Joe Friedrichsen wrote:
Hi! I have a DV stream that I'm trying to encode for DVD. I have an NTSC
DV camera, and it shoots in 30000/1001. I can encode it perfectly well
into 30000:1001 MPEG-2.
As an experiment, I wanted to make a 24000/1001 encoding (I didn't
really care about the output quality, I just was curious). When I
encode, I use mplayer to pipe the video's yuv stream to a named fifo
(stream.yuv). While that is going on the background, I cat the fifo
through yuv2fps and then to mpeg2enc.
After reading mpeg2enc's man page, I decided that changing -F 4 to -F 1
and adding -p would work. However, I was wrong and mpeg2enc changed the
frame rate number to 4 (30000/1001) when it started. The man page gave
me the impression that -p would work for 24fps _source_ material and
tell the decoder to do pulldown at playtime. yuv2fps changed the
framerate to 24000/1001, but mpeg2enc still encoded at 30000/1001.
By removing -p, I got things to work and mpeg2enc kept the source's
24000:1001 frame rate. What am I not understanding about using -p?
24000:1001 is not a valid frame rate for DVD. Thus 3:2 pulldown is used and
the frame rate becomes 29.97.
Programs like mplayer detect the pulldown flags and will play the file back
at 23.976 progressive, even though the mpeg claims to be encoded at 29.97.
It is confusing, and it took me several attempts to convert film rate files
into something suitable for DVD.
So this is my assumption. That the pulldown option adds an MPEG flag that
causes a stream to go from 23.976 to 29.97, with the correct interlacing 3:2
pattern. I assume it's encoded somehow into the stream (I'm guessing null
motion interlaced p frames) so that programs that do not detect it will just
play it as 29.97 interlaced, but it is distinct enough that some programs can
detect it and revert to 23.976 progressive.
I have a program that will allow me to convert a 23.976 mpeg elementary
stream to 29.97 very quickly (about 1000 fps) by simply adding the pulldown
flags, so I assume it's not a complex procedure.
Hopefully someone else can explain it better.
So you are correct, adding the -p option makes your mpeg appear as if it is
29.97 interlaced even though it's 23.976 progressive. This is perfectly
normal behaviour.
Mark
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