[mythtv-users] HD3000 and Myth. Please help me understand

[Steve Adeff]

On Sunday 20 November 2005 14:50, Michael T. Dean wrote:
> Steve Adeff wrote:
> >you must be doing something wrong if you can't get an SD 1hr show into
> > 700mb with such a loss in quality. A clean SD source is easy to fit 1hr
> > onto 700mb with no discernable quality loss using XviD. I don't know what
> > the space savings are though, I haven't played around enough in this area
> > to check.
> >
> >As for HD:
> >use IVTC to cut the frame rate to 24000/1001, then scale the resolution to
> >960x544. Using the high quality settings and 2-pass for XviD you'll get a
> >great encode that can fit onto a 700mb CD with the original AC3 track.
> >Noticable issues arise around network logos and for shows with lots of
> > high movement scenes you'll lose some background information in slower
> > areas. BUT you can burn a 1hr show to a CD.
> >
> >Another option is to just use IVTC keep the resolution and convert to XviD
> >using high quality settings and use constant Q set to 4 or below (2 being
> > the best choice). this will cut the size down by roughly 1/2. IVTC is a
> > huge space savigs as it basically gets rid of 6fps (30000/10001 ->
> > 24000/1001).
> >
> >Note with IVTC though, WB uses a weird patterning and normal IVTC methods
> >won't work. ABC, NBC, CBS, FOX and UPN all seem to stick with proper IVTC
> >patterns and shouldn't be a problem.
>
> This would seem to imply that all high-def content is filmed at 24fps
> and then converted to 30/60fps using telecine.  (Otherwise, how could
> you do an inverse telecine?)  Is that right?  If so, then what's the
> point of 720p60 and 1080i60?  Why not just transmit everything at 720p24
> or 1080p24?  This would mean that the whole benefit of 720p60
> (additional frame rate) is lost because there are only 24 real frames...
>
> I could see that being true for movies, but what about normal TV dramas,
> etc.?  I would think that since CBS standardized on 1080i, they would
> shoot at 1080i60/1080p30/1080p24.  (Do they actually switch frame
> rates?  I know they stick with either 1080i or 720p, but each includes 3
> different frame rates.)  Similarly, ABC should shoot at 720p60.  (And,
> if networks do switch frame rates, they could just transmit
> 720p24/1080p24 for the movies that are shot at 24fps.)
>
> Or, when you're talking about IVTC, are you just saying that you're
> telling it to throw away some frames or completely re-sample or something?
>
> Mike

All HD is 60 fields/sec., its the whole 60hz thing (1/2 of 120hz, which was 
used for timing. I don't know if in europe they will do a 50hz timing for 
HDTV, my assumption is not since HDTV's no longer use the power frequency as 
a timing device like the old NTSC and PAL tv's did/do).
All HD content is filmed at 24fps, at least from what I've seen. This is 
because FILM is done at 24fps and they can then use the same equipment in 
filming(when they do use film as source, which is being done) and editing, 
etc. saving HUGELY on production costs.

So, HD content is a mix of telecined (24->30) and non-telecined (stays at true 
24). It usually switches back and forth every few seconds, sometimes it might 
stay at one or the other, but from my experience, it usually flops around. 
The reason for all this, as noted above, is that the shows are filmed at 24 
but HDTV needs 30 (again, 60hz). Why they don't just send out as pure 
telecined I don't know, but they don't and that requires the mpeg2 decoder 
chip to telecine the 24fps content before outputing to the TV's chips. Not 
that telecineing video is hard, its stupid easy, its IVTC thats more 
difficult.

So,for proper conversions HD requires IVTC for the 30fps parts. Playback then 
takes the 24fps and telecines it for proper timing.

The IVTC fitlers are smart enough to tell the difference between the telecined 
and non-telecined parts in dropping frames. This is because of the 
duplication of a frame and the 3:2 pattern. Donald Graft's page has some 
interesting information on all this (www.neuron2.net).  The mplayer man also 
has some notes in the descriptions of its available IVTC filters.

From my experience, CBS has a pretty pure telecined pattern(though I haven't 
done much with them), FOX doesn't, UPN doesn't. WB has a truely f'd up 
pattern that is a mix of telecined 3:2 and non-telecined interlaced (and boy 
do the IVTC filters HATE that....). I haven't done enough ABC/NBC to be able 
to speak definitevely on them (I only get them in 2.0 so I rely on others to 
do the processing for me).

-- 
Steve