International television standard systems
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 versus NTSC
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.
The display size
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.
DISPLAY HEIGHT
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.