Introduction

What are codecs? 
The word "codec" is short for "coder-decoder".  It can either be hardware or software that can compress or decompress data. Because of the limitations of the Internet and the fact that some people still have slow connections, minimizing the files size while maximizing the amount of media became paramount. 

Digital movies in raw format are very large files, So movies are compressed using various codecs, each codec has its own unique advantages and disadvantages. Some codecs make very small movies, but with low image quality, While others make small movies of quality but decompress much slower.

Unlike file compression packages (WinZip for example) that requires you to decompress a file before being able to use its contents, codecs decompress media files "On-the-fly" so you can view the movie from its original compressed format.

There are many different types of codecs in use on the internet, and new ones arriving regular. This guide concentrates on the codecs that the majority of people are using. If some other codec becomes more popular in the future then it would need to be undated again, but for now it contains what people are using to mainly share movies.

Identifying codecs

The simplest way to identify what codec is in use would be to use a hex editor and view the headers of the movie. The headers contain important information See this guide on FourCC . However there are programs around that make this task much more user friendly.

Gspot
By far the most widely used program is Gspot, you only need to load into it a media file for it to tell you what codec was used to make that file. It will also try to discover what your system is using to decompress that file and play it. However this does not always work to plan, and sometimes your told that your system contains everything needed to play the file, but it still does not play, or it may play video but have no audio.

Virtual Dub
File > File information

Finding out what codecs are being used in movies, is usually needed because you have a problem. The movie may play without any video, or without sound. It is then necessary to find out what codecs are in use so that you can install them.

In the passed people recommended codec packs. The idea was that it was not necessary to find out what was missing, if you installed them all. The "Cure-all" idea comes with many problems, where one particular home movie was playing just fine, you now find it does not play at all. In addition to these types of problems, codec packs are released with Demo codecs, and some of those demos install spyware. Always purchase the codec, and install separately.

There have been reports of people seeing a red logo displayed throughout a home movie they made, or their firewall asking if you want to block some application that wants to access the internet. There have been thousands of people with problems, and they have all been caused by codec packs. You don't need them !

Playback
If you only want to playback the movies you have downloaded, there are some players that come with their own filters and codecs, so using those might mean you wont need to install a codec at all. There are also some codecs that will decompress other formats, for example DivX 5 could be used to decompress Xvid, this again has its own set of unique problems.

Some players that could be used
BSplayer
This player is popular among those who use subtitles
Zoom Player
Popular with those who playback movies via a TV out card to watch your their TV
Stamp3 VideoLAN and DivX Player and others like Power DVD

By far the most widely used player is the one that comes installed with Windows. Windows Medial Player, but new released of this have also come with problems. Windows Media Player V6 is still installed on most systems, even if you have an updated version of it.

Encoding
If you intend to do any encoding of media files, then you will need to install the popular codecs, and you will probably want a player just to verify things are working as they should. Windows Media Player v6 uses less resources than any of the others, and is ideal for testing your playbacks at each stage.

Codecs in use
These are the popular codecs and are mostly used to share movies
DivX 5 Pro
DivX ;-) MPEG4 Hi-Res Video Codec - 3.11alpha release (Low motion, fast motion, DivX audio)
Microsoft MPEG 4, V1 V2 V3 Install V4.00.0.3688
Mpeg 2 Ligos Technology LSX-MPEG Suite v2.0 for Mpeg 2 playback and for TMPGEnc
Note: I have heard that this is a hacked Illegal Microsoft codec, but not able to confirm this.
Fraunhofer Mp3 Pro
Moonlight Odio Dekoda V1.23, Universal AC3/MPEG Audio (Layer 1/2/3) Decoder
Mpeg 1, Installed by Windows
Xvid

Others, you don't need these installed
DivX 4.11
DivX 4.8
Morgan Multimedia (M-JPEG)
WMV M$
Mov Quicktime
ASF M$
RM RealProducer

Installation

DivX Pro, Current V5.0.5
This is downward compatible for DivX 4 (Not 3.11 alpha)
DivXPro505Bundle.exe
Installation:
Run, DivX.Pro.v5.0.5.exe
See readme.txt for the rest

DivX ;)- 3.11 Alpha
Warning, I have heard this may be an illegal hacked Microsoft codec, but cannot confirm this
This has Low motion, and fast motion
DivX.inf
DIVX_c32.ax
DivXa32.acm
DivXc32.dll
DivXc32f.dll
L3codeca.acm
Register_DivX.exe
Installation:
Run, Register_DivX.exe
If asked for disk, give location where you ran the file from.

M$ MPEG 4, V1,2,3
V4.00.0.3688, Unlocked for use of V3 in AVI, not just ASF
mpeg4fix.inf
Mpg4c32.dll
Installation:
Right click mpeg4fix.inf and select install

Mpeg 2 filter
Use this when you want to convert Mpeg 2 in TMPGEnc +
LSX MPEG Encoder Suite 2.0.exe
Installation:
Run LSX MPEG Encoder Suite 2.0.exe

Fraunhofer Mp3 Pro
Get higher bitrates with this MP3 codec.
Mpeg 3 layer codec Fraunhofer.exe
Installation:
Run, Mpeg 3 layer codec Fraunhofer.exe

AC3 Audio
Moonlight Odio Dekoda DirectShow® filter V1.23, binary (71,855 bytes, ZIP) Freeware
Download MoonLight Odio Dekoda v.1.23
Updated with WinZip self installer
mlcom.ax
reg.bat
regsvr32.exe
Installation:
Make a folder on your hard drive at C:\Program Files\AC3OdioDec
Unpack the contents of the zip file to that folder.
Now double click the Reg.bat to register the filter and its location.
Don't delete the folder afterwards.

Xvid
Download XviD-04102002-1.exe Freeware
Use only the stable release
XviD-04102002-1.exe
Installation:
Run, XviD-04102002-1,exe
Un-install after use.
 

You can also test your system by using my test files. Download and play the files. They should all look and sound the same. If one of them gives you problems, you can tell by its name what codec to install. If you are doing any encoding, you should also try to encode them, this also may show up a problem.
 

Test files
These should be played in Windows Media Player 6 to get a better indication that your system is working correctly.

Each movie clip has been created in stereo with channel 1 having a 800hz tone and channel 2 a 1000hz tone. Not meant has a test of your stereo
AC3 has 3/2 (L, C, R, l, r) LFE again not meant has a Dolby test, if you can hear the buzz then you have an AC3 codec installed and working.

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