Full vs Zoom aspect ratio question

Archived from groups: alt.tv.tech.hdtv (More info?)

I've got a Samsung Sir 351 that I display on a 4x3 tv via the
composite input ( I know, it's not HDTV, but it's digital, and the
monitor will have to do until i learn a little more about displays)

The question is on the difference between the Full and Zoom settings.
PBS defaults to Full on their digital SD transmissions, and it looks
great. In fact the receiver will not let me change the aspect ratio.

However, when I use the Full apect ration setting on a letterbox
transmission ( NBC for example) everybody looks tall and thin, and
there are black bars on the side (none on top). If I switch to zoom,
the picture fills the screen, but everyone looks short and squat

Clearly the Full setting work for PBS, but neither full nor zoom work
quite right for "ordianry" letterbox transmissions.

Can some one give an explanation of what the settings were inteded
for?

Why is it that the treceiver allows changes on some stations but not
others?

TIA

--G