15 Mar
2006
15 Mar
'06
12:58 p.m.
From: "Jules Richardson"
<julesrichardsonuk at yahoo.co.uk>
Barry Watzman wrote:
Hi
For files, I believe it gets the location from the directory
and then calculates about how many tracks to skip. Once
there it looks for the headers ( just like soft sectored ).
I don't think it spends too much time doing this.
Even not knowing the track spacing, a couple of header reads
on a few tracks would determine the rate of change relative
to the disk diameter. It is relatively easy to calculate
the data block location from that ( linear equation ).
No need to do a truly random search unless one doesn't
know the byte/bit offset into the disk.
Dwight
On recordable CDs, the data is on the label side
and scratches can remove
the data. On DVDs, the data is inside the media, between two layers of
plastic. Scratches cannot permanently destroy the data proper as they can
with CDs. The risk is that DVDs can delaminate, which will destroy them.
Also, air getting in between the two plastic layers laminated together (with
the data between them) can destroy the data by oxidizing the dye layer.
This form of failure occurs from the outside in.
I've certainly seem the same thing on data (rather than video) laserdiscs too,
although on 20 year old discs the damage doesn't seem to extend in more than a
centimetre or so.
I don't buy the "spiral track"
argument that was made against optical media.
These are "random access" media, not sequential access.
Don't the drives work by some sort of successive approximation when seeking?
So in theory, sprial recording or not, an error could affect a fairly large
chunk of the disc... I have a vague memory of reading something like that once
somewhere.
cheers
Jules