convert video/audio file to audio format with ffmpeg
ffmpeg can convert between different audio formats including MP3, AAC, AIFF, AU-SUN , DTS, WMA, M4A,
MP2, RA, WAV, AAC, FLAC, OGG, APE.
FFmpeg mp3
This page describes how to use the "lame" encoder within ffmpeg to creates mp3's (ffmpeg apparently has no native mp3 encoder). See also other codecs you could use, and/orAACEncodingGuide for background on FFmpeg audio encoding in general.
VBR Encoding
Example to encode VBR MP3 audio with ffmpeg using the libmp3lame library:
ffmpeg -i input.wav -codec:a libmp3lame -qscale:a 2 output.mp3
Control quality with -qscale:a (or the alias -q:a). Values are encoder specific, so for libmp3lame the range is 0-9 where a lower value is a higher quality. 0-3 will normally produce transparent results, 4 (default) should be close to perceptual transparency, and 6 produces an "acceptable" quality. The option -qscale:a is mapped to the -V option in the standalone lame command-line interface tool.
| LAME Bitrate Overview | |||
|---|---|---|---|
| Option | Kbit/s | Bitrate range kbit/s | ffmpeg equiv. |
| -b 320 | 320 | 320 CBR (non VBR) example | -b:a 320k (NB this is 32KB/s, or its max) |
| -V 0 | 245 | 220...260 | -q:a 0 (NB this is VBR from 22 to 26 KB/s...) |
| -V 1 | 225 | 190...250 | -q:a 1 |
| -V 2 | 190 | 170...210 | -q:a 2 |
| -V 3 | 175 | 150...195 | -q:a 3 |
| -V 4 | 165 | 140...185 | -q:a 4 |
| -V 5 | 130 | 120...150 | -q:a 5 |
| -V 6 | 115 | 100...130 | -q:a 6 |
| -V 7 | 100 | 80...120 | -q:a 7 |
| -V 8 | 85 | 70...105 | -q:a 8 |
| -V 9 | 65 | 45...85 | -q:a 9 |
In our example above, we selected -qscale:a 2, meaning we used LAME's option -V 2, which gives us a VBR MP3 audio stream with an average stereo bitrate of 170-210 kBit/s.
CBR Encoding
If you need constant bitrate (CBR) MP3 audio, you need to use the -b:a option instead of -qscale:a. Here you can specify the number of bits per second, for example -b:a 256k if you want 256 Kbit/s (25.6 KB/s) audio. Available options are: 8, 16, 24, 32, 40, 48, 64, 80, 96, 112, 128, 160, 192, 224, 256, or 320 (add a k after each to get that rate). So to get the highest quality setting, it would be like -b:a 320k.
Troubleshooting
Sometimes it will give you *less* bits per second than you request. If you're doing CBR, it could be because you chose a value "in between" its allowable settings (if defaults down to the next lower). If you're doing VBR, it could be because the input itself is already in a lower bitrate than the requested, in which case, it basically just re-encodes it at the bitrate the input already was. Or possibly that the input was CBR and the VBR aspect is able to reduce bitrate by quite a lot.