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What options are there to encode at variable bit rate with aac_at?

aac_at seems to be the best aac encoder. I have used it to convert opus audio (because macOS older than Sonoma do not support opus) in some movies as CBR aac with something like:

ffmpeg -i movie.mkv -c:v copy -c:a aac_at -b:a 128k movie.mp4

Some sources recommend using VBR for better quality. But there are few examples how to do it.

This seems to work OK:

ffmpeg -i movie.mkv -c:v copy -c:a aac_at -aac_at_mode vbr -q:a 2 movie.mp4

-q:a range is 0-14. With my sample movie according to MediaInfo -q:a 0 and 1 output CBR at 152 kb/s and 132 kb/s, with 2 VBR average 116 kb/s, with 4 VBR average 66 kb/s and with 14 VBR average 27 kb/s (all VBR max at 128 kb/s).

https://wiki.hydrogenaud.io/index.php?title=Apple_AAC

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