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• Transcode HLS Segments individually using FFMPEG

  27 mai 2013, par rayh

  I am recording a continuous, live stream to a high-bitrate HLS stream. I then want to asynchronously transcode this to different formats/bitrates. I have this working, mostly, except audio artefacts are appearing between each segment (gaps and pops).

  Here is an example ffmpeg command line :

  ffmpeg -threads 1 -nostdin -loglevel verbose \
  
     -nostdin -y -i input.ts -c:a libfdk_aac \
  
     -ac 2 -b:a 64k -y -metadata -vn output.ts

  Inspecting an example sound file shows that there is a gap at the end of the audio :

  

  And the start of the file looks suspiciously attenuated (although this may not be an issue) :

  

  My suspicion is that these artefacts are happening because transcoding are occurring without the context of the stream as a whole.

  Any ideas on how to convince FFMPEG to produce audio that will fit back into a HLS stream ?

  ** UPDATE 1 **

  Here are the start/end of the original segment. As you can see, the start still appears the same, but the end is cleanly ended at 30s. I expect some degree of padding with lossy encoding, but I there is some way that HLS manages to do gapless playback (is this related to iTunes method with custom metadata ?)

  
  

  ** UPDATED 2 **

  So, I converted both the original (128k aac in MPEG2 TS) and the transcoded (64k aac in aac/adts container) to WAV and put the two side-by-side. This is the result :

  
  

  I'm not sure if this is representative of how a client will play it back, but it seems a bit odd that decoding the transcoded one introduces a gap at the start and makes the segment longer. Given they are both lossy encoding, I would have expected padding to be equally present in both (if at all).

  ** UPDATE 3 **

  According to http://en.wikipedia.org/wiki/Gapless_playback - Only a handful of encoders support gapless - for MP3, I've switched to lame in ffmpeg, and the problem, so far, appears to have gone.

  For AAC (see http://en.wikipedia.org/wiki/FAAC), I have tried libfaac (as opposed to libfdk_aac) and it also seems to produce gapless audio. However, the quality of the latter isn't that great and I'd rather use libfdk_aac is possible.

• FFmpeg benchmark option

  12 septembre 2019, par chronosynclastic

  FFmpeg allows measuring CPU and wall-clock times using the -benchmark option. It gives e.g. an output like this :
  bench: utime=0.689s stime=1.350s rtime=4.036s.

  Does the value rtime contain also the file reading and writing times ? If yes, is there a way to measure solely the encoding time ?

• How to swap package configuration for environment

  24 juillet 2018, par Finn Maunsell

  I’d like to change the location that I get ffmpeg from for my anaconda environment. The problem has occured because I want to use the ffmpeg configuration for my base system instead of the configuration meant for the environment.

  If I use ffmpeg in ubuntu bash then I get this for my active environment :

    ffmpeg version N-91510-gd134b8d Copyright (c) 2000-2018 the FFmpeg developers
  
    built with gcc 5.4.0 (Ubuntu 5.4.0-6ubuntu1~16.04.10) 20160609
  
    configuration: --prefix=/home/notebook/ffmpeg_build --pkg-config-flags=--static --extra-cflags=-I/home/notebook/ffmpeg_build/include --extra-ldflags=-L/home/notebook/ffmpeg_build/lib --extra-libs='-lpthread -lm' --bindir=/home/notebook/bin --enable-gpl --enable-libaom --enable-openssl --enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame --enable-libopus --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-nonfree

  and for non-environment :

  ffmpeg version 3.4.1 Copyright (c) 2000-2017 the FFmpeg developers
  
    built with gcc 4.8.2 (GCC) 20140120 (Red Hat 4.8.2-15)
  
    configuration: --prefix=/home/notebook/anaconda2/envs/tensorflow --disable-doc --enable-shared --enable-static --extra-cflags='-Wall -g -m64 -pipe -O3 -march=x86-64 -fPIC -I/home/notebook/anaconda2/envs/tensorflow/include' --extra-cxxflags='=-Wall -g -m64 -pipe -O3 -march=x86-64 -fPIC' --extra-libs=
  
  '-L/home/notebook/anaconda2/envs/tensorflow/lib -lz' --enable-pic --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --enable-libx264

  Solving this will allow me to use the 2nd ffmpeg installation in my conda environment.