With millions of Amigas is use all over the world, it is vital to ensure that programs written with the AMOS Professional system can operate in as many territories as possible. For historical reasons, different standards of television systems have evolved in different territories, which creates a major problem for computer programmers. For example, a "British" television set will work perfectly well in Hong Kong and Ireland, but will fail to operate anywhere else in the world! That is because the UK has adopted the PAL (I) standard, a third of the planet has adopted other PAL standards, half the world uses the NTSC system, and the rest use another system called SECAM! Luckily the SECAM system is identical to PAL as far as AMOS Professional is concerned, so you need only be concerned with the differences between PAL and NTSC.
Fortunately, AMOS Professional is an international language, and is designed to work automatically under the two major television standards that exist where the Amiga is marketed, namely PAL (all standards) and NTSC. If you are serious about designing your programs for specific territories, there is a list of world-wide standards at the end of this Appendix. Otherwise, rest assured that AMOS Professional will take care of most Amiga users on this planet!
When AMOS Professional code is run on a system that differs from the one used by the original author of the program, it undergoes two obvious changes. These are the size of the screen display and the speed at which the program runs. Here are the crucial differences between the two systems.
PAL updates images 50 times a second, and can theoretically display up to 312 lines on screen. The actual limit will vary depending on your TV set or monitor, but it is likely to be about 270 lines.
NTSC updates images 60 times a second, but only displays a maximum of 256 lines, which is normally restricted to 220 horizontal lines. This maximum restriction of 220 lines is also imposed on the height of Sprites.
It might seem that PAL has a clear advantage over the NTSC system, but this is not necessarily so. Although NTSC screens are about one fifth shorter than their PAL cousins, their faster updating time provides a higher quality picture, which is brighter, more stable and more restful for the eyes. Here is a synopsis of the compatibility problems between the two systems.
Supposing you have created a 320 wide by 256 high screen. Under the PAL system, the picture occupies the entire display, but on NTSC, the bottom section of the screen is hidden from view. Any error messages are not visible and wandering Objects are likely to fall from view, through the bottom of the screen. Fortunately, AMOS Professional provides a function to assess the situation.
function: report maximum available screen height
height=Display height
This function returns a value of 311 if you are in PAL mode, or 261 if an NTSC machine is being used.
These are theoretical maximum heights,, and do not take into account any limitations of you! television or monitor. To be safe, 56 lines should be subtracted, giving the actual working screen height. For example:
Screen Open 0,320,Display Height-56,16,Lowres
Print Display Height
It is obviously impossible to slow down an NTSC television, just for the benefit of AMOS Professional! If you want your programs to work at the same speed in either mode, special action has to be taken. Look at this routine:
For T=0 To 60*50
Wait Vbl
Next T
Accounting for the difference in updating speeds, that last example delays a program for exactly one minute in PAL, but for only 50 seconds under NTSC. This is because the WAIT VBL command halts the program for one fiftieth of a second on a PAL machine, but for only one sixtieth of a second for NTSC.
Because NTSC machines are faster, all AMOS Professional programs will speed up dramatically when PAL versions are executed. Obviously this works the other way round as well, slowing down NTSC programs when run under the PAL system.
Also, AMAL will run more quickly under NTSC, and there will be a noticeable increase in the speed of animation sequences.
The good news is that music speed is not affected by these alternative modes. The bad news is that synchronised audio-visual sequences may well become out of step!
Even worse, what used to be a smoothly animated PAL display is likely to be transformed into a jerky NTSC animation, that stops at random intervals. The cause of this is almost certainly the fact that you are using loops for animation, graphic drawing, joystick tests, and so on, and that you assume they will be completed at each turn of the loop. However, if there is only one sixtieth of a second to execute the entire procedure instead of one fiftieth, the routine is overrunning its allotted time. This forces AMOS Professional to wait for the next VBL, throwing your entire sequence out of synchronisation.
To cure such synchronisation problems, you must ensure that all routines can be accomplished within the sixtieth of a second limit. This will guarantee that your program can run under either system, albeit at slightly different speeds.
If you are moving Objects directly from AMOS Professional Basic, you should try using AMAL for extra speed. Furthermore, if there are many Bobs on screen, replace the smaller, faster Bobs with Sprites, and be prepared for some encouraging results!
Many AMOS Professional programmers may decide to ignore these problems completely, satisfied that their work will not be seen outside of their immediate circle of contacts, let alone outside of their country. However, this is not a professional attitude, and if you have any intention of reaching a wider audience with your programming, there is nothing worse than allowing your work to fall apart before the eyes of an unsuspecting user.
So, if compatibility problems are not to be ignored, they can at least be avoided. This is achieved by adding a simple test at the start of a program, which will warn other users of potential problems, and abort the program if it is run on an incompatible machine.
function: identify NTSC or PAL machines
mode=Ntsc
The NTSC function is provided to identify whether or not an NTSC machine is in use, and will return a value of -1 (True) if this is so. Otherwise a value of zero (False) is given, when a PAL machine is identified. The following example gives an idea of its use, and similar routines are essential for professional releases aimed at an international audience:
If Ntsc=0
Print "Sorry, PAL version only!"
Print "NTSC version coming soon!"
End
End If
Whereas synchronisation problems can be overcome, the difficulties caused by the two different sized screens causes a bigger problem. The smaller working area of NTSC displays has to be taken into account at the beginning of your program. To open a perfect screen in either display mode, you are recommended to save the screen height and position as global variables, which may be set at the start of a program, as follows:
Global YSIZE,YPOSITION
YSIZE=256 : YPOSITION=49
If Ntsc
YSIZE=200 : YPOSITION=55
End If
Screen Open 0,320,YSIZE,16,Lowres
Screen Display 0,,YPOSITION,,
NTSC users can easily provide their PAL cousins with a dormant screen area at the bottom of the display.
To return the complement, PAL users should restrict the size of their menus, activity buttons, dialogue boxes, and similar features, to no more than a quarter of the total screen area.
Even better practice is to use pull-down menus rather than dialogue boxes. Before "releasing" a program, you should simulate an NTSC screen by changing a simple screen variable, and checking that everything appears to be in order.
An examination of the Object Editor and all of the ready-made AMOS Professional HELP programs will show how they adapt themselves to the current display mode automatically!
Certain recent Amiga models have special monitors which are designed to accommodate both systems. For example, the A3000 can emulate both PAL and NTSC, and you should check your Amiga manual for details. As far as AMOS Professional is concerned, the current mode is dictated by the type of screen which occupies the front of the display when the machine is booted. Supposing you have an A3000 in PAL mode, and have set its preferences to NTSC. The Amiga will be in NTSC mode when the Workbench screen is the front screen, but it will return to its default PAL mode as soon as another screen is brought forward.
AMOS Professional relies on the current mode to establish the maximum display size, so if it is run from an NTSC Workbench, it will be in NTSC mode of 200 lines maximum, cycling at 60 hertz. However, if you flip to Workbench by pressing Amiga + A, and start another program which opens a new screen, the A3000 will revert to PAL without informing AMOS Professional. This means that when you return to AMOS Professional, the size of the display will be limited to 200 lines, but the update frequency will be 50 hertz. A hybrid PAL/NTSC mode!
Nothing can prevent this, other than the purchase of a genuine NTSC machine!
The following lists catalogue the different standard systems adopted in various territories around the world.