Aspect ratios

I’m fed up doing the same calculations every six months, so I’ll post them here in the hopes I remember where I left them. I’m also fed up with whoever decided that television pixels would be rectangular, and whoever decided not to include any conversion system in QuickTime, but there we go.

Conventional 4:3 aspect PAL transmissions are taken to be 720×576 pixels. Which the Carol Vordermans amongst you will instantly recognise as being 5:4, not 4:3 at all, so to make a full-frame PAL capture look right on a computer monitor, it should be scaled to 720×540 pixels.

Now, a 16:9 widescreen capture is still stored at 720×576 pixels, simply stretched more at the display (which is the sensible approach when you can’t change the transmission bandwidth, to be fair). So to make a widescreen frame display right on a computer, scale it to 720×405 pixels.

Some notes: I’m assuming the pixels on the target computer are square – It used to be the case that Macs would only do square pixels, but now we have more choice of monitors there’s more scope for the user to screw things up. 1280×1024, for example, isn’t the same aspect ratio as 1024×768, so it’ll look squished on (4:3) CRT monitors; 1280×960 is a ‘better’ resolution. Windows users have been able to mess things up like this since SVGA came along, of course.

Also, you may wonder why I’m only scaling the vertical resolution – why not interpolate horizontally rather than throw away vertical data? Well, because the vertical data isn’t really there in the first place; the highest-resolution capture format that’s common is (so far as I’m aware) DigiBeta, which is what most of my shows originate on. DigiBeta record decks typically handle about 500 vertical lines, maximum. When running widescreen, many cameras simply subsample the (4:3) CCDs, giving an even lower ‘true’ vertical resolution. Which sounds ghastly, but bear in mind that people happily watch VHS recordings, and VHS staggers to only about 240 lines vertically; even DV is only about 360 lines, I think. And funnily enough, all the stuff I’m playing with is DV-sourced; so scaling to 720×405 is still interpolating upwards from the true capture resolution.

There. I hope that’s clear. The magic numbers are 540 and 405; on my monitor, they look right. Thanks.

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