QVC

I think so too, and that's why you can get a lot of problems with some graphics kit (and Facebook video in particular) refusing to show widescreen 720x576 stuff in widescreen. Even with youtube you have to put in a bit of code in the tags field to get it to stretch 720x576 widescreen stuff. 720x576 widescreen PAL is sometimes referred to as having rectangular pixels rather than square pixels for 4:3 PAL and HD.

Err - 4:3 PAL 720x576 isn't square pixels either.

Square pixels would give you 768x576 - which has been used in some 4:3-based video systems (particularly PC capture cards) - but is seldom used for broadcast. Similarly 16:9 square pixels would be 1024x576.

NB 720x576 isn't 4:3 or 16:9 - both systems are wider than this to allow for a little bit of timing latitude and to keep things nicely modulo-16. The 4:3 and 16:9 portions of the signal are the central 702x576 region - which corresponds to the active analogue 52us width of a standard 576i analogue signal. Of course some manufacturers ignore this - which can cause some nasty width twitches with SD/HD up-downconversion... (Very visible on BBC One HD at the end of live HD shows - at least for a while)

It is in most SDI implementations (conventionally both systems use the same 13.5MHz sampling rate - though there was an additional 18MHz rate introduced for 16:9 that remains pretty much unused) - so in a studio you're likely to find both 4:3 and 16:9 signals distributed at 720x576. However the actual 4:3 and 16:9 region is in the central 702x576 portion - as the 720x576 standard is a little wider than 4:3 and 16:9.

When it comes to broadcast you can resample though - so some 4:3 and 16:9 content is broadcast as 544x576 for instance - which reduces bandwidth requirements a bit (ITV1 was broadcast in this format on DSat for ages, and a lot of the Freeview channels also use this to reduce their bandwidth).

Actually the US has gone backwards in this regard since analogue switch off.

When the US networks distributed separate analogue SD 4:3 feeds and a 16:9 digital HD feeds to their affiliates, they had control over how 16:9 content was displayed to 4:3 SD viewers - and often chose to letterbox 16:9 content on this outlet. This allowed them to shoot 16:9 safe and ignore 4:3 CCO issues.

However when analogue OTA broadcasting was switched off (in fact just before) the networks ditched their 4:3 SD network feeds. However there are still lots of 4:3 SD viewers in the US - either watching via cheap analogue converter boxes (the government provided subsidy vouchers for analogue SD-output boxes) or on 4:3 SD cable feeds.

The networks now expect 4:3 viewers to watch a permanent 4:3 centre-cut - so have shifted a lot of their shows back to 4:3 safe - with things like the network bugs shifting in from the edges. Whilst using AFDs could have solved this (AFDs allow the broadcaster to signal how a 16:9 image should be viewed in 4:3) most US boxes aren't AFD compatible, and even internally only some networks are using AFD technology.

Most European countries who have switched off analogue 4:3 OTA broadcasting appear to now take a view that 16:9 safe is the way to go - I suspect the UK will move this way (though having Sky boxes default to 4:3 CCO hasn't helped)