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Well, 480p is a 4:3 picture, and is technically part of the HD specification. I don't know if anyone does it, oreven if it's possible for a single channel to switch back and forth, but it's vaguely possible that Tom's local Fox station does.

I imagine you're right, though, and that they pillar box the 4:3 picture into a 16:9 frame.

Not that it's particularly relevant to hi-definition broadcasts in the US, but for whatever it's worth, standard PAL broadcasts in the UK definitely switch between 16:9 and 4:3 as required by the content, and there's a side-channel flag of some sort that tells the TV what it's up to. This is however not the panacea one might think, as (a) the aspect-ratio flag is often broadcast the wrong way round, and (b) if reception is a bit iffy, the TV will keep losing and re-finding the aspect-ratio flag and switching aspect ratio. John's TV, which is a CRT, blanks for about half a second on each switch. This gets annoying rather rapidly.

Peter