ffmpeg - output 5.1 AAC without lowpass on the LFE channel

blendmaster 0
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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 "[1:a][1:a][1:a][1:a][1:a][1:a]join=inputs=6:channel_layout=5.1:map=0.0-FL|1.0-FR|2.0-FC|3.0-LFE|4.0-BL|5.0-BR[a]" -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]' testlfe.wav

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

output spectrogram

(from ffmpeg -i testlfe.wav -lavfi showspectrumpic=s=640x320 testlfe.png)

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).

aac mp4 audio ffmpeg
1 Answers

Turns out, the AAC codec inherently limits the LFE bandwidth, so there's no way around it.

(thanks to kesh in the comments) Wikipedia's Advanced Audio Encoding article claims the upper limit is 120Hz which matches my spectrogram, but doesn't cite a source. The actual ISO/IEC 13818-7:2006(en) Standard costs a bunch of money to read as usual, but in the free glossary there is an entry:

low frequency enhancement ( LFE ) channel: limited bandwidth channel for low frequency audio effects in a multichannel system

blendmaster0
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