[Robert Payne]

I'm trying to get my HTPC set up properly. Reading through the multitude of threads here, I see messages saying that my dvd player should be set to "force bob" for "film sources".


What is the difference between "film" and "video" sources?


Thanks,


-robert

 

[RyanDinan]

Robert,


"Film source" is material that originated from 24fps film, and converted to 60fps of video by a process called 3-2 pulldown. With 3-2 pulldown, each frame of video is represented by 2 or 3 fields of video. Each frame is made up of two fields (even and odd). If 3 fields are needed, one field gets repeated. The pattern goes like this:

3-2-3-2-3-2-3-2 etc...


Frame 1 is represented by 3 fields, A, B, A.....Frame 2 is represented by 2 fields, C, and D......Frame 3 is E, F, E

So there's really only two fields of information per frame.....The extra redundant field must be in there to adapt the framerate difference between film (24) and video (60).


Now, if a DVD is mastered correctly, there are "flags" that tell the MPEG decoder which fields are redundant, so the image can be reconstructed from the two necessary fields. These fields get "weaved" back together to form one 480-line, de-interlaced image - Which then gets displayed either 2 or 3 times to adapt it back to the video refresh rate. Software DVD players rely on these flags to de-interlace the image "perfectly". If the flags are set incorrectly, the software must use a lower quality video de-interlacing method, which doesnt produce as good results.


Video source, is simply anything shot on video equipment (60 individual fields/frames per second).


-Ryan

 

[Yvonus]

We're talking of interlaced video here. In this mode, a video feed is a sequence of "fields". Each field is half a frame.


Displaying this with sufficient speed makes a good interlaced image.


A frame comes into 2 different fields.


Then comes the recording process ; for a movie, each frame is fully on film. So the 2 fields extracted from this movie frame comes from one same frame.


With video, the device doing the recording does interlaced recording. So the first field is captured at time "T", and the next field is captured at time "T"+ 1/50th of second.


The big difference is here ; in film mode, a couple of fields come from a unique image. In video mode, a couple of fields doesn't come from a unique image !


With film mode, especially in PAL, where fields comes in 2:2 type sequences, it's not too much difficult. Find 2 fields, rebuild a single frame from those 2 field and it's ok because the 2 fields come from the same image.


With video mode, this is not so easy. If you use simple algorithms like bob or weave, you lose resolution or you get artefacts.

You need to implement complex algorithms that will try to rebuild intelligently a frame from those 2 fields.


That's one area where the difference between deinterlacing devices appears. Basic tv app do that badly. Software dvd players do that poorly. Dscaler does it not so badly (see Tom's Barry Greedy high motion plugin). Faroudja leads the pack .. they do it VERY well.


I hope the explanation is not too bad, the thing is not easy to explain

 

[Yvonus]

See the schematic on this (french) page http://www.dvdpascher.net/guide/dossier2.php3


You see even and odd field combined to make a unique frame.


With film = quite easy to do.


With video = quite difficult.


In this exemple, this is purely video (Eagles live) and the thing is not moving a lot so even a basic deinterlacing algorithm will do well.


But live sport, with high motion is one hell of a mess to deinterlace.

 

[Robert Payne]

Thanks for the great information. Now let me see if I understand what you are telling me.


"Film" is anything shot at 24 fps --> any DVD movie can be considered "film" and I should set PowerDVD to "force bob"


"Video" is anything shot with video equipment --> my home movies, concert DVDs (Metallica S&M), and TV shows on DVD(Sopranos, Simpsons) are "video" and I should set PowerDVD to "force weave"


Is that about right?


-robert

 

[JohnAd]

Robert


You've got it! Except that some TV shows are also shot on film. Both the Sopranos and the Simpsons are film type DVDs. Basically most "quality" drama is shot on film or made to look like film.


John

 

[David Panko]

You've almost got it.. except that you have the two reversed.


For film-source DVD's you should use "Force Weave".


With video-source DVD's "Force Bob" will prevent combing artifacts but will also soften the picture. An adaptive de-interlacing algorithm would be best, but we're still waiting for the ideal software DVD player.


The smart-detection in PowerDVD 4.0 is pretty good so you can just leave it on "Auto" rather than choosing "Force Bob" or "Force Weave".


Dave.