2
No.14
2
Welcome to another batch of 8-bit's
technical tips. These pages are not
just for hardware problems but also
software and programming difficulties.
Send your problems to:
Andy Nibbs Andy Nibbs
Tech-tips Tech-tips
2 Seaview c/o Duncan Webster
Hoylake 7 Ashdale
Wirral Thringstone, LEICS
L47 2DD LE6 4LW
I am told by Duncan that the response
to the last episode was good, thanks to
all of you for that.
If at any time you feel you do not
totally understand Tech-tips or want
something explaining, do not hesitate
to drop me a line.
In this edition of Tech Tips
Master Compatibility
Envelope programs
View spellcheck dictionary size
Adventure creators
Watford solderless RAM
Producing animated digitised pics
Subject: Master Compatibility.
One of the most simple problems with
master compatibility to solve is one
of the most complex to explain.
CHARACTERS Most things printed on
your beeb's screen are characters
these include all letters, numbers
and symbols.
USER DEFINED CHARACTERS These
characters can be altered and so
made to look entirely different
to how they did before with a
command in a program.
BBC B software developers defined
the characters from 224 to 255 as
shown in the user guide but unwittingly
start PRINTING their defined characters
from ASCII 128.
This works fine on the BBC B but on the
Master when the program prints
characters from 128 the Master prints
not the characters from 224 but the
characters from 128.
CURING THE AILMENT:
(1) Modifying the program not to
print characters 128 onwards
but 224 onwards.
(2) Using the ACORNSOFT CONVERT
ROM on the welcome disc.
This does about the same thing.
Subject: Envelope designers
A sound envelope designer was featured
in BEEBUG MAG a really long time ago.
I'm too lazy to look through back
issues but maybe some reader has that
mag to hand. Maybe somebody could write
a similar program for 8-bit software!
Subject: View Spellchecker
A PD program by Alan Blundell
R.Pashby wanted to know what is the
largest dictionary file available on
the 32k BBC B.
The answer is, as big as you filing
system can handle:
DFS: Just under 200K (80tracks)
Just under 100K (40tracks)
ADFS: Around 600K (ADFS 80TDS)
Some alterations to the program do
have to be made to allow such large
expansion. Refer to the manual for
details of this, any problems drop me
a line.
He also mentioned the dreaded Can't
extend error.
CAN'T EXTEND
This error is produced when the DFS
tries to add data to a datafile
and there is not enough space on
the disc immediately after the file
to accomodate the extra data.
The trouble is, when the computer
is first instructed to OPENOUT a
new file for writing to it gets
the largest space on the disc for
this file. Normally this is the
last file on the disc and so CAN
EXTEND to the end of the disc.
But, if a file is saved after the
file which will be extended,
extending the file would overwrite
it. So it can't extend!
Well done to reader 1K1 for
supporting Liverpool, I do too!
CAN'T EXTEND, THE CURING OF:
If you stick to having one disc
for the Spellcheck programs and
datafiles and you regularly perform
a *COMPACT command on this you will
not suffer from this program.
Subject: Watford solderless SWRAM
+ Yorkshire boys discs.
As I do not have access to, or any
experience of this board I am asking
other readers to write in. However you
should check:
a) You do in fact have any sideways
RAM installed, I am not sure that
this is supplied with the board.
If you do not Watford will sell
you the RAMs.
b) Any READ PROTECT or WRITE PROTECT
switches are turned OFF.
Subject: Adventure Creators
Since way back there have been several
adventure creators for the beebs GAC
from Incentive software, The Quill,
and (I think) a beeb version of ALPS
(which pre-dated the Arc version). You
could probably get a copy of any of
these through classified ads in 8-BIT
and other mags.
I am at the moment submersed in the
rigours of writing my own creation
system and a manual for it. More info
on that as it develops. I'll probably
use it to write some adventures for
8-BIT PD.
SOUND SAMPLING
You may have noticed that in the
August Micro User there is an
article about using ATARI ST
samples.
You could probably use the LOG
conversion program on output from
a sampler attached to a Beeb.
Hello to Dave Stillman.
Subject: Animated Digitised Pictures
Perhaps the first place to start when
doing these is to obtain a digitiser,
the Watford Beeb Digitiser appears to
be the only suitable one on the market
but if you have a friend with another
computer (eg Arc) and a digitiser it
might be possible to convert screens.
Watford Electronics advertise in BBC
Acorn User and The Micro User, the
digitiser is about £120.
Maplin Electronics also produce a video
digitiser kit which could be interfaced
to the Beeb, but you would have to
interface this and write software for
it to get a working digitiser. Maplin
catalogues can be bought at WHSMITH
stores around the country, the kit
is quite cheap at around £35 though
extra expenditure would be involved in
interfacing etc. Certainly the right
option for the person who can write
machine code and is prepared to spend
time building and writing software for
it, but has a limited budget.
VIDEO DIGITISERS
A video digitiser is a device which
converts video signals into digital
data to be processed by computer.
Video digitisers on the beeb are
limited by the beeb's lack of good
graphics, compared to PCs, ARCS
Amigas etc.
The whole 8-bit range of BBCs are woe-
fully short of one commodity vital for
any animation especially digitised
pictures, MEMORY.
Some compromises have therefore to be
made. You're never going to get full`
screen, 25 frames per second animation
on the beeb, or most other computers
for that matter.
For example, in the star wars animation
demos, which don't use sideways RAM at
all!, the screen size is small. The
screen size and overall mode is
controlled by the 6845 CRTC chip and
understanding the operation of this
chip is a must for anybody wanting to
animate digitised pictures.
CATHODE RAY TUBE CONTROLLER (CRTC)
Displays on monitors and TVS
(except pocket LCD TV sets) are all
produced by a scanning cathode ray
tube, basically this shoots little
subatomic particles (electrons) at
a screen of phosphor dots on you're
screen which glow when hit making
the a display you can see.
There are dots for red, green and
blue points on colour screens and
three separate guns firing the
electrons at these all scanning the
screen producing the whole display.
This cathode ray tube is controlled
by one small chip, the 6845 CRTC
which controls (amongst other
things!):
Number of characters across and
down the screen, vertical and
horizontal adjustments to position,
whereabouts in memory the screen
data starts and interlacing.
Anyway you'll need The Advanced User
Guide, The New Advanced User Guide or
A Master Reference Manual to find the
information about this chip that you
will need. These books can be obtained
from mail-order or from your local
dealer.
To animate lots of pictures you'll need
to hold all those pictures in memory
at once - maybe using sideways and/or
shadow RAM. Then you need to switch the
display between the individual pictures
to animate.
The switching entails either physically
copying the picture data or switching
the 6845 registers to get the same
effect. Only a certain amount of frames
can be switched to this way as not all
memory can be switched to and so you
might also require some copying of
memory to supplement this.
DELTA COMPACTION
A way of storing more frames for
animation in memory at once is to
store only the changes between
frames. This is more complex to
code but saves memory.
SUMMING UP
The 8-bit range of BBCS are sorely
limited as a vehicle for digitised
animation, but for some people this
in itself is a good reason to try
it. It might be interesting to see
any results anybody has at this.
If you need any help with any
aspect of this or any other area
drop me a line.
If you asked a question in either
issue 11 or in issue 13 any answers
will be in the third Tech Tips.
AJN, 14th July.
Which brings me to the end of the
second Tech Tips I hope you
enjoyed and understood it! Keep the
questions coming in!
This was written and produced by
Andy Nibbs using Fantastic 7 editor
by Andy Nibbs on a BBC Master 128.
(In case you wanted to know)
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From Andy Nibbs
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