Interlacing progressive content during encoding

[microchip8]

Hi,

I'm at the moment in the process of learning interlacing and how it is done. In another thread, which helped me a lot about telecine/inverse telecine (many thanks to Tack for that), my question didn't get answered, so I apologize for starting this new thread.

What I want to know is how to interlace progressive content during encoding. I use MEncoder under Linux for the encoding process and I'm aware that it can do such thing with the tinterlace filter.

Now, I have here a 25fps PAL, a 23.976fps progressive NTSC and a 29.97fps progressive NTSC content. I figured out how to use the tinterlace filter (-vf tinterlace=4) but what I don't know is if I have to change frame rates when doing a interlacing.

I did a small test here on the PAL content with -vf tinterlace=4 and without providing a frame rate (encoding took place at 25fps). MEncoder encoded the content and made it interlaced (though it was spewing a lot of "duplicate frames" messages). Upon playback of the interlaced encode, everything was fine. I could see the interlacing and A/V sync was not a issue. Is the interlacing procedure the same for NTSC or is it different? Can someone give me a easy "guide" on how to interlace progressive content during encoding?

Thanks in advance

[Tack]

What I want to know is how to interlace progressive content during encoding.
Why do you want to do this? If the goal is to lower the bitrate needed for a given quality, I think you'd be much better off just scaling the video down and keeping it progressive. Interlacing is a relic that is best relegated to history.

I figured out how to use the tinterlace filter (-vf tinterlace=4) but what I don't know is if I have to change frame rates when doing a interlacing.Yes. With tinterlace=4, your output frame rate will be half the input rate. This is documented in the man page.

MEncoder encoded the content and made it interlaced (though it was spewing a lot of "duplicate frames" messages).That would be your hint that you need to cut the output frame rate in half. :)

Upon playback of the interlaced encode, everything was fine. I could see the interlacing and A/V sync was not a issue.Everything may have looked fine, but if you step through the frames you'll see that every frame is duplicated.

Is the interlacing procedure the same for NTSC or is it different? Can someone give me a easy "guide" on how to interlace progressive content during encoding?I guess the straightforward guide that ought to work in all cases is -vf tinterlace=4 with -ofps of half the input frame rate. You might need to fiddle with the phase filter here. I've never actually done this though. :)

But I must say, the notion of converting progressive content to interlaced offends my sensibilities. :)

[microchip8]

Thanks Tack... I will give it a try. I'm not actually trying to encode my stuff here to interlaced content. I'm trying to learn how interlacing is made from progressive content, how the whole procedure works. I don't like interlacing and telecine either but out of curiosity, I like to know how it is made :D

[Guest]

You appear to be confused. You have to distinguish the nature of the content from the nature of the display device. You can display progressive content on an interlaced display device. There is no need to "interlace" the content!

I think you have got this notion from the description of "re-interlacing" which was part of the discussion of resizing an interlaced content video. In that case, the desire is to retain the temporal resolution of the original content, so the trick was to bob it to progressive, do the resize, and then convert back to interlaced content by "re-interlacing" (which is really just a strategic decimation and recombining of the fields).

Converting progressive content to interlaced in the same way does not increase the temporal resolution so it is pointless. And the frame rate will be half what it is supposed to be because you have decimated fields.

One possible application might be where you have 59.94fps progressive content (no frame dups) and need to make an NTSC DVD. The reduction in frame rate there is welcome.