[x264-devel] X264 encoding interlaced output

[laasa]

I try to recode a part of an 1080i at 25 interlaced recording.
Therefore I am using ffmpeg and x264.

I get every 20 ms a new packet with alternating top or bottom field.
After decoding  this packets I get 1 picture every 40 ms (after decoding 
bottom field).
Put this to the x264 encoder (interlaced mode with TFF) I get one packet 
every 40 ms which contains only the top field.
What I want to get from encoder are one packet every 20 ms with 
alternating top or bottom field (same as the input).
I have checked the properties of the encoder and have no other idea.

Thanks for help.