Codecs

Raw audio and video files are very large so a variety of schemes have been developed to make them smaller without significantly degrading quality. This shrinking process, called compression and decompressions, is accomplished by complex algorithms stored within installed system components called codecs.

COmpression/DECompression = CODEC

There are hundreds of codecs in circulation. Some codecs are written for various audio formats while other are written for video formats. It might also be important to note that some codecs are very good and some are very bad. If your source movie was created with one or more resource demanding codecs your conversion process will be slower because every video frame and/or every sound sample will have to be passed through the codec before it can be processed by the converter.

For the purpose of this document we'll limit our discussion to some video codecs used in AVI, QuickTime, Real Media and Windows Media video formats.

The codec works in harmony with playback or editing software to compress files when they are created and to decompress them for playback or for editing. For example, when you double click on an AVI file, your default media player will open the file and read through the file header until it locates the 4 character code (called the FOURCC code) which tells the player what codec was used to create the file. The below image illustrates an AVI file opened in a HEX editor. In the right column you will note the characters vidsmp43. This tells us, and your media player, that the video in this AVI file was compressed using the Microsoft MPEG-4 V-3 codec,

After your media player has located the FOURCC code it searches through the codecs on your system for a match. If it finds a match it calls on another component that we'll call the stream reader, which reads the data stream and passes it to the codec. The codec now creates a new stream of data that the player can interpret as a sequence of images much like the frames in a film strip.

When a new movie is created the process is reversed. First the source must be decompressed and turned into a frame sequence, then the editor makes whatever changes are required to the stream and feeds it back through the codec before writing the file to disk.

If you don't have the codecs that were used to create a particular movie (audio and video) you won't be able to play your movie. If you don't have the read/write versions of the codecs you won't be able to save it to the original format. Some non Microsoft codecs give away free playback versions of their codec but charge for the writable version.

MPEG-2 codecs are not part of the Windows operating system and are usually distributed with DVD playback software.

Versions Windows Media codecs are difficult for the casual user to identify since the wrong codec will often play a movie almost perfectly. However, when you want to convert or edit a particular Windows Media file you must have the correct codec. The best way to handle this problem is to install all the Windows media codecs. The below page contains links to the Microsoft codecs.

http://www.jhepple.com/support/KB/wmv_conversion.htm

Movies which follow the Microsoft Video for Windows standard have separate data streams for audio and for video. DV video, Type I which is used by some digital movie cameras, combine the audio and video to a single stream. Type I files must first be converted to Type II before they can be edited with VFW software.

If you have any early DivX ;-) or Angel Potion codecs installed on your computer anything might happen including system crashes. You should remove any and all hacked codecs from your computer and replace them with the most recent, legal versions.

Confusingly enough, the last entry under installed codecs is usually "Full Frames (Uncompressed)". This is not a codec but is instead the absence of a codec.

Keep in mind that the quality and size of the output file is completely determined by the codec so you should choose carefully.