Video Codecs

Movies use compression algorithms to reduce the size of files much like a zip file compressor reduces the size of a text file. Generally speaking video compression removes redundant pixels and stores a map of where the pixels belong while audio compression removes silence and overlapping sound.

The codec is a separate program that Windows loads with the multimedia file. It has two parts: the encoder and the decoder. Fx MPEG Writer will use the codec that created the file to decode it and will use the codec that you choose to re-encode it.

The above image may not accurately reflect the codecs installed on your system.

A codec is to a multimedia application what a font is to a word processor. In raw uncompressed form, multimedia files are huge taking up an inordinate amount of drive space. To minimize the drive space occupied by a multimedia file it may be compressed using a compression codec.

There are an unknown number of codecs that may number in the thousands. Some are excellent and can reduce the size of a raw multimedia file to a tiny fraction of the uncompressed size with little quality loss; some are terrible, producing poor quality, large files or even crashing your operating system. Codecs are registered with Microsoft by their internal FOURCC header code. This is not to be confused with the file type as specified by the file extension.

Four-Character Codes (FOURCC) were introduced by Microsoft to clearly identify video data stream formats. The unique FOURCC value assigned to every compression format and pixel layout allows video frames to be passed between file and codec by ensuring the FOURCC of the source frame matches a FOURCC supported by the codec. FOURCCs describe the software technology that was used to produce multimedia data. By implication, they also describe the format of the data itself. FOURCCs are supposed to be registered with Microsoft by the vendors of the respective multimedia software technologies but not all vendors play by Microsoft's rules. Below is a DivX 4 encoded movie open in a binary editor. As you can see there are two places in the visible Window where the word divx can be seen.