How are interlaced broadcasts made?

[comrinec]

Is say 1080i material broadcast as 60 fields from different points in time, or 60 fields in which there are 30 lots of 2 fields from the same point in time?

And then do lcd tv's deinterlace by taking 2 fields and combining them into one, or by displaying each field seperatley?

I've heard some people say it's 60 fields from different times because it's smoother for things like sports and some people say it's 30 pairs so it can be perfectly reconstructed to 30 1080p frames.

When I used vlc media player to bob deinterlace some 576i50 footage I recorded from tv, there was a huge increase in the smoothness of the video over the blend mode, which makes me think that the fields are all from different points in time.

[reepa]

It can be either, even within the same field (sports broadcast with cgi overlays for example) - it depends on the source. That's what makes deinterlacing so hard. If you use bob deinterlacing on film-based material, you'll lose vertical resolution but you won't gain any smoothness.

Traditional television and video cameras record fields which exist at different points in time. Newer digital cameras can record at "film" rates where pairs of fields occupy the same point in time. Same goes for everything sourced from film. Of course progressive video has none of these problems :-)

[comrinec]

So does a tv decide to deinterlace using different ways based on the source or does it always do it the same way?

[Blue_MiSfit]

Depends on the TV :) Most are rather stupid and will treat everything the same. Exactly HOW this is also depends on the TV.

Some high end TVs and projectors have very good adaptive deinterlacers that can perform inverse telecine (60i to 24p reconstruction), and good quality bobbing (60i to 60p), AND switch on the fly to the ideal method. To really get good quality in all situations, a video processor (also called a scaler) is required, which is a separate standalone component that does this all in hardware. They range in price from $1000 to ~$100,000 for the ultra high-end production grade format converters.

PCs can also do this (to a certain extent) in real-time, by using ffdshow's AviSynth interface. Multithreading makes lots of formerly impossibly slow things feasible.

~MiSfit