The demo machine described the other day could easily be generalized to PAL output. There are some complexities though. Because of the missing quarter cycle of the carrier frequency per line, the PAL signal for a still image repeats every 4 frames. This means that in order to do the same "extremely simple, highly standards-compliant demo" that was possible on the original demo machine for NTSC, we need 2.7Mb of sample data. Let's run with this and round up the PAL machine's memory to 4Mb - rather than making the PAL machine as similar as possible to the NTSC machine, we should take the opportunity to introduce some variety.
Similarly, the CPU clock speed for the PAL machine should be (exactly) 17.734475MHz.
Generating interesting standard PAL signals does have some complications that NTSC signals don't have. Because of the 25Hz offset, the colour carrier frequency starts at a different phase on each line, meaning that a sprite needs to have different sample data depending on its vertical position. I expect that most demos written for the machine would use one of three simplications of the PAL standard (as most if not all computers and consoles that generate PAL signals did):
- eliminate the 25Hz offset so that the colour carrier phase repeats every 4 lines
- use a whole number of subcarrier cycles per line (making the chroma patterns vertical)
- eliminate interlacing, doubling the frame rate at the expense of halving the vertical resolution
These simplifications change the horizontal and vertical retrace frequencies slightly from the standard 15.625KHz and 50Hz rates, but not so much that real hardware is likely to fail to display the intended image.