ffmpeg - warnings when i try to merge several audio files with an .mp4 file

Question

I'm trying to merge 8 audio (.wav) files with a video (.mp4) file. I have the following command:

ffmpeg -i video.mp4 -i audio1.wav -i audio2.wav -i audio3.wav -i audio4.wav -i audio5.wav -i audio6.wav -i audio7.wav -i audio8.wav -filter_complex "[1:a][2:a]amerge=inputs=2[a0]" -filter_complex "[3:a][4:a]amerge=inputs=2[a90]" -filter_complex "[5:a][6:a]amerge=inputs=2[a180]" -filter_complex "[7:a][8:a]amerge=inputs=2[a270]" -map 0:0 -map "[a0]" -map "[a90]" -map "[a180]" -map "[a270]" -c:v copy -c:a:0 libfdk_aac -vbr:0 5 -c:a:1 libfdk_aac -vbr:1 5 -c:a:2 libfdk_aac -vbr:2 5 -c:a:3 libfdk_aac -vbr:3 5 -movflags +faststart output.mp4

I can successfully run the command and the output is good, but i get the following warnings displayed (3 times each):

[Parsed_amerge_0 @ 0x7fcaca708da0] No channel layout for input 1 [Parsed_amerge_0 @ 0x7fcaca708da0] Input channel layouts overlap: output layout will be determined by the number of distinct input channels

And:

[libfdk_aac @ 0x7fcacc87c800] Note, the VBR setting is unsupported and only works with some parameter combinations

What do they mean and how can i "get rid of them"?

Thanks!

Answer 1

The first warning means one of the WAV inputs has no channel layout defined i.e. channel 1 is Front Left, channel 2 is Front Center..etc. amerge decides the channel layout of the output stream based on the input, but since that designation is missing for one of the inputs, amerge will assign a channel layout which is default for the number of channels present in the output.

One thing you could try is rewrap all of the audio inputs i.e.

ffmpeg -i audio1.wav -c copy audio1-new.wav

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The 2nd warning is displayed whenever VBR mode is invoked. Unless you get the error: Unable to set the VBR bitrate mode, you should be fine.