Kevin Cui's Blog

From the fdk_aac encoder to automated static FFmpeg builds

3 min read#ffmpeg

Intro

I have been doing some video-processing tasks at work, and ran into a requirement to extract the audio from an MP4 into AAC.

My first thought was that it was simple: just ffmpeg -i source.mp4 -vn -acodec copy sound.aac.

That turned out to be wrong. The AAC duration and the MP4 duration were completely different. See:

I also tried the methods on the internet, but none of them fixed it.

Analysis / fix

After testing, I found that different bit rates produced different AAC Duration values.

But how was I supposed to know what the bit rate was? So I traced it from the start.

First, the MP4 came from Webm, and the Webm video came from MediaRecorder.

I looked at the MediaRecorder API again and found this property:

  • audioBitsPerSecond: specifies the audio bit rate

OK, found it. I changed the code, added the audioBitsPerSecond property on the MediaRecorder interface, and set it to 128000, which is 128K.

Then I converted with the command below and checked the result:

ffmpeg -i source.mp4 -vn -acodec aac -b:a 128k -y sound.aac

That did not improve things…

Then I wondered whether, even with a constant bit rate, there was still some drift, so I changed 128k to 200k and tried:

Not only did it not fix it, it increased duration even more.

When I was close to giving up, rurico suggested trying the libfdk_aac encoder. So I recompiled FFmpeg to install the libfdk_aac encoder (extra encoders in FFmpeg require a rebuild).

It worked…

Statically compiling FFmpeg / automation

But that only worked on a Mac. I needed it to run on Ubuntu, and the thought of compiling all of FFmpeg on Ubuntu Docker already made my scalp tingle. (I spent 4 hours trying to compile FFmpeg with libfdk_aac on Ubuntu, and failed in the end.)

After I got home from work and finished dinner, I searched ubuntu build ffmpeg again and saw static build. It clicked.

So I looked for a ready-made statically built FFmpeg. There are some, but because of the libfdk_aac LICENSE, they generally do not include libfdk_aac.

Then I found an open source FFmpeg static build project on GitHub: ffmpeg-static

Testing showed it was fine, but from inside China every download, install, and compile was slow, so I remembered GitLab has a free runner. I created a project on GitLab for automatic builds.

You only need to add .gitlab-ci.yml to the project:

image: ubuntu:18.04


stages:
  - build


build-ubuntu:
  stage: build
  script:
    - apt-get update
    - apt-get install -yq bzip2 xz-utils perl tar wget git bc
    - apt-get install -yq autoconf automake build-essential cmake curl frei0r-plugins-dev gawk libfontconfig-dev libfreetype6-dev libopencore-amrnb-dev libopencore-amrwb-dev libsdl2-dev libspeex-dev libtheora-dev libtool libva-dev libvdpau-dev libvo-amrwbenc-dev libvorbis-dev libwebp-dev libxcb1-dev libxcb-shm0-dev libxcb-xfixes0-dev libxvidcore-dev lsb-release pkg-config texi2html yasm
    - git clone https://github.com/BlackHole1/ffmpeg-static
    - cd ffmpeg-static
    - chmod 777 *
    - ./build-ubuntu.sh -B
  artifacts:
    name: build
    paths:
      - ./ffmpeg-static/bin/*

Then push to GitLab, CI triggers automatically. Half an hour later, the result:

gitlab project: https://gitlab.com/BlackHole1/ffmpeg-static-build

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