Can't read DVD

[Tim]

Allegedly, on or about 24 August 2013, g sent:
> something very surprising, years ago, someone i know had a commercial
> music cd that when you held it up to a light, it looked like a sieve,
> but played without anything noticeable. 

Back when I was young, and dinosaurs roamed the earth, audio CDs came
out as a brand new thing.  Just about any that I looked at, in the same
way, had that same look to them, and they all played fine.  What we can
see of the disc is nothing compared to how the laser read the disc, both
in the microscopic size of the pits (that we can't see with our naked
eyes), and how it automatically compensates for random small missing
bits of data *scattered* throughout the stream - it's not a huge chunk
of consecutive data being lost.

On that note, it always amazed me that data CDs were ever possible.  For
a start, you can't just lose a bit in your data (or program) and carry
on as if nothing happened, it all has to be precisely correct, and audio
CDs are continually working through playback errors, all the time the
disc is playing.  Checksums can say the data is wrong, but if there's
more than one bit wrong, you couldn't work out what the error actually
was.  With audio, you can make a guess, to fit a missing value between
the two adjacent surviving values, but computing data doesn't have that
sort of relationship with adjacent data.  Storing the data more than
once, as the old C64 floppy disc drives supposedly did, is an approach
that lets you read a corrupted disc, so long as at least one copy of the
data survived, but that halves your data storage size, and data CDs
didn't do that.

-- 
[tim at localhost ~]$ uname -rsvp
Linux 3.9.10-100.fc17.x86_64 #1 SMP Sun Jul 14 01:31:27 UTC 2013 x86_64

All mail to my mailbox is automatically deleted, there is no point
trying to privately email me, I will only read messages posted to the
public lists.

George Orwell's '1984' was supposed to be a warning against tyranny, not
a set of instructions for supposedly democratic governments.