I am using following commands to make aac/mp3 files with 256kbps bitrate and constant bitrate mode.

For aac:

ffmpeg -i test.wav -b:a 256k -y test.aac

For mp3:

ffmpeg -i test.wav -b:a 256k -y test.mp3

I need to make an aac file in CBR mode from a wav file. As per the documentation available at https://ffmpeg.org, CBR should be made using above mentioned commands but it is always making VBR.

I need help to see if I am doing something wrong, or there are some other steps to be followed. I have tested with ffmpeg versions 2.8 and 3.2.

Can anyone help me fix it?

  • I think that the -b option behavior depends on the codec? i.e. for libfdk_aac, you'd specify bits, for Lame, kilobits etc. – Yorik Jul 26 '17 at 17:46
  • @Yorik, Lame within ffmpeg also takes bits/s. The standalone tool may be different. – Gyan Jul 26 '17 at 19:35

The native AAC encoder doesn't do CBR. It does ABR but can drift. libfdk_aac can do CBR.

  • Hi, thank you for your reply. Can u please guide me how can I achieve this, I followed the documentation but it didn't work – Sajag Singh Jul 27 '17 at 22:59
  • You need a build with lbfdk_aac compiled. What command did you use? – Gyan Jul 28 '17 at 4:53
  • I have already done that, command used is ffmpeg -i test.wav -c:a libfdk_aac -b:a 256k -y test.aac – Sajag Singh Jul 30 '17 at 11:25
  • How are you checking the output? – Gyan Jul 30 '17 at 11:28
  • using mpck and mediainfo command – Sajag Singh Jul 30 '17 at 12:32

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