[Libav-user] AAC decode problems

[Info || Non-Lethal Applications]

> On 14. Sep 2017, at 13:22, Anton Shekhovtsov <shekh.anton at gmail.com> wrote:
> 
> 
> 
> 2017-09-14 14:06 GMT+03:00 Info || Non-Lethal Applications <info at non-lethal-applications.com <mailto:info at non-lethal-applications.com>>:
> Hi guys,
> 
> I just received a movie file with AAC audio which doesn't play correctly in my player using the ffmpeg libraries.
> It sounded “robotic” and the audio ran out of sync rather quickly.
> 
> Ffprobe output:
> 
> ffprobe version 3.3.1 Copyright (c) 2007-2017 the FFmpeg developers
>   built with llvm-gcc 4.2.1 (LLVM build 2336.11.00)
>   configuration: --prefix=/Volumes/Ramdisk/sw --enable-gpl --enable-pthreads --enable-version3 --enable-libspeex --enable-libvpx --disable-decoder=libvpx --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libx264 --enable-avfilter --enable-libopencore_amrwb --enable-libopencore_amrnb --enable-filters --enable-libgsm --enable-libvidstab --enable-libx265 --disable-doc --arch=x86_64 --enable-runtime-cpudetect
>   libavutil      55. 58.100 / 55. 58.100
>   libavcodec     57. 89.100 / 57. 89.100
>   libavformat    57. 71.100 / 57. 71.100
>   libavdevice    57.  6.100 / 57.  6.100
>   libavfilter     6. 82.100 /  6. 82.100
>   libswscale      4.  6.100 /  4.  6.100
>   libswresample   2.  7.100 /  2.  7.100
>   libpostproc    54.  5.100 / 54.  5.100
> Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/Users/flo/Downloads/wetransfer-6ec47f/IQT Capture 170911 Unity Round1.mov':
>   Metadata:
>     major_brand     : qt  
>     minor_version   : 512
>     compatible_brands: qt  
>     encoder         : Lavf57.66.102
>   Duration: 00:07:58.81, start: 0.000000, bitrate: 2672 kb/s
>     Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720, 2499 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
>     Metadata:
>       handler_name    : DataHandler
>     Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 162 kb/s (default)
>     Metadata:
>       handler_name    : DataHandler
> 
> By examining the file I found out that the packet durations of the AAC stream were not 1024 as they are with all the other files I have but rather either 1000 or 1100.
> While this by itself would not be a problem, it’s very confusing that the AAC decoder outputs AVFrames with a duration of 1024.
> 
> So now I’m a bit stuck as I really don’t know what to do.
> If the packets were all 1000 I would try discarding the last 24 samples of each AVFrame but as the frame apparently only holds 1024 samples although the packet says it contains 1100 samples, I’m stuck.
> I tried sending a NULL packet in the case of longer packets hoping to get the remains 76 samples out of the decoder but I got this error message:
> 
> [aac @ 0x10539da00] Got unexpected packet size after a partial decode
> 
> The file plays fine in both QuickTime and VLC so there must be a correct way of handling these kind of files..
> 
> I’d be very grateful for any insight anyone of you might have!
> 
> Thanks and best,
> 
> Flo
> 
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> 
> 
> What do you mean "packet contains 1100 samples", how is that specified? AVPacket->duration is in time_base units, not in samples.

The audio stream has a time_base of 1/48000. So does the audio codec context.
That’s what I find so weird.

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