power without the price
SEBRA version 1.33
The High-Res Monochrome Monitor Emulator
Patrik Persson 1992-1994
The ST's graphics are great. Lots of colour and fast animations -
but there is one problem: you can't use the high resolution mode on a
colour monitor or a TV set. Of course you can buy a second high
resolution monitor as well, but it still costs about ú100. A Multisync
monitor capable of displaying all three ST resolutions costs about four
or five times as much. Atari never mentioned this problem in their
So I wrote a program called SeBra which allows the previously
impossible: to run the high resolution (640x400) mode on a colour
monitor/TV set! It is intended for use with all ST/STe/Mega computers.
It doesn't actually send the monochrome signal to the colour monitor
(it can't cope with the 71.25Hz vsync frequency) but instead uses a
number of different algorithms to convert the 640x400 picture to a medium
resolution 640x200 picture.
You may wonder why I called it SeBra? (Oh yes, you do...) Well, the
word sebra is (you guessed it) the Swedish spelling of the word zebra
(black & white) but can also be split into the two words 'se' and 'bra',
meaning 'see' and 'well' respectively. Interesting, isn't it?
I have tried some other other small utilities that try to do the
same thing as SeBra (MONOEMU5, GFA-MONO etc) but I'm happy to tell you
that SeBra is faster, more compatible (they both have problems with the
STE), takes advantage of a blitter chip (if you have one) and has lots of
other nice options.
You install SeBra by putting it in the AUTO folder of your boot disk
or hard disk. Upon booting a message is displayed and SeBra allows you to
skip its installation by pressing the Esc key. Pressing any other key
Your ST now thinks that it's equipped with a high resolution
monochrome monitor, but that's not all: in some respects, it's even
better. By pressing Control, Alternate and another key, you can make
SeBra do lots of nice tricks.
THE THREE DIFFERENT MODES
To begin with, Control+Alternate and the three keys "(", ")" and "/"
on the numeric keypad allows you to switch between three different
display modes. The first (Ctrl-Alt-"(") is the 'shaded' mode. This
converts the 640x400 monochrome screen into a 640x200 screen in three
colours. The 'shaded' mode is the mode that gives the best overall view,
but it is also the slowest. It is similar to the display produced by
other 'Mono Emulators'.
The second mode (Ctrl-Alt-")") is the 'fast' mode. It was invented
to give the same speed as the 'magnified' mode (see below) and show the
whole screen at once. It is very simple: it converts the 640x400 screen
to a 640x200 screen by skipping every other scan line.
The third mode (Ctrl-Alt-"/") is the 'magnified' mode. It displays
200 of the 400 scan lines. By moving the mouse you can scroll vertically.
This mode is the fastest and clearest of the three.
THE SPEED CONTROL
The process of converting the high resolution screen to a medium one
takes some effort from the processor (although a blitter chip can reduce
the load). The more processor time you for the emulation, the less can be
used for the actual program. If you use less processor time for the
emulation, the display gets jerkier (especially in the 'magnified' mode).
SeBra allows you to choose the amount of processor time to use for the
emulation. Press Control, Alternate and one of the keys "1", "2" or "3"
on the numeric keypad to select speed. "1" gives a jerky screen, but the
processor is not not slowed down very much. "2" is normal and gives a
good screen update but a slight speed reduction. "3" gives a very solid
display, but the speed reduction is significant.
I have included an option to improve the mouse movement by updating
the mouse pointer whenever it is moved. This means that the slow speed
mode often can be easier to use, and also improves the normal speed mode.
It can be switched on with Control, Alternate and the "+" key on the
numeric keypad and off with Ctrl-Alt-"-". The only drawback is that it
needs about 2-3% of total processor time if the mouse is moved about a
lot. You wouldn't have noticed if I didn't tell you!
Another thing that affects computing speed is the vertical
synchronisation frequency. In Europe this is normally 50Hz but can be
changed to 60Hz (USA). Most programs don't mind, but some TV sets and
possibly some monitor can't cope. Anyway, the 60Hz display is more
pleasant to the eyes. It also increases emulation speed (and reduces
processing speed) slightly. Press Control, Alternate and the "*" key on
the numeric keypad to toggle vertical synchronisation frequency.
In order to keep loading and saving as fast as possible, the
emulation speed is automatically reduced to a minimum when a disk drive
is working. The speed is then restored to normal when the drive stops.
Even if SeBra emulates a mono monitor, you can use colour! Press
Control, Alternate and Enter to choose between four different colour
schemes (try that on a mono monitor) and Control, Alternate and "0" on
the numeric keypad to invert the screen.
When I wrote SeBra I included an option to reset the computer from
the keyboard (like in TOS 1.04 and later). Press Control, Alternate and
Delete to reset. If you hold down the right Shift key as well, memory is
If you are lucky enough to have a blitter chip in your machine,
SeBra can use it. If you turn the blitter OFF in the Desktop menus, this
usually means that the operating system leaves the blitter passive (what
a waste). SeBra detects this and takes control. So, to make SeBra use the
blitter, you first have to tell the operating system to leave it alone by
deselecting the blitter in the Desktop menus (or the XCONTROL Control
Panel, or any program that has a blitter option).
Once you have discovered you favourite settings, it would be very
annoying if you had to select them on every boot-up. Because of this I
have included a menu with which you can select the settings you want and
save them. Simply double-click on the SEBRA.PRG file from the Desktop,
make your selections in the dialog box and click on SAVE. Your settings
will be saved in the program file itself. If you are already running
SeBra when you start the menu, you will see the selections take effect as
you click the buttons.
This menu contains an option labelled 'Install automatically'. If
you activate this option (click in the little box and a check mark
appears) SeBra won't wait for a keypress when you run it from the AUTO
folder, but start automatically.
Please note that while the menu is running, the SeBra keyboard
controls are disabled to make sure that all information in the menu is up
SeBra is compatible with most programs that require a monochrome
monitor, such as Calamus, Calamus Outline Art, Calamus Font Editor, CuBase, Creator,
DynaCadd, Quick ST 2 & 3 and Signum!2. In general, run most GEM-
based programs run well, but games (which don't use TOS) usually don't
run. Of the programs I have tested only Flight Simulator II and TurboAss
(a brilliant German assembler) have refused to run correctly. This isn't
too much of a problem since they both run perfectly in colour...
Many other programs that work in both colour and mono modes work
with SeBra. This means that you (for instance) can use HyperPaint to edit
monochrome images (.PC3, .PI3 & .IMG).
It works with all versions of TOS that I have tested it on (1.02,
1.04, 1.62, 2.06). My guess is that it works on other versions too. I
have been informed that it works with a 16MHz processor and cache. However,
computers with 68030 computers (TT, Falcon and various accelerators) don't
like SeBra. But I guess that anybody with enough money for such an
accelerator can afford a real monochrome monitor...
Previous versions have contained a slight bug that made SeBra go
berserk with 4 meg machines in the 'magnified' mode. Other peculiar
things also happened (not only to 4 meg machines). I HOPE it will work
better now. Still, I only have a 2 meg STE myself, so don't know for
sure. But I think I've found the bug this time.
* Most 'serious' applications work.
* Most games don't work.
* Most (probably all) TOS versions work.
* Some accelerators (68000-based) and such things _probably_ work.
SeBra is in the Public Domain. This means that you can make as many
copies as you like and give to your friends at no cost. All financial
donations will be gratefully accepted, though... 8-)
If you find any bugs or have some idea of how to improve SeBra,
please drop me a letter. If it concerns bugs, please try to enclose the
* A detailed description of the situation in which the error occured.
* What computer (ST/STE/MegaST/MegaSTE etc) you were using, TOS version,
memory size, peripherals, other programs in memory etc.
OK, that's what I wanted to say. I hope you find a use for SeBra and
maybe don't have to rush out and buy a new monitor. Enjoy!
S-226 45 Lund
© marko latvanen / atari forever 1999-2006