Encoding DNxHD using ffmbc

Question

I have decided to re-encode my footage as DNxHD for editing. I am using ffmbc (downloaded here) to do the encoding. My source footage is HDV 1080i50.

Therefore I think the best format to trancode into would be DNxHD 1080i50. According to this document ffmpeg (which ffmbc is derived from) has a limited selection of formats of which 1080i50 is one.

Here is the command I am using (%1 and %2 are the in and out filenames):

ffmbc -i %1 -s 1920x1080 -vcodec dnxhd -b 185M -mbd rd -tff -acodec copy -threads 0 %2

Questions:

1. To encode interlaced media I need to specify -tff or -bff. I don't really understand the difference. Which would be the best to choose? Currently I have chosen tff as that is how the source is encoded.

2. Is there any benefit to doing a two pass encode? My understanding of a two pass encode is that it helps keep the bit-rate constant. Is that helpful for editing. I can do a two pass encode with the following command, but it's so slow I'm wondering if it's necessary?

   ffmbc -i %1 -s 1920x1080 -vcodec dnxhd -b 185M -mbd rd -tff -an -f rawvideo -an -threads 0 -y NUL ffmbc -i %1 -s 1920x1080 -vcodec dnxhd -b 185M -mbd rd -tff -acodec copy -threads 0 %2

3. My source is actually encoded as 1440x1080i. Can any one comment on the drop in quality when converting to 1920x1080i. (I believe that the image will still be the same size but the source has wider pixels)

Answer 1

1) If you're not going to deinterlace it then stick to the source's field order.

2) I wouldn't bother with the two pass encoding (if indeed it even does anything) - dnxhd will only encode at certain specific fixed bit rates anyway so you're actually pretty constrained for options.

3) You'll see some softening, but it's unavoidable. ffmbc's filtering is ok though.

What software are you editing in?

And the 185M bitrate might be overkill if your source is HDV. Try 120M and save yourself a bit of space.