Saturday, September 19, 2009


After having a look at what PAL looked like on images that were broadcast to your television or arrived from a home computer, I wanted to have a look and see if I could simulate how a PAL VHS image would be mangled.

From what I could gather, a PAL VHS picture was 576 lines but it only stored a maximum horizontal resolution of around 333 pixels. On the back of an envelope I worked out that meant that the image stored on a VHS tape had about 46.25% of the horizontal resolution of a broadcast PAL image.

Therefore, suppose the original image was this Telefusion Yorkshire one (click to see it at full size):

Before: the original 720 x 576 pixel image

Then it would be stored as this much red information (click to see at full size):

Red - 100 x 576 pixels

This much green information (click to see at full size):

Green 195 x 576 pixels

and this much (or, rather, little) blue information (click to see at full size):

Blue 38 x 240 pixels

And when combined you'd get an image like this:

After: Simulated PAL VHS image

Incidentally, PAL images have non-square pixels. So in order to see the "before" and "after" images on this page in the correct aspect ratio I have scaled them horizontally from 720 pixels up to 788 pixels.

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