> With it’s different VIC-II chip generating a different master clock signal, the PAL C64 has a few more cycles available per frame than the it’s NTSC counterpart.
Just for additional clarification:
If I remember correctly the C64 had only a single quartz.
All clock signals were derived from that single quartz.
That meant that also the clocks driving the raster beam were derived from that single quartz.
As a consequence the main processor in NTSC and PAL versions ran at different frequencies. The PAL version was 0.98 MHz and if memory serves me correctly the NTSC version was slightly above 1 MHZ.
Even back then some people overclocked their machines, but it was different than today. You had a hardware switch for the frequency and when you were in fast mode no IO was possible. No video, but also no serial communication. To see the results of your calculation you had to switch back to slow mode.
> All clock signals were derived from that single quartz. That meant that also the clocks driving the raster beam were derived from that single quartz.
Yes, and that VIC clock also drives the SID sound chip. So while the same code will play a song back "in tune" (relative pitch) on both a PAL and NTSC C64, it will be tuned lower (absolute pitch) on a PAL C64. Think of it like the concert pitch difference between A = 443 Hz (Germany -- common) and A = 430 Hz (Verdi tuning a.k.a. Schiller Institute -- uncommon).
To play the same absolute pitch on both a PAL and NTSC, code could use an alternate lookup table to map a note to different SID frequency register values for PAL or NTSC. In practice, I don't think this was done, but someone can correct me. What was, sometimes, done was skipping every 6th frame of a raster vblank music playback routine on an NTSC C64 (@ 60 Hz) to match the tempo (not pitch) of the same song played on a PAL C64 (@ 50 Hz).
While a program is being loaded from tape on the C64, the screen remains blank, to avoid memory reads issued by the video chip blocking the time-sensitive code on the CPU.
I want to keep my C=128D NTSC for testing that my code works on both PAL and NTSC correctly, but I'm running on 50Hz, 220-240V. What in the power supply do I have to replace?
As the C128D power supply is based on mains transformer, first thing you need to replace is said mains transformer. It's probably easier to just remove the whole original power supply and replace it with some small industrial 5VDC SMPS and 9V transformer (assuming that nothing depends on the frequency of the 9VAC input, which from looking at service manual seems to be the case).
The 6526's time-of-day clock uses it, on the C64, aftermarket or hobbyist power supplies that leave it out make the variables TI and TI$ stop updating.
Just for additional clarification: If I remember correctly the C64 had only a single quartz. All clock signals were derived from that single quartz. That meant that also the clocks driving the raster beam were derived from that single quartz.
As a consequence the main processor in NTSC and PAL versions ran at different frequencies. The PAL version was 0.98 MHz and if memory serves me correctly the NTSC version was slightly above 1 MHZ.
Even back then some people overclocked their machines, but it was different than today. You had a hardware switch for the frequency and when you were in fast mode no IO was possible. No video, but also no serial communication. To see the results of your calculation you had to switch back to slow mode.