ffmpeg -i %s -ab %bk -v 0 -f mp3 -

[shinsui]

OK. I have read about transcoding and I think I understand it. However, I can't find the meaning of the %bk I know it stands for the used bitrate, but where is this value defined?
//dick

[GJ51]

https://www.ffmpeg.org/ffmpeg.html

[daneren2005]

It is defined by whichever client is doing the transcoding. The web flash player probably has some default value it uses, and the mobile clients all have some way to control what value it uses. You could hard code it if you wanted, but I don't see any reason to.

[rancur3p1c]

https://www.ffmpeg.org/ffmpeg.html

doesn't cover it. I've been googling and looking all over for -ab, can't find it.
I guessed that it was %b as in insert bitrate to the command line here, followed by k, eg "128k"

stack overflow confirms.