Resurrecting this old thread since I was thinking about aspect ratios the other day...
Quote:
Originally Posted by absence
Okay, let's try this again. Some data first: The visual part of a scanline lasts 52 μs for PAL and 4739/90 μs for NTSC. The number of visible lines are 576 for PAL and 486 for NTSC. The aspect ratio of the visible part of the signal is 4:3. The Amiga pixel clock frequency in "hires" mode is 28,375,160/2 Hz for PAL and 28,636,360/2 Hz for NTSC (are these figures exact?).
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For NTSC Amigas the high-res pixel clock is 4 times the NTSC subcarrier frequency, i.e. 4×315/88 = 315/22 MHz. Plugging that into your NTSC calculation you end up with an exact NTSC pixel aspect ratio of 28512:33173 = 0.85949...
That's fairly close (within 0.06%) to 0.86:1 = 43:50.
My calculations gave the same 9,600,000:9,221,927 result for the PAL aspect ratio as you. A probable close-enough (within 0.1%) ratio for emulators to use when scaling, or if you're trying to draw a circle on a PAL Amiga would be 1.04:1, i.e. 26:25.
So in summary:
Actual Amiga pixel aspect ratios: NTSC 28512:33173, PAL 9600000:9221927
Good enough for most purposes aspect ratios: NTSC 43:50, PAL 26:25 (= 56:50)
The 44:52 NTSC aspect ratio reported by AmigaOS is further from the correct value than 43:50, but still fairly close. However the aspect ratio reported by AmigaOS for PAL modes is 44:44, i.e. 1:1; about 4% off the correct value!