Sustainability of Digital Formats: Planning for Library of Congress Collections

Extended version of the AVI file format for moving image content that wraps a video bitstream with other data chunks. The most important impact of the extended version is increased file size, and most modern AVI files employ the DML extensions. The original 1992 Microsoft-IBM-defined AVI file consists of one RIFF "chunk" tagged as AVI, with an effective limit in a FAT16 filesystem of 2 GB, small for a high resolution video file. The 1995 OpenDML extensions, developed by an OpenDML group associated with Matrox Electronic Systems Ltd., specify multiple RIFF chunks that bring the capacity to whatever level the filesystem supports; one specialist reports (link available through Internet Archive) that NTFS can carry AVI files that reach 32 terabytes. The added RIFF chunks are tagged AVIX. Informally, the AVI OpenDML files are sometimes referred to as AVI 2.0, and the original format as AVI 1.0.

The tagging of the file's chunks and subchunks employs Microsoft's FOURCC four character code. Both the AVI and AVIX chunks contain a movi subchunk that carries the actual picture and sound data that may employ a wide range of codecs, including those listed under Relationships below, also identified by FOURCC tags. Like the original AVI file, the AVI chunk in an OpenDML file contains a hdrl or header subchunk that provides metadata about the video. Both the original AVI and OpenDML AVI files contain an index to the offsets of the data chunks within the file. In the original AVI file, the index is tagged idx1 and identifies the location of video frames; in the OpenDML version, it is tagged indx and, among other changes, identifies the locations of video fields as well as frames. The field-level identifications are required to support playback of interlaced picture in professional video systems. One commentator (link available through Internet Archive) notes that the "meta index file structure [is] at the beginning of the [OpenDML] file as opposed to AVI 1.0, which uses an index at the end of a file, therefore AVI 2.0 has significant "Seeking" advantages- a must for large AVI files."

OpenDML was initially developed to support the carriage of a special form of MJPEG (Motion JPEG), whose frames are encoded as JPEG-DIB (JPEG with Device-Independent Bit compression; see OpenDML specification page 36), a constrained version of JPEG specified by Microsoft, with moving image usage detailed in the OpenDML specification. See also comments in the Notes below.

Developed as an open specification by a committee working in association with Matrox Electronic Systems Ltd. of Canada.

Documentation of the OpenDML extension version 1.02 available from Morgan Multimedia (available through Internet Archive) and other sites.

Widely adopted for video production and filemaking. Adoption may not extend to all permitted codecs and other features. The Wikipedia AVI article describes the OpenDML extensions as widely adopted; however, in 2000, one commentator wrote that the introduction of Microsoft's Advanced System Format or ASF had "pushed to one side" the OpenDML effort. The National Archives and Records Administration (NARA) uses AVI_OpenDML_1_02 as the preservation master format for their reformatted video materials. For more information see NARA's Digitization Products and Services.

The AVI_OpenDML_1_02 file wrapper is transparent; the encoded video bitstreams carried in the wrapper require tools and documentation to decode.

Technical information is carried in header chunks (e.g., hdrl, movi, and others); descriptive metadata may appear in a RIFF INFO chunk. The OpenDML extensions add a Video Properties Header (vprp) that provides metadata beyond that in the original AVI format, e.g., concerning aspect ratio; OpenDML also offer table structures for improved description of timecode, including discontinuous instances.

Depends upon the codec selected, with uncompressed or lossless codecs providing excellent clarity.

The OpenDML extensions were intended to increase quality in professional applications by supporting the representation of interlaced video (60 fields per second), 24 fps for content from motion picture film, and other features.

Version 1.02 of the OpenDML specification is in general circulation, with "last revision" given as 1996 and "reformatting" as 1997. The cover identifies the author as "OpenDML AVI M-JPEG File Format Committee." On the first inside page, a note provides the name Matrox Electronic Systems Ltd., and disclaims any patent infringements as well as stating that no license to Matrox patents is granted by the publication.

Section 7 of the specification is titled MJPG Motion JPEG DIB Extensions, and it provides detail about the implementation of this encoding in the file. The compiler of this format description did not carry out exhaustive research but the evidence in the specification, together with other scraps of information, leads to the following speculative conclusion: in 1996, some interested parties sought to take advantage of the still image JPEG compression algorithm for moving images. (The JPEG standard for still images, ISO/IEC 10918-1, was first published in 1994.) These parties concocted a method to bring together JPEG and AVI, called the OpenDML extensions. Although Motion JPEG was the focus at the time the specification was drafted, in the years that followed the set of extensions have been applied as much or more to other codecs. Comments welcome. See also the history note in AVI.

A video specialist at the U.S. National Archives and Records Administration reports that her commercial-market production system "decides" when to make AVI with or without the OpenDML extension: "When we make a small file (less than 2GB), it's a 'version 1' avi. When we make anything over 2GB, it's OpenDML ('version 2'). We only make the small files when capturing our color bar test pattern so no actual records are stored in the 'version 1' avi format. I believe that other commercial systems also make this type of decision. Meanwhile, I have heard that there are some older tools that only work with 'version 1' avi, but they're no longer being developed." (Personal communication to compiler, March 28, 2016.)