Originally Posted by Schoenfeld
Hmm.. some misinformation in this thread.
The A500+ has the ECS chipset, which means it's software-switchable between PAL and NTSC, so all games will work right if they don't fiddle with the NTSC/PAL bit in the chip registers. Unfortunately, some older games do.
Connecting to a video monitor is a different story. If you only own a video monitor, then the colour carrier frequency is critical. The whole Amiga chipset was designed around the NTSC video standard, and therefore the clock rate has been chosen to be 28.63636MHz for NTSC. This divides down to the proper 3.579545MHz for the NTSC colour carrier. If an NTSC machine is software-set to PAL, then this colour carrier still remains the same, so if the monitor can handle the different number of lines of the PAL picture, you'll still have color.
PAL machines came later. The PAL system was kind-of hacked into the Amiga chipset: They somehow had to come up with a clocking scheme that allowed syncronous generation of the PAL colour carrier, without compromising the rest of the chipset too much. The PAL colour carrier is about 1.25 times higher than the NTSC colour carrier. This numer is sufficient enough to create a circuit that generates the right frequency from something that's timed almost like the original Amiga NTSC timing:
PAL colour carrier is 4.43361875MHz. Divide this by 1.25, and you get 3.546895. This is the "new" chipset clock CCK (also known as "colour clock"). Multiply this by 8 and you get the crystal frequency of the PAL model of 28.37516MHz.
Multiply the CCK by two, and you get the CPU frequency of 7.09379MHz - as opposed to the NTSC CPU frequency of 7.15909MHz. This shows that the CPU of the NTSC version is clocked almost 1% faster. Further, the NTSC video system has less video lines, but comparable syncing periods, giving a higher sync time vs. picture time rate, essentially lowering memory performance requirements. This should give another tiny performance boost, because memory is statistically free for more CPU cycles than video cycles (that's in systems without fastmem).
Conclusion:
You will need the right crystal frequency to operate the Amiga on a video monitor, but you will not have to exchange the Agnus chip. Only if you want your system to *cold*start* in your desired mode, changing the Agnus is required.
AmigaManiac's video converter may give you a colour picture on a video monitor even if you have the wrong crystal in the computer. However, his colour clock is NOT in sync with the rest of the chipset. This is bad design practise, as it'll produce constantly moving interference of the colour carrier on the picture. Both the NTSC and PAL video system were designed to get around such interference by syncing the colour carrier with the HSync signal, and the Amiga chipset is making good use of it. I'd suggest to look for an A520 of the right video system, and have the correct crystal frequency in your Amiga - this will give the best possible picture, one that's worthy of an Amiga.
Jens
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