Does ffmpeg's aac encoder need a audio bitrate?

What happens if you do not set a audio bitrate in the aac encoder ?

-ab 128k

Does it automaticly select the highest bitrate it can or something i am curious if i should do this or if it will be bad i see no difference.

It totally depends on the AAC encoder you are using. ffmpeg can use several AAC encoders:

• aac (ffmpeg-internal) – defaults to 128 kBit/s for both mono and stereo.
• libfdk-aac – defaults to 128 kBit/s for stereo and 96 kBit/s CBR for mono.

In general, you should choose a higher bitrate than the default for aac. This is because it produces somewhat lower quality than libfdk-aac at the same bitrate. Do that by setting -b:a appropriately, e.g. -b:a 192k.

I tried to summarize the VBR options on my homepage, and the FFmpeg Wiki has good info on AAC encoding as well as some example commands.

Also, when you see a line similar to this in ffmpeg's log output:

Stream #0:0: Audio: aac (libfdk_aac) (mp4a / 0x6134706D), … 96 kb/s

Then the kb/s shown here do not necessarily reflect what the encoder will really use, as it's up to the specific implementation.

There are also some encoders that are not supported anymore by ffmpeg:

• libfaac – defaults to VBR setting of 100, which results in about 128 kBit/s for stereo audio (reference).
• libvo-aacenc – defaults to 128 kBit/s for both mono and stereo.
• libaacplus