[FFmpeg-user] Performance: Multiple output, encode audio only once

[Claudiu Rad]

Hello,

Trying to figure this one out but after searching over the internet and 
looking through ffmpeg/filter documentation, still isn't clear for me 
how to do it optimally:

I want to take a video input source with the purpose of transcoding it 
to three different levels/bitrates. I also want to change audio codec, 
thus, need to reencode, but should be the same in all output streams. I 
know that the audio encoding doesn't take much CPU power, but, I still 
want to do it efficiently also in the cases where I have 5 or 6 outputs.

Command line sample with two levels only of what I am doing now:
ffmpeg -i input.mkv \
     -acodec libvo_aacenc -ac 2 -b:a 112k -vcodec libx264 -s 1280x720 
output1.mp4
     -acodec libvo_aacenc -ac 2 -b:a 112k -vcodec libx264 -s 640x360 
output2.mp4

How can I avoid encoding to AAC twice in this case (5 or 6 times in 
real-case scenarios)?
Is somewhat of a filter_complex recommended or should I just try to pipe 
from one ffmpeg instance to another? I am worried about eventual 
instabilities in this case involving a system pipe and that two ffmpeg 
instances may eat more system resources so this wouldn't be optimal from 
other points of view. This is why I would favor a filter_complex-like 
solution.

And one more question to extend my performance worries a bit:
How many times the input is demuxed/decoded when using multiple output? 
More precisely, in the above case, is the input video stream decoded 
once or twice?
If twice (or N times in case of N outputs), how can this be improved, 
because it may involve a lot of CPU.

Thanks.

-- 
Claudiu