How convert High bitrate mp3 to lower rate using ffmpeg in android

Question

We want to convert 320kbps mp3 file to 128kbps mp3 so currently we are using below ffmpeg command but its not working.

ffmpeg -i input.mp3 -codec:a libmp3lame -qscale:a 5 output.mp3

Result:-the output bitrate same as input mp3.

And we are following the FFmpeg Encoding guideline for that here is the link :- https://trac.ffmpeg.org/wiki/Encode/MP3

so please suggest any solution.

Answer 1

I tried your shown command (tested on Windows / commandline) :

ffmpeg -i input.mp3 -codec:a libmp3lame -qscale:a 5 output.mp3

Result : It works for me. However the -qscale:a 5 makes FFmpeg decide on an average bitrate for you. With one (320k) MP3 file I got it giving a close convert of 134kbps. This is expected since :

lame option   Average kbit/s  Bitrate range kbit/s    ffmpeg option
   -V 5             130           120-150                -q:a 5

Solution :
Instead of making the internal mp3 frames hold different bitrates (that vary to acommodate the "current" perceived audio, eg: think "silent" parts using smaller rate of bits/bytes compared to "busy" audio parts), so just set a constant bitrate of 128kbps as you need.

I would just set it to constant 128kbps manually and explicitly with :

ffmpeg -i input.mp3 -codec:a libmp3lame -b:a 128k output.mp3

Answer 2

I use this shellscript in order to not visit this stackoverflow-page over and over again :)

#!/bin/bash
[[ ! -n $1 ]] && { 
    echo "Usage: mp3convert <input.mp3> <output.mp3> <bitrate:56/96/128/256> <channels> <samplerate>"
    exit 0
}
set -x # print next command
ffmpeg -i "$1" -codec:a libmp3lame -b:a "$3"k -ac "$4" -ar $5 "$2"