[Xiph-Advocacy] Ogg and WebM technical alert: default FFmpeg Vorbis encoder is broken

[Monty Montgomery]

Overview:

FFmpeg's built-in Vorbis encoder produces low enough quality output to
be considered broken.  This encoder is used by default in the majority
of FFmpeg builds, and will produce .ogv and WebM videos with low to
unusably poor audio quality.

This alert is intended for all users of FFmpeg (via the command line
or GUI wrappers) and all application developers that make use of the
FFmpeg command line tool.  Application developers that use the FFmpeg
libraries should also take care that the libavcodec built-in Vorbis
encoder library is not used by accident.

Scope:

All past and present builds of FFmpeg and libavcodec up to but not
including the upcoming 0.6 release.  Default builds of the upcoming
FFmpeg 0.6 release will not use the built-in encoder by default, but
it will still be possible to accidentally use or restore the built-in
encoder to default status during the FFmpeg build.  It should be
assumed that any build of FFmpeg and any application using FFmpeg
could be producing videos with substandard Vorbis audio unless the
FFmpeg build and usage is verified to be using system Vorbis
libraries, such as those provided by Xiph.Org or aoTuV.

Workaround / Fix:

FFmpeg can be forced to use the external/system libVorbis library by
passing:

-acodec libvorbis

as part of the FFmpeg command line.

Note that passing '-acodec vorbis' is incorrect and requests the
low-quality built-in FFmpeg-internal Vorbis encoder.  Also, FFmpeg may
be built without libvorbis support, meaning that many FFmpeg builds
only have the internal encoder available.  In this case, requesting
'-acodec libvorbis' will fail with the error 'Unknown encoder
'libvorbis''.

FFmpeg can be built with working libvorbis support and the internal
Vorbis encoder disabled as follows:

./configure --disable-encoder=vorbis --enable-libvorbis; make; make install

Such a build completely removes the internal Vorbis encoder from
libavcodec, eliminating the possibility of accidental use on the
command line or in libavcodec-based applications.

Verification:

Use of a good Vorbis encoder in .ogg, .oga, .ogv and WebM files may be
verified as follows.  This test will work on any Ogg or WebM file to
verify the encoder that produced the audio.  Note that 'Vorbis' is
case-sensitive:

strings file_to_be_checked | grep Vorbis

A file that was encoded using a good encoder will output a line
containing 'Xiph.Org libVorbis' or 'aoTuV', such as:

boatanchor$ strings test2.ogg |grep Vorbis
Xiph.Org libVorbis I 20100325 (Everywhere)

or

boatanchor$ strings test3.ogg |grep Vorbis
AO; aoTuV b5d [20090301] (based on Xiph.Org's libVorbis)

A file encoded by FFmpeg's internal Vorbis encoder will produce no
grep output as it does not set a vendor string.

Monty
Xiph.Org