[FFmpeg-trac] #5910(undetermined:new): AAC to PCM conversion inserts extra silence in the beginning
FFmpeg
trac at avcodec.org
Tue Nov 1 17:32:46 EET 2016
#5910: AAC to PCM conversion inserts extra silence in the beginning
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Reporter: | Owner:
jwilhelmsson | Status: new
Type: defect | Component:
Priority: normal | undetermined
Version: unspecified | Resolution:
Keywords: aac pcm | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
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Comment (by jwilhelmsson):
I figured.
I don't know what the expected delay is, or if there is any, only the
difference between what ffmpeg produces when extracting and converting and
what Nuendo/Protools/Quicktime produces.
As I wrote, the source material we receive is mp4 and/or mov files, but
when we work with it we need to have the audio (in wav format) on a
separate track so I need to extract and convert the aac audio streams.
During this conversion, ffmpeg seems to insert silence in the beginning of
the files, as follows (all numbers are approximate and measured by marking
areas in Audacity):
The "tj" files get 44 ms delay (about 2100 samples, which sounds close to
the 2112 value).
The "td" files get 108 ms delay.
The "g" file gets 32 ms delay *at the start*, but 44 ms overall. This is
easy to see since it starts with color bars and a tone which should start
at the very beginning and has a clean end. Comparing the end points you
get 44 ms, but the start is for some reason shorter.
And as I've written, these delays appear regardless of whether the audio
is extracted and converted from the original file, with {{{ffmpeg -i
source.mp4 -c:a pcm_s24le -ar48k out.wav}}}, or if the audio is first
extracted ({{{ffmpeg -i source.mp4 -c:a copy source.aac}}}) and then
converted ({{{ffmpeg -i source.aac -c:a pcm_s24le -ar48k out.wav}}}).
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Ticket URL: <https://trac.ffmpeg.org/ticket/5910#comment:17>
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