[FFmpeg-user] ntsc-dvd doesn't default to interlaced.
andycivil at gmail.com
Fri Jul 26 08:08:46 CEST 2013
Short description: I'm moaning that the default output for "ntsc-dvd" is
progressive, even if I give it enough information to make it interlaced.
First, an aside on interlacing. I know that in general, people here don't like
interlacing and attempt to deinterlace at the earliest opportunity. I can see a
day when interlacing is pointless, when all CRT displays are on the scrap heap
and no one uses DVDs any more. I used to think that interlacing was a viable
alternative to compression for saving data, but I now realise that where
software compression is available, the advantage is moot because compression can
reduce the data rate more efficiently than the interlacing process, and although
interlacing might appear to halve the amount of data required, it simply makes
the datastream less compressible so you gain nothing in the end.
Having said that, we still live in a world where people use DVDs to store and
share movies. And like it or not, we have to work within the constraints of what
is a valid DVD-video stream. Within that constraint, a video of 30fps interlaced
(60 fields per second) does indeed look better than a straight 30p because there
is more temporal information. Pointing out that a 60p movie might compress to
smaller than the 30i is irrelevant, because 60p is not a valid mode for DVD-video.
I am in the position of wanting to make DVD videos of material I have shot with
an HD camera. After days of head-scratching, I have found that the interlacing
of HD video is opposite in field order to that required by DVD. Further, the
video editor that I'm using to edit my videos seems to do a poor job of scaling
the interlaced content (seriously). I therefore turned to trusty FFmpeg to solve
the quality problem. I got the editor to output a rather large temporary file in
HD resolution and the full 60p. (The quality is OK because it seems able to
de-interlace fine.) I then asked FFmpeg to convert this to DVD-video. It's a
c:\ffmpeg\bin\ffmpeg -i E:\temp\2013-06-07-hdp.mp4 -target ntsc-dvd -acodec mp2
-ac 2 -b:a 128000 output-dvd.mpg
Now here's my complaint: this works fine, but it makes a 30p video, simply
throwing away half of the frames. 30p is a valid mode for DVD-video, but it's
not as smooth as it could be. I found that I had to specifically add the flags
to make it create a proper interlaced video. (Which it does, for sure.)
Therefore the default action is not the best it can do (with a 60p source). Is
that a fault, or simply a limitation of the 'preset' system (not knowing if the
source video has enough frames to make interlaced or not)?
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