What are the exact screen refresh rates on the PAL and NTSC systems? I know that on the PAL system it's about 50 times per second (50 Hz) and on the NTSC system it's about 60 times per second (60 Hz), but what are the exact rates?
I tried to calculate the refresh rates but something is very wrong with my math.
The Plus/4 Encyclopedia says that a PAL Plus/4 has a 17.73447 MHz clock and an NTSC Plus/4 has a 14.31818 MHz clock and all working frequencies are derived from that clock.
To get a CPU clock frequency (single clock), you need to divide the clock rate by 20. On the PAL system that's 17.734470 / 20 = 0.8867235 MHz (886723.5 Hz) and on the NTSC system it's 14.31818 / 20 = 0.715909 MHz (715909 Hz).
The PAL screen has 312 raster lines and the NTSC screen has 262 raster lines.
The width of a raster line is 504 pixels. (I'm not sure about this.)
During 1 CPU cycle the raster beam moves 8 pixels forward on a raster line. Therefore it takes 504 / 8 = 63 CPU cycles to complete a whole raster line.
To draw the whole PAL screen (312 raster lines), it takes 312 * 63 = 19656 CPU cycles. On the NTSC system it takes 262 * 63 = 16506 CPU cycles.
To get the PAL and NTSC screen refresh rates, I divide the CPU clock frequency by the CPU cycles needed to draw the whole screen:
Re: Screen refresh rates Wrong. NTSC clock divider is 16. One rasterline is 57 cycles. Frame rate is about 49.86 Hz in PAL and 59.92 Hz in NTSC. No monitor or TV would have been able display those refresh rates back in the days without more or less extreme detuning. Nowadays it would be completely impossible.
Re: Screen refresh rates Another note. Not CPU cycles. CPU cycles depend on clock settings and TED DMA interference. Those are, for a lack of better term, TED displaying base cycles.
Re: Screen refresh rates @Spektro: In the Ultimate map I've put some slightly different frequencies, but I don't remember where I found them: PAL: 17.734472 (instead of 17.734470) NTSC: 14.318181 (instead of 14.318180)
If you are sure of yours, I can correct them for the next issue. Thank you.
Re: Screen refresh rates I found the frequencies from this Plus/4 Encyclopedia article: https://plus4world.powweb.com/plus4encyclopedia/500244. The frequencies in your Ultimate map are probably the right ones.
Re: Screen refresh rates Theoretically, those frequencies should be the quadruple of the colour sub-carrier of the given TV-standard. In practice, they are the value of the available crystal oscillators with the closest frequency.
Screen refresh rates
