8.1) How may a program behave on NTSC/PAL/SECAM computer versions?

Every Atari computer unit is designed for compatibility with either the NTSC,
PAL, or SECAM analog color video encoding standards.  Some functional
characteristics vary among these versions of the computers, so that software
may behave differently depending upon whether the computer it is run on is an
NTSC, PAL, or SECAM computer.

PAL B and PAL I Atari computers differ only in the TV channel frequencies used
by the RF signal produced.  So in terms of software compatibility, all PAL
Atari computers are indistinguishable.

The FGTIA found in SECAM Atari computers is designed to be 100% software
compatible with the PAL GTIA.  This fact, along with the fact that SECAM
computer models include a PAL ANTIC, mean that the PAL and SECAM versions of
the Atari computers are completely software compatible, but with one practical
exception: in GTIA Graphics Mode 1 (BASIC Graphics mode 9), while the GTIA can
display 16 distinct luminances, the FGTIA can only display 8 distinct
luminances.

Thus the situation essentially simplifies down to just two sets of Atari
computers that may have potential software compatibility issues between them:
NTSC computers vs. PAL/SECAM computers.  What might happen if you run a
software program designed with an NTSC Atari on a PAL or SECAM Atari, or a
program designed with a PAL or SECAM Atari on an NTSC Atari?  There are a
number of possibilities:

1) The program may run faster or slower than intended.

In order to work with the different timings of the NTSC and PAL/SECAM video
signal standards, components of the NTSC versions of the Atari computers run
at slightly different speeds than they due on PAL/SECAM Atari computers.

The CPU clock rate of the PAL/SECAM Atari computer is slightly slower than
that of the NTSC Atari:
       NTSC machines: 1.7897725MHz
  PAL/SECAM machines: 1.7734470MHz
Software timing that is based exclusively on the CPU clock rate would thus run
nearly 1% faster/slower on the opposite type of Atari.  This effect, while
small, can be significant in applications that are computation- or timing-
sensitive, such as music players, or in any programs designed to simulate real
time.

The screen refresh rate of the PAL/SECAM Atari computer is considerably slower
than that of the NTSC Atari:
       NTSC machines: 59.94Hz
  PAL/SECAM machines: 49.86Hz
Software that operates as a Vertical Blank Interrupt (VBI), that is, software
that is repeatedly executed during the times between screen frame refreshes,
is thus executed at considerably different frequencies on NTSC machines vs.
PAL/SECAM machines.  Based on this effect alone, a VBI programmed on an NTSC
machine would run 16.8% slower on PAL/SECAM machines.  Conversely, a VBI
programmed on a PAL/SECAM machine would run 20.2% faster on NTSC machines.
(59.94Hz-49.86Hz=10.08Hz ; 10.08Hz/59.94Hz=16.8% ; 10.08Hz/49.86Hz=20.2%)
These calculations ignore the above-mentioned CPU clock rate differences,
which would also come into play.

2) The program may exhibit some sort of "screen flickering" effect.

The ANTIC display list is the software program responsible for the video
display, horizontal scan line by horizontal scan line.  There are 262 lines
available in the (non-interlaced) NTSC video signal, while there are 312 lines
available in the (non-interlaced) PAL/SECAM video signal.  If software written
on a PAL/SECAM machine sets up an ANTIC display list that is made up of more
scan lines than are available in the NTSC video standard, the program will
exhibit a "screen flickering" effect if run on the NTSC Atari.

3) The system may crash.

NTSC and PAL/SECAM machines have different numbers of machine cycles available
for execution of software routines designed as vertical blank interrupts
(VBIs).

An Immediate VBI must complete execution within the number of machine cycles
available during the vertical blank time:

  NTSC: 2508 machine cycles
    (262 NTSC scanlines - 240 Atari scanlines) * 114 cycles/scanline
  PAL/SECAM: 8208 machine cycles
    (312 PAL/SECAM scanlines - 240 Atari scanlines) * 114 cycles/scanline

  8208 - 2508 = 5700
  PAL/SECAM machines have a total of 5700 more machine cycles available for
  Immediate VBIs than are available on NTSC machines.
 
A Deferred VBI must complete execution within the number of machine cycles
available from one vertical blank to the next.  The number of machine cycles
available for a Deferred VBI depends upon the ANTIC Display List in use, but
the upper limit may be derived from the total number of machine cycles per
frame:
       NTSC: 29859 machine cycles / frame
  PAL/SECAM: 35568 machine cycles / frame

  35568 - 29859 = 5709
  PAL/SECAM machines could have as many as 5709 more machine cycles available
  for Deferred VBIs than are available on NTSC machines.

If there are not enough machine cycles available on an NTSC machine to execute
a VBI that was developed on a PAL/SECAM machine, the NTSC system will crash.

4) The colors displayed by the program are not what was intended.

When utilizing ANTIC graphics modes 2, 3, or 15, NTSC Atari computers exhibit
unique color artifacting effects that are not present on PAL/SECAM Atari
computers.  (Artifacting is discussed elsewhere in this FAQ list.)  As a
result, software that utilizes one of these high-resolution graphics modes can
appear to be using very different colors on NTSC machines in comparison to
PAL/SECAM machines.

Also, the additional color frequency generation circuitry present in PAL/SECAM
machines produces a color palette that is similar to, though different from,
the color palette of NTSC Atari computers.  These differences are subtle
enough that they are generally not problematic.

5) The program may explicitly refuse to run on incorrect hardware.

Software may be designed to determine whether the Atari is NTSC or PAL/SECAM,
and refuse to run if the hardware present does not match what is expected.

6) The program may not load correctly at all.

This would mostly likely result from copy protection techniques based upon
precise hardware timing associated with disk drives, cassette recorders, or
components of the computer itself, where the timing was not anticipated to
vary depending upon NTSC vs. PAL/SECAM hardware.

According to Jindroush (2/26/02), two examples of programs that run on NTSC
machines but not PAL/SECAM machines as a result of timing-based copy
protection techniques (probably based on vblank timing) are Transylvania and
The Quest, both by Penguin Software.

7) The program may run fine on both NTSC and PAL/SECAM machines.

Either the differences are too slight to matter, or the software may be
sophisticated enough to detect NTSC vs. PAL/SECAM hardware, as described
above, and act accordingly.

An example of a program that alters its behavior depending upon detection of
NTSC versus PAL/SECAM is Ghostbusters by Activision (checks the GTIA type).

=-=-=-=-=-=-=
Bottom line:

Software written for NTSC machines (North America) will (almost) always work
on PAL/SECAM machines (Europe), but software designed on PAL/SECAM machines
sometimes won't work as intended on NTSC machines.

Replacing the NTSC ANTIC chip in an NTSC Atari with a PAL ANTIC changes the
screen refresh rate to 50Hz, allowing most of the PAL/SECAM-only European
software to run on a North American NTSC Atari.  However, make sure your
display device can support a 50Hz video refresh rate first!

North American Atari users might also obtain and use real European PAL or
SECAM Atari machines, with a similar caveat concerning the display device.
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