Date : Thu, 04 Jan 2001 20:54:44 -0000
From : "Chris Thornley" <C.J.Thornley@...>
Subject: Re: VFS: (BBC Video Disk) : Doomsday Project
Hi,
Since I posted the message I sent to the group I have come across some
information which might be of use related to the Video Disk.
First I have found some links
http://www.cix.co.uk/~jgriffiths/cv/CV.html
The first appears to be to a CV from someone who worked at acorn on the data
retrieval from the video disk.
The second link
http://www.atsf.co.uk/dottext/domesday.html
Has some detail about the Doomsday project and the laser disk player.
I have also track some FAQs about laser disk players and an article in a
technical TV/Video book about laser disk players and it has a extra bit
concerning the one which was used in the acorn project.
Apparently acorn needed a way of storing data on a laser disk so it could be
retrieved and fed into a computer and possibly overlaid onto video images
from the laser disk. This data was stored in one of the two audio tracks on
the laser disk and could either be retrieved via scsi or RS232. This data
could not be retrieved if the disk was in freeze frame mode.
Also according to some of the links I have found the VFS filing system was
an extension to the ADFS filing system which will allow access to the laser
disk computer data which was stored in the audio band.
Also laser disk are considerably different to CD-rom or DVD disk in that
video information which is stored on them is in a analogue form. Basically a
TV signal is made. This has composite video data and two or more audio side
bands. The whole lot is FM encoded and this FM data determines the length of
the and duration of the tracks on the disk. There is one track per disk. 1
Frame of video data is stored per revolution which lasts approximate 1hr per
side depending if it was encoded via CAV or CLV methods.
In CAV mode the disk spins at 1500 rpm in PAL mode and it can accommodate
7Mhz of video data easily with out loss.
Apparently a lot of laser disks from this period which where used in TV,
Arcade Games (Dragons Lair), Doomsday Project suffered from laser rot due to
bad pressings techniques so it might be a interest to find a way to transfer
all this raw data of the disk to another media.
I was thinking if a way can be found to tap into the FM data which is read
from the disk. This is in a digital form could some how be clocked into a
computer at 7Mhz. Then we will have a perfect record of the original raw
data just in case we needed to make another pressing of the laser disk.
Software can be used to decode FM data to analogue, and I have a few
computer programs which will decode a colour image from a digitised raw
composite video signal. So this might be a feasible method of conversion to
another format without loss.
Or you might have to result to expensive digitising and mpg encoding
equipment like Real Magic DVD recorder which is a hefty £2000.
I can send you a solidisk program which will allow you to save a rom from
ROM/RAM to floppy. I will dig this out and send to you if you want?
Chris