Date   : Thu, 14 Sep 2006 21:28:21 +0100
From   : "Ian Wolstenholme" <BBCMailingList@...>
Subject: Re: Domesday Disaster

A bit of soapy water seems to have done the trick partly.  It
appeared to have the effect of moving the disc error further away
from the original place, each rub down with a soapy teatowel was
giving me about 2,000 extra sectors!

*DUMPing the file from a couple of sectors before the "bad" sector
also seems to help, I am not sure why this would be.  Maybe *DUMP
is more tolerant than an OSGBPB call but that wouldn't explain why
OSGBPB continues past the error on the next attempt.

Next silly question - if the laser is reading the underside of the disc,
how does it do this given that the drive tray obscures most of
the disc surface?  There is probably a very simple explanation to
this given that this is most likely how CDs & DVDs work as well,
I just can't think of it at the moment.

Best wishes,



Ian

----- Original Message -----
From: David Hunt
To:  <bbc-micro@...>
Sent:  Thu, 14 Sep 2006 17:52:55 +0100
Subject: RE: [BBC-Micro] Domesday Disaster

> Quite possibly. Hmm, don't LV discs track from the outside 
> inwards (unlike CDs that go from the inside outwards)? Which 
> if you're doing a raw dump probably means any fault is likely 
> near the centre given how far through you got...

IIRC you're right, the only problem being with CAV discs, more data has to
be compressed into a smaller space on the inner tracks.

If you've read another full disc already, the machine is probably fine, I
remember other people's machines getting big vertical lines in the picture
towards the end of each side which turned out to be a dirty and misaligned
head.
 
>  > and if so, what would be the best way to clean
> > it and which surface is being read when the disc is in the drive?
> 
> The laser reads the underside of the disc. Careful use of a 
> non-abrasive cloth and some warm soapy water should do fine 
> for cleaning off dust I'd have thought.

I'd avoid water (and humid storage areas) as some discs have separated and
allowed the water in between the layers of disc causing them to separate
thus oxidising the "silver" data layer. Also, if you live in a hard water
area you'll leave limescale stains on the disc surface which are hard to
completely remove.

> 
> At recommendations of others I've used brass polish before to 
> remove scratches from CDs - I'd imagine that LV discs are a 
> similar plastic material and it'd work just as well for those 
> (I've not tried it on a LV disc, but I'd be prepared to risk 
> it, if that helps!). Silver polish should be fine too - just 
> don't use anything like Jif (or Cif or whatever it is these 
> days!) as that'd be too abrasive.

I used to use a lens cleaning cloth to remove fingerprints and dust, more
dirt than that, I used spectacle lens cleaner (a blue gel). For removing
scratches I used Duraglit or Brasso, it looks scary but, the data becomes
recoverable. I used it on several of my old house/rave CDs that had been
DJ'ed to death so I could copy the music into iTunes.

> 
> I've seen Domesday discs rot from the outer edges - it seems 
> like the sealing sometimes gives up along the rim and then 
> they corrode. Having said that the museum has one disc like 
> that where the corrosion goes in about 1cm, yet it's still 
> readable - I guess that there's quite a big gap from the edge 
> before the data starts. If I'm right about the discs tracking 
> from the outside in then that's not your problem anyway...

I think it is a thumb width from the edge, but don't quote me on it, a
reasonable gap would allow you to handle the disc without fear of not being
able to watch your movie without cleaning the disc on the side you can't
see! (discs are always read from the lower side - some players could play
both sides, but it had to reverse the motor)

I don't think BBC Worldwide ever relinqushed their Copyright on the content,
I remember someone being paid by the Government to migrate the applications
and data to Windows (some nightmare involving serial ports and frame
grabbers), but I've never seen them. Perhaps it's another Government thing
that we all pay for, but when we want to see it we have to pay for it
again... If anyone out there knows of what happened to the Windows app, I
like to see it.

Dave ;)