pit were shallower or deeper,
the effect would be lost

https://en.wikipedia.org/wiki/Wobble_frequency

https://www.myce.com/article/Why-DVD...to-DVD-RW-203/

Pre-pits versus ADIP

Conclusion

During my study of rewritable DVD formats it seemed very clear to
me that DVD-R(W) standard was not as well designed as DVD+R(W)

There's info out there.

Paul