Posted by ~P~ on 04/15/06 19:37

I am not sure what Jeff is talking about, and his lack of any documentation
to back up what he is saying implies to me that he is pulling it out of thin
air.

The DVD information is very specific:
"MPEG-2 progressive_sequence is not allowed, but interlaced sequences can
contain progressive pictures and progressive macroblocks. In the case of 24
fps source, the encoder embeds MPEG-2 repeat_first_field flags into the
video stream to make the decoder either perform 2-3 pulldown for 60Hz NTSC
displays (actually 59.94Hz)"

MPEG-2 can't be encoded as progressive frames, it MUST be encoded
interlaced. But, it can be tagged to let the player know 2 things...

1. The material is progessive in nature, so the decoder can put two fields
together to make a single progressive frame.
2. There is 2-3 pulldown used by the decoder of the DVD player - which it
says (second time this has been quoted by the way) in several locations
within the DVD FAQ at: http://www.dvddemystified.com/dvdfaq.html#3.4

I don't know what a DVD header is because I haven't heard mention of it. I
do know that there are flags that are set throughout the DVD to mark the
type of material that is currently on the DVD. I am not specifically aware
of this being part of a specific header. Though, it could be. But, where
is information that says a 'header' must be anything? Where is information
about MPEG-2 headers? It isn't in the DVD FAQ, which is pretty darn
thorough... so... where is this piece of information you are relying on? I
really am interested for my own knowledge.

Now, the fact that DVD players output at 480i/60 as a requirement does not
have anything to do with what is actually on the disc. What is on the disc
can be a number of formats and resolutions.

THERE PARAGRAPHS FROM SECTION 3.8 of the above listed DVD FAQ explains
exactly what I have been saying, and is in complete support that the actual
information on a DVD disc is not 480i/60, but is 480i/24.

"
DVD is specifically designed to be displayed on interlaced-scan displays,
which represent 99.9 percent of the more than one billion TVs worldwide.
However, most DVD content comes from film, which is inherently progressive.
To make film content work in interlaced form, the video from each film frame
is split into two video fields -240 lines in one field, and 240 lines in the
other- and encoded as separate fields in the MPEG-2 stream. A complication
is that film runs at 24 frames per second, whereas TV runs at 30 frames (60
fields) per second for NTSC, or 25 frames (50 fields) per second for PAL and
SECAM. For PAL/SECAM display, the simple solution is to show the film frames
at 25 per second, which is a 4 percent speed increase, and to speed up the
audio to match. For NTSC display, the solution is to spread 24 frames across
60 fields by alternating the display of the first film frame for 2 video
fields and the next film frame for 3 video fields. This is called 2-3
pulldown. The sequence works as shown below, where A through D represent
film frames; A1, A2, B1, and so on represent the separation of each film
frame into two video fields; and 1 through 5 represent the final video
frames.

Film frames: | A | B | C | D |
Video fields: |A1 A2|B1 B2|B1 C2|C1 D2|D1 D2|
Video frames: | 1 | 2 | 3 | 4 | 5 |For MPEG-2 encoding, repeated
fields (B1 and D2) are not actually stored twice. Instead, a flag is set to
tell the decoder to repeat the field. (The inverted order of C2 and C1, and
D2 and D1 are because of the requirement that top and bottom fields
alternate. Since the fields are from the same film frame, the order doesn't
matter.) MPEG-2 also has a flag to indicate when a frame is progressive
(that the two fields come from the same instant in time). For film content,
the progressive_frame flag should be true for every frame. See 3.4 for more
MPEG-2 details.

As you can see, there are a couple of problems inherent in 2-3 pulldown: 1)
some film frames are shown for a longer period of time than others, causing
judder, or jerkiness, that shows up especially in smooth pans; 2) if you
freeze the video on the third or fourth video frame when there is motion in
the picture you will see two separate images combined in a flickering mess.
Most DVD players avoid the second problem by only pausing on coherent frames
or by only showing one field, although some allow you to freeze on
flicker-frames. (This is what the frame/field still option in the player's
setup menu refers to.)"

Thanks for the fun reading!

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