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.
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
MPEG-4 Version 1
MPEG-4 Version 2
DV (Type 1 format)
- DV (Type 2 format)
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).