Originally Posted by Daniel Dare:
“I don't know if anyone noticed on broadcast but flicking from standard BBC Four to BBC Four HD the picture ratio went from full-screen (stretched) to original 4:9 ratio.”
Thinking about this a bit more, you don't say which version, SD or HD, was stretched.
If it was the SD version that was stretched you've probably got the wrong aspect ratio setting in your equipment somewhere, most likely in the TV set.
You only get control over the shape of the image on your TV for SD broadcasts so settings like "fullscreen" (4:3), "widescreen" (16:9), "zoom" (4:3 enlarged to fill the width of a 16:9 screen, but losing top and bottom of the original image) and "smart" (stretch a 4:3 image increasingly towards the edge to fill a 16:9 screen) only apply for SD signals.
If you have the aspect ratio correctly set, the SD image should switch correctly between 4:3 and 16:9 when broadcasters send the right signal.
If your set top box is auto-upscaling, the TV setting shouldn't come into it, as all HD is broadcast as 16:9, with the broadcasters setting the image shape and size within the 16:9 frame. If it's wrong then, the broadcasters have probably got it wrong.
My Sky box is set to
not upscale SD broadcasts which means I can change the aspect ratio myself in the TV settings when the broadcasters get it wrong.
My preference is to always watch in OAR (Original Aspect Ratio) as anything else either distorts the image or too much image is cropped off the top and bottom edges. I find it annoying that many documentaries these days choose to zoom and crop historical 4:3 footage.