[FFmpeg-user] ntsc-dvd doesn't default to interlaced.

[Andy Civil]

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.

Long description:

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 
simple command:
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 
-flags ildct+ilme
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)?

--
Andy