AVI files and associated compression algorithms (file extension .avi)

AVI stands for “Audio Video Interleave”.  (Note that interleaving is a general practice, not limited to AVI files.)  The AVI format was first used on PCs with a very simple compression algorithm – Microsoft Video 1 – which produces a low quality image with clearly visible “blocks” whenever there is any significant motion in the scene.  However, the AVI file format has been used more recently (the mid-1990s onwards) with a variety of different compression algorithms.  Video player software that plays AVI files (such as Windows Media Player) determines which codec to use to decompress the data by reference to codec information in the file header.

The AVI format may also be used to store uncompressed (or only slightly-compressed) video.  This arrangement is commonly used by video capture boxes and video capture cards.  The captured digital bit-stream is written directly into an AVI file without compression, so that the user can review and edit it in the highest possible quality format and then perform compression later during the “render” phase of production.  In such an uncompressed AVI file the codec definition field in the file header will point to a no-compression codec that is typically bundled with the software that comes with the capture device.  The more expensive video capture devices store the captured video as an AVI file using the DV codec (explained later), which involves a use of JPEG-like algorithm for intraframe image compression, but no interframe compression.  This format is also used by digital camcorders.  DV is becoming the generally accepted format for storing “raw” material in a high-quality form.

You can generally (although not always) view the codec definition in an AVI file using Windows Explorer: right-click on the filename, select Properties, then select the Summary tab and look at the Video Compression entry.  Note that a codec name may be shown under “Video Compression” even where the codec is a non-compression type of codec.  For example, Videowave video capture software inserts a Video Compression codec identifier of “NUVYUW” in the uncompressed AVI files that it creates.

In summary, a file with an extension of “.avi” may contain compressed or uncompressed video.  If it is compressed then any one of the several common compression algorithms, or vendor-specific variants of those algorithms, will have been used in preparing the file.  The most common compression formats used for AVI files include:

 -         Microsoft Video 1

-         Intel Indeo

-         Radius Cinepak

-         MPEG-4 Version 1

-         MPEG-4 Version 2

-         DV (Type 1 format)

-         DV (Type 2 format)


The size of an AVI file relative to the number of minutes of video material it contains may vary greatly, depending on (a) the compression algorithm used, and (b) the quality of the pre-compression image, in terms of pixels per frame and frame rate.  The earlier standards like MPEG-1 are generally used with a much lower quality image, so MPEG-1 files are smaller than MPEG-2 files, even though MPEG-2 is a more efficient compression algorithm than MPEG-1.  For example, one minute of video material will produce the following file sizes:

§         High-quality, uncompressed video: about 300 Mbytes

§         High-quality, DV (intraframe-compressed) video: about 250 Mbytes

§         Low-quality, Microsoft Video 1 compressed: about 9 Mbytes

§         VCD-quality, MPEG-1 compressed: about 9 Mbytes

§         DVD-quality, MPEG-1 compressed: would be about 70 Mbps (but there is no generally-available codec to do this)

§         DVD-quality, MPEG-2 compressed: about 46 Mbps

§         DVD-quality, MPEG-4 compressed: about 7 Mbytes.

Note that MPEG-1 compressed material can be stored as an AVI file, but it is more common to give it an extension of “.mpg” when stored on a PC.  MPEG-1 files always have an extension of “.dat” on a VCD disc (see next section).