[mythtv-users] CBS/NBC 1080i and telecine

[Andre Newman]

On 16 Jul 2009, at 17:53, Tom Dexter wrote:
>
> I wasn't complaining about the change at all...the mixed frame rate
> stuff is one of the bizarre practices I was referring to.  I meant
> that the change could be a sign that they're getting their act
> together.  I really don't understand why they're mixing progressive
> and interlaced frames...that's really odd.

As I understand it at a conceptual level, the stream is always  
1080i29.97, with most broadcast kit it has to be, very little supports  
format changes without 3 months of testing & a major signoff! The  
source material is labelled in metadata at ingest to the playout  
centre as from progressive 24fps source, eg. it came from film shot at  
24fps. The 1080i stream will contain 1080p progressive frames split  
into interlaced fields, unlike "normal" 1080i if you just put the two  
fields together you get a perfect 1080p frame, this is where the  
progressive flag comes from, it's not really a progressive stream but  
the decoder can interpret it as one. On top of this is 3:2 pulldown,  
every 4 frames one is repeated but instead of just sending the extra  
frame it seems the encoder is sending a 3:2 pulldown repeat last frame  
flag, which I'm sure makes encoding more efficient. Much nicer than  
the old way of 3:2 field repeats where the CRT and eyeball is used to  
fill in the gaps with the extra field being odd then even each time,  
works ok on CRT but it's a pig for a de-interlacer to pull apart for a  
modern progressive display, especially after it's been edited or even  
just cut in with other material in a TV playout centre.

In theory a clever decoder can do perfect deinterlacing, ignore the  
3:2 pulldown flag and output perfect 1080p24, haven't seen a HDTV or  
STB that does yet but I'm very happy to be shown one.

Of course the Interstitials and adverts are most likely plain ole  
1080i59.94 which will be where the mixed interlaced/ progressive flags  
are coming from, the output would look pretty foul if real 1080i59.94  
was de-interlaced as if it was 1080p24.

Things are a bit simpler in the UK, we generally shoot Drama and high  
value TV shows on film at either 24fps or 25fps, then it's easy to  
transmit 1080i50 that de-interlaces perfectly to 1080p25 and the  
speedup from 24fps to 25fps is not too massive and something we are  
used to here, slightly squeaky voices and slightly shorter running  
times :-) Before a TV pedant steps in, yes I know it's not really  
24fps but 23.976fps.

A friend works on Neighbours in Melbourne (poor sod) and they shoot  
that at 1080p25 which is recorded in a 1080i50 stream (called  
1080i25psf) then it's good quality for a convert to European SDTV or  
US HDTV by slowing slightly and applying 3:2 pulldown, the creative  
types love it too because it looks like it's been shot on film.

I was recently involved in shooting some sports at 2k at 59.94fps and  
using conventional standards conversion the 1080i50 version looks far  
more pleasing than 30fps converted to 25fps. Some people are  
experimenting with 1080p300 which converts beautifully to 1080i50 or  
1080p50 as well as 1080i60 or 1080p60, one day this is how  
international TV will be made, I wonder if VDPAU can play 1080p300,  
lets see ;-)


I'm going to have a dig in some WBGH CSI recordings and see if I can  
see this going on.

Andre