[FFmpeg-user] 5.1 AAC without lowpass on the LFE channel

Steven Ruppert steven at waywiser.org
Mon Sep 12 18:33:41 EEST 2022


I'm trying to encode 6 arbitrary mono audio streams into a single AAC 
5.1 track in an mp4 container (here with test streams):

$ ffmpeg -f lavfi -i testsrc=duration=10:size=100x100:rate=30 -f lavfi 
-i aevalsrc="-2+random(0)" -filter_complex 
-map '0:v' -map "[a]" -c:a aac -channel_layout 5.1 -t 10 testlfe.mp4

5 of the channels replicate the input audio just fine (modulo encoding). 
However, the LFE channel is lowpassed. Extracting with:

$ ffmpeg -i testlfe.mp4 -filter_complex 
"channelsplit=channel_layout=5.1:channels=LFE[LFE]" -map '[LFE]' 

I get a lowpassed rumble, instead of the original full white noise.

Is there a way to prevent the lowpass from happening?

I couldn't find any references whether that's inherent to the AAC 5.1 
encoding, something that ffmpeg does, or inherent to the decoding 
process. (I did decode my same test files using something that uses 
Microsoft MediaFoundation and the LFE channel was still lowpassed).


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