[FFmpeg-user] Audio "bumps" in concatenated file

[Soeren Balko]

On Wed, Oct 23, 2013 at 8:40 PM, Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:

> Soeren Balko <soeren <at> zfaas.com> writes:
>
> > I just tested this scenario with the libfaac and mp3 encoders
>
> Please test audio-only with mp2, ac3 and native aac encoders.
>
> Please do not top-post here, Carl Eugen
>
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I tested some more variants: For one, I tried invoking the segment muxer
with a list of split frames (-segment_frames) but still have audible
"bumps".

As per your suggestion, I first "spliced out" the audio stream:

ffmpeg -i input.mov -codec copy -map 0:1 audio.mov
then split the audio-only file into segments (again at time-based offsets):

ffmpeg -i audio.mov -codec copy -map 0 -f segment -segment_time 1
audio%02d.mov

then transcoded these with the alternative audio codecs, e.g.:

ffmpeg -i audio00.mov -f mpegts -acodec mp2 audio00.ts
ffmpeg -i audio01.mov -f mpegts -acodec mp2 audio01.ts
...
and finally concatenated these together:

ffmpeg -i "concat:./audio00.ts|./audio01.ts" -codec copy output.mp4

For AAC-coded streams, I have to add the -absf aac_adtstoasc option,
otherwise ffmpeg will not process the files. Interestingly, the "audio
bumps" problem seems to be codec-dependent. My observation was:

MP2, AC3, libfaac: no audible "bumbs"

AAC, MP3, libfdk_aac: there are audible "bumps"

Interestingly, libfaac is in the "good" group this time - I used a
different file for testing in this posting.

ffplay is so kind to even generate a visual appearance of this phenomenon
(see attached screen shot).The arrows point to thin vertical lines where
the audio is silent for a split second.

 [image: Inline image 1]
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