[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?).

The basic formula for pixel aspect ratio is display aspect ratio divided by data aspect ratio. The display aspect ratio is 4/3. The data aspect ratio is width in pixels divided by height in pixels. The width is the visual scanline duration multiplied by the pixel clock frequency, and the height is the number of visible lines. The result is:

PAL:
4/3 / (28,375,160/2 Hz * 52 μs / 576) = 9,600,000/9,221,927 ≈ 1.041

NTSC:
4/3 / (28,636,360/2 Hz * 4739/90 μs / 486) = 2,916,000,000/3,392,692,751 ≈ 0.8595

(For the curious, substituting the Rec.601 pixel clock frequency of 13.5 Mhz for the "hires" Amiga pixel clock frequency results in correct pixel aspect ratios for digital PAL/NTSC.)

Does this look right?

Of course, the numbers don't make sense for monitors that let you stretch the standard resolution to the edges. In those cases it is probably better to use the following pixel aspect ratios:

PAL:
(4/3) / (640/512) = 16/15 ≈ 1.067

NTSC:
(4/3) / (640/400) = 5/6 ≈ 0.8333