Date   : Fri, 15 Sep 2006 00:54:11 +0100
From   : "Ian Wolstenholme" <BBCMailingList@...>
Subject: Re: Domesday Disaster

Of course if I had pulled out the drive tray fully instead of letting it stop
when it bumps into the A5000 keyboard sitting on top of the Master then I
would have seen that there is a cut-out at the back of the tray which
would allow the laser to read any part of the disc when it is spinning!  How
very clever!

I am now getting a consistent error at the same sector, so either the "drift"
has resolved itself or the dust has settled!  I wish I knew where this sector
was on the disc surface, I could make sure it's clean!

The *DUMP thing may be a bit of a red herring.  My logic was that you
deduct the start sector of the file (as reported by *INFO or *EX) from
the sector number of the error, multiply by 256 and DUMP from that
point within the file and that's the same location on the disc as where
the error was reported.  Of course, it's not always the case since
the file might not be a contiguous block of sectors, and probably less
likely so on a Domesday disc given the way they are laid out with file
structure and video content.

So, a bit of an Eric Morecambe/Andrew Preview moment there: "I'm
reading all the right sectors, but not necessarily in the right order...."

Best wishes,



Ian
----- Original Message -----
From: Jules Richardson
To:  bbc-micro@...
Sent:  Thu, 14 Sep 2006 21:52:01 +0000
Subject: Re: [BBC-Micro] Domesday Disaster

Ian Wolstenholme wrote:
> 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.

Hmm, that is odd - it almost sounds like an alignment problem drifting over 
time, where if you seek to about the right place first it does better than if 
you seek to block 0 and do a linear read of the whole disc from there.

If you've got the remote, you can kick the player into a diagnostic mode where 
it'll show failure information on-screen whilst you access it over the SCSI 
bus. Unfortunately I can't remember how the heck you do it now, but someone 
else on the list may recall (if not I can go hunting via google groups; I'm 
sure I've mentioned it over on c.s.a.h in the past).

Having said that, memory says that the VFS error codes are pretty good - a lot 
better than just "read error" - and so if you have the manual it may give some 
useful info as to the exact problem.

> 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.

:-)

Think of a 5.25" floppy - that manages with only a tiny window through which 
to access data too. The tray in the VP-415 just has a cut-out in the right 
place so that the laser can see the disk, and the laser tracks in and out on a 
'sled' much the same way as a floppy drive's head assembly does (it's actually 
a little more complicated, as there are various servo-controlled mirrors in 
the LV player to focus the laser beam a lot better than a floppy drive's head 
can 'focus' on the disk surface)

Incidentally, I presume that CD and DVD players have a similar mirror 
arrangement - don't know for sure, though.

cheers

Jules