[Ffmpeg-devel] Why I cannot get the results I want ???

[Thomas Parmegiani]

Hi all !

I would like to implement ffmpeg library in my application to schedule
conversion tasks.
User will choose an encoding profile (type : Always MPEG2, constant bitrate
: 10->80 Mbs, GOP : I only, IP or IBBP (with fixed gop size)).

I tried first using ffmpeg command line but most of my encoding don't
produce that I want :

- I cannot reach the desired constant bitrate :

ffmpeg -i /G/test-50mbs.ts -minrate 25000K -maxrate 25000K -bufsize
2500K -bt 1 -intra /G/test-25mbs-intra-only.ts

It does not work. I get too many buffer underflows and the final bitrate is
4mbs instead of desired 25mbs.
I tried to change bufsize value without any success.
Can anyone help me to find the correct command line to have the desired
constant bitrate ?

- I cannot have a fixed gop size :

ffmpeg -i /G/test-50mbs.ts -minrate 25000K -maxrate 25000K -bufsize
2500K -bf 2 -g 12 -flags2 sgop /G/test-25mbs-intra-only.ts

It works for some files I have but not all only a few. I have read somes
posts and someone wrote to set scenechange_threshold = 1000000000. It does
not fixed my problem.
Can anyone help me to find the correct command line to have a fixed gop size
?

I am novice with MPEG2 format. And I am wonder if it possible to have a
destination file according the profile I choose, in a very simply way. If
not, will I have to play with all paramters such as qmin, qmax, max_qdiff,
bufsize... until I get the target format I want ?

Thanks in advance for your clarifications...

Best Regards,

Thomas.P