Digital Video Formats

    ...Akkana Peck

Introduction and Disclaimer

I am not an expert in video formats.  I wrote this page because I was having trouble getting the information, and couldn't find any good explanations of video formats on the web.  The information here is gleaned from reading pages I found via google searches (references to some of the useful sites I found are given at the bottom).

This is a work in progress.  If you find errors, please let me know.

What's a Video Format?

Video formats are confusing because most video files have at least two different types: the container, and the codec(s) used inside that container.

The container describes the structure of the file: where the various pieces are stored, how they are interleaved, and which codecs are used by which pieces.  It may specify an audio codec as well as video.

A codec ("coder/decoder") is a way of encoding audio or video into a stream of bytes.

To make life even more confusing, some names, such as "mpeg-4", describe both a codec and a container, so it's not always clear from context which is being used.  You could have a movie encoded with an mpeg-4 codec inside an avi container, for example, or a movie encoded with the Sorenson codec inside an mpeg-4 container.

The Linux file program is a fast way to find out the container format of a video file.

You can use the mencoder program (part of mplayer) to tell you the container and video codec of a file (you'll have to wade through a lot of other output).

For mpeg files, you can find out the audio codec with mpginfo, part of the mpgtx package.  For other formats, try
mplayer -identify -frames 0 filename | grep ID_

Common Container Formats:

AVI (.avi): Most commonly contains M-JPEG (especially from digital cameras?) or DivX (for whole movies), but can contain nearly any format (not Sorenson).  Sometimes you'll see a reference to the "fourcc": this is a four-character code (such as "divx" or "mjpg") inside the AVI container which specifies which video codec is being used.

Quicktime: Most often used for the locked Apple Sorenson codec, or for Cinepak (free), but can also hold other codecs such as mjpeg, etc.

WMV (.wmv): More or less MPEG4; can contain nearly any codec, including several Microsoft spinoffs of MPEG-4 which vary in their freedom and licensing requirements.

ASF ("Advanced Streaming Format", .asf): a subset of wmv, intended primarily for streaming: an early Microsoft implementation of an MPEG4 codec.

Common Codecs:

MPEG ("Moving Pictures Expert Group"): three video formats, MPEG 1, 2, and 4.
MPEG-1: Old, supported by everything (at least up to 352x240), reasonably efficient.  A good format for the web.
MPEG-2: A souped-up version of MPEG-1, with better compression.  720x480.  Used in HDTV, DVD, and SVCD.
MPEG-4: A family of codecs, some of which are open, others Microsoft proprietary.
MPEG spinoffs: mp3 (for music) and VideoCD.

MJPEG ("Motion JPEG"): A codec consisting of a stream of JPEG images.  Common in video from digital cameras, and a reasonable format for editing videos, but it doesn't compress well, so it's not good for web distribution.

DV ("Digital Video"): Usually used for video grabbed via firewire off a video camera.  Fixed at 720x480 @ 29.97FPS, or 720x576 @ 25 FPS.  Not very highly compressed.

WMV ("Windows Media Video"): A collection of Microsoft proprietary video codecs.  Since version 7, it has used a special version of MPEG4.

RM ("Real Media"): a closed codec developed by Real Networks for streaming video and audio.  Maybe also a container?

DivX: in early versions, essentially an ASF (incomplete early MPEG-4) codec inside an AVI container; DivX 4 and later are a more full MPEG-4 codec..  No resolution limit.  Requires more horsepower to play than mpeg1, but less than mpeg2.  Hard to find mac and windows players.

Sorenson 3: Apple's proprietary codec, commonly used for distributing movie trailers (inside a quicktime container).

Quicktime 6: Apple's implementation of an MPEG4 codec.

RP9: a very efficient streaming proprietary codec from Real (not MPEG4).

WMV9: a proprietary, non-MPEG4 codec from Microsoft.

Ogg Theora: A relatively new open format from Xiph.org.

Dirac: A very new open format under development by the BBC.

There are many others; this document does not attempt to list them all.

References

Advanced Video Compression - Part 1 and Part 2.  A most excellent overview.
Codec/Container Table -- The page is partly in French, but the table is in English.
Linux A/V (Co)Decs -- says it's out of date, but a useful table nontheless, showing containers, codecs, and players which support them.
FAQ of AVI MPEG Video Converter
FourCC list of video codecs

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