About AVI files

A short article with an explanation what AVI files are, what types exist and what you can do if you have trouble to open them.

1. What is an AVI file?

First of all, to avoid possible misunderstanding, AVI is not a video and audio compression standard like for example MPEG. AVI, or Audio Video Interleave, is a file type developed by Microsoft to store video and audio stream for playback on PC's. It does not define the actual contents of he video and audio but just the 'container' in which these streams are stored. Inside the AVI container you can store video and audio using any available type of compression. The AVI file also contains a header in which information is stored describing the compression method used for the video in the file. This piece of information is a Four-Character Code (FOURCC); each codec (compressor/ decompressor) on the market has a special assigned FOURCC.

If you ever experienced the problem that you could not open an AVI file, most likely cause is that you did not have the proper codec, which was used to compress the stream, installed on your system. You can check which codec's are installed on your system Configuration Panel->System->Hardware tab->Sound video and game controllers->Video codecs A nice freeware program to check which compression is used inside an AVI file is the freeware program G-spot.

There are two types of AVI's:

• In Type-1 AVI files, video and audio are combined in one stream, also called ivas, for interleaved video/audio stream.
• Alternatively, in Type-2 AVI files, video and audio are stored in separate streams.

2. Capturing and accessing AVI files

In order to capture and access AVI file, Microsoft developed a system, consisting of a set of routines and drivers, called Video for Windows (VFW). However, this system does not have the ability to read files in which video and audio are placed in one stream (Type-1). Later on, Microsoft replaced VFW by another system called DirectShow. DirectShow can read Type-1 AVI files and is also backwards compatible with VFW, so also Type-2 files can be read.

Video Editing programs written for Microsoft WIndows platforms make use of these systems to capture and access video files. Most current programs use the DirectShow interface and can therefore access both type of AVI files. Some older programs however, such as the freeware program VirtualDUB, make use of VFW and can therefore not open Type-1 files.

In case you have trouble opening Type-2 avi files, there are several freeware programs to convert from Type-2 to Type-1. Some examples are the Canopus DV File converter, or the DV Type-1 to Type-2 Converter form Ulead (both free).