To encode VBR mp3 audio with ffmpeg, using libmp3lame library, we would specify a command like this:
ffmpeg -i input.wav -q:a 2 out.mp3
Option -q:a needs a parameter, that tells ffmpeg something about expected "audio quality". This parameter has a different meaning for each audio encoder used, and we are now describing the libmp3lame encoder.
The table for LAME option -V <number> is shown here (and that option is mapped to the -q:a option in ffmpeg): VBR (variable bitrate) settings for LAME.
| Switch | Kbit/s | Bitrate range kbit/s |
|---|---|---|
| -b 320 | 320 | 320 CBR |
| -V 0 | 245 | 220...260 |
| -V 1 | 225 | 190...250 |
| -V 2 | 190 | 170...210 |
| -V 3 | 175 | 150...195 |
| -V 4 | 165 | 140...185 |
| -V 5 | 130 | 120...150 |
| -V 6 | 115 | 100...130 |
| -V 7 | 100 | 80...120 |
| -V 8 | 85 | 70...105 |
| -V 9 | 65 | 45...85 |
So, in our example above, we selected -q:a 2, meaning we used LAME's option -V 2, which gives a VBR mp3 audio in the approximate range of 170-210 kbit/s.
If you want constant bit rate (CBR) mp3 audio, instead of using -q:a option, you need to use the -b:a option to set the preferred "audio bit rate" in bits per second, like -b:a 256k if you want 256 kbit/s audio.
