ffmpeg: how to encode AAC @ CBR instead of VBR?

[marcorocchini]

Hi, yes: I'm a cat

but I wonder if is there a way to encode CBR because if I use

ffmpeg.exe -i input.mxf -filter_complex "[0:1] [0:2] amerge" -vf scale=interl=1 -pix_fmt yuv420p -c:v libx264 -profile:v main -level:v 4.1 -g 33 -bf 2 -crf 20 -flags +ildct+ilme -top 1 -c:a aac -b:a 320k -ar 48000 -aspect 16:9 OUTPUT.MP4

is alwais VBR. How can I resolve? thanks