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Container
The main types of video file formats are mp4, avi,
ogm, and matroska. These files are in fact only envelopes (or
containers) for compressed video and audio streams. There are
several ways to compress this data, we will call codec the
compression and decompression software responsible for reading
this data. An avi file can therefore very well contain a video
stream with an xvid or even mpeg2 codec.
Each container has in the header the information that
describes the streams it contains, as opposed to .mpeg files
which can only contain mpeg. The .avi container is still one of
the most used, it is however limited, we cannot put ogg stream,
or even several sound tracks and chapters as for DVDs. This is
why new containers have appeared to offer new possibilities, I
am thinking in particular of the ogm container ( http://www.bunkus.org/videotools/ogmtools/index.html ), matroska ( http://www.matroska.org ) and of course mp4
Codec
A number of
codecs are directly inspired by a standards body called MPEG
for Motion Picture Expert Group. It developed the MPEG1
standard for the production of video laser discs. MPEG2 was
developed for digital television. MPEG3 was stillborn, because
its functionalities were largely taken over with the
development of MPEG2. MPEG4 allows the broadcasting of video
with a low bitrate medium.
The oldest and most
universal codes are:
VCD compatible with MPEG1
standard, resolution 352*288, 25img/s PAL/SECAM, bit rate
1123kb/s, quality slightly lower than a VHS tape
XVCD compatible with MPEG1
standard, resolution 352*576 or 352*288, 25img/s PAL/SECAM,
bit rate up to 2500kb/s
SVCD compatible with MPEG2
standard, resolution 480*576, 25img/s PAL/SECAM, bit rate
2520kb/s, SVHS quality
DVD compatible with MPEG2
standard, resolution 720*576, 25img/s PAL/SECAM, bit rate of
28400Kb/s higher DV quality
The VCD codec is the most
universal, it can be read by all video playback software on PC
(and others) and by all home DVD players.
The SVCD codec can be read
by some playback software and some home DVD players.
The DVD codec is obviously
read by home DVD players.
The most recent and most
used video codecs today are:
The x264 which is based on
the MPEG4 AVC/H264 standard is currently the most fashionable and
popular codec, compact while preserving very good image
quality, to give an idea for a DVD quality file the size is
half as small. The x265 which is based on the MPEG4 HEVC/H265
standard which should eventually
replace the x264, it significantly improves coding/compression
compared to AVC, in other words, the files obtained are much
more compact for the same quality. Theora is a completely free and
patent-free video codec, it is not fully compatible with the
MPEG4 standard which means that its use does not result in the
payment of a royalty to the MPEG consortium.
DivX is a codec that has
fallen into disuse today, it is an implementation of a
non-final version of the MPEG4 H263 and H264 standards. It is
not a free codec (in the OpenSource sense), it is developed by
DivXNetwork. It appeared a few years ago under the name
DivX;-) in its version 3.11, by digging a little we realized
that it was a pirated version of a Microsoft codec
(MS-MPEG4-v3, which is a misnomer since it is not compatible
with the MPEG4 standard). So DivXNetwork was forced to develop
its own codec, compatible with the MPEG4 standard, and in
addition decided to make it OpenSource, from there was born
OpenDivX. Its existence was more than ephemeral, it very
quickly gave way to the DivX 4.0 codec which had nothing
OpenSource about it. However, the OpenDivX sources were the
basis for the development of a fully OpenSource MPEG4
compatible codec, namely XviD (the opposite of DivX...). In
summary, Xvid is an MPEG4 compatible codec under the GNU GPL
(OpenSource) license and DivX (4.0 and 5.0) is a codec that is
not fully MPEG4 compatible and is not free.
audio codec is still widely used, it is
the audio specification of the MPEG1 and MPEG2 standards,
purists will find that it degrades the audio quality, there
are now other codes that can better preserve the audio
quality. The AAC audio codec is one of
these new codecs, it is said that a file encoded in 128kb/s in
AAC is equivalent to an MP3 file encoded at 256kb/s. The Vorbis audio codec , which unlike the two
previous codecs is completely free and patent-free, is more
efficient than MP3 but remains largely less popular, wrongly
so.