Sustainability of Digital Formats: Planning for Library of Congress Collections

IMF, the file extension and acronym for the Interoperable Master Format, is defined by the ST 2067 family of SMPTE (Society of Motion Picture and Television Engineers) standards in order to streamline the management and processing of multiple versions of primary or "master" audiovisual content, such as feature films, short-form content, television or episodic content, advertisements, and more to be "efficiently represented, managed, played back, processed and transformed on file-based systems." IMF is designed to facilitate the internal or business-to-business (B2B) distribution chain and is not intended to function as the mechanism for final delivery to the end consumer. Note: See General for details about the use of the term "master" and alternative options including "primary" in SMPTE publications and in this document as well as the rationale for "IMF_Package" as the short name instead of "IMF".

IMF draws its inspiration from the Digital Cinema Initiative Distribution Package (DCP) which is a set of encrypted files representing digital moving image content packaged for shipment to theaters. The two formats are not directly related from a standards perspective but the concepts of a packaged set of all the track files, a separate XML packing list and other housekeeping files for B2B distribution is the same.

Conceptually, an IMF package (known as an IMP) is not a single file although it may be delivered as one ZIP archive file. An IMP is a bundle of components for a piece of media content including all the audio and video essence along with full metadata including, if applicable, subtitles and captions. Each component is treated as a separate and individual asset. The flexibility of an IMP is that it allows for these assets to be combined in various ways to create different versions of the content in a Composition, such as the theatrical cut for a specific country or an airline edit.

IMP/IMF packages can vary but typically comprises:

MXF OP1a file containing

• image track(s) which current implementations include options for lossless or lossy JPEG2000 or ProRes
• audio track(s) which can be Broadcast Wave or Multichannel (Dolby Atmos or 5.1 Surround). Each Audio Track File is intended to carry a single soundfield group. If the audio that is to be carried consists of more than one soundfield group, then each is carried in its own Audio Track File(s).
• Timed Text (TTML) track files for captions and subtitles (optional)
• Ancillary data track files for VBI (vertical blanking interval) and/or ANC (ancillary) data contained in source material (optional)
• Caption Description Packet (CDP) Track File for CEA-708 captions and CEA-608 data (optional)

Additional IMF components are:

• 1 Asset Map XML file which provides a mapping between UUIDs (universally unique identifiers) and actual filenames
• 1 Packing List (PKL) XML file which provides general information including file sizes and hash or fixity values for all other files contained in the IMP
• 1 or more Composition Playlist (CPL) XML file(s) which specify how media components (audio, video, timed text, etc) are assembled on a timeline to compile specific content representations. The CPL also lists technical metadata for the EssenceDescriptors that are present within the SegmentList (Playlist).- see below for more details
• Output Profile List (OPL) which may include optional Volume Index and Output Profile List XML files.

For an example IMF file using the RDD 59-1:2022 - SMPTE Registered Disclosure Doc - Interoperable Master Format — Application DPP (ProRes), see the BBC example via Dropbox (and linked from within RDD 59-1).

The Library of Congress Recommended Formats Statement (RFS) lists IMF as a preferred format for file-based video.

IMF is highly adopted as a packaging format in broadcast environments. It is required as a delivery specification for Netflix, Warner Brothers (both with JPEG2000 lossy) as well as for UHD programs in the BBC iPlayer (with ProRes 422 HQ), for Disney Master Packages and Distribution Packages.

The Packing List identifies each file, includes file-integrity checks ("hash") for each file, as well as a public key and digital signature that is part of the overall security system. Optional "MIC" (media content integrity) hash values can be created at the KLV triplet level using a structure which parallels that for encryption defined in SMPTE 429-6:2006. The MIC value within the KLV triplet is calculated only on the V portion of triplet - not the K and L.  The hash algorithm type is defined in the "MIC Algorithm" metadata item within the Cryptographic Context Set. The default algorithm is SHA-1. The actual hash value is stored on the "MIC" metadata item within the Encrypted Triplet Variable Length Pack.

Accessibility Features

The IMF/IMP (Interoperable Master Format / Package) offers good accessibility support, enabling alternate audiovisual description across a number of track options specifically for use by assistive technology, including: Timed text track file (SMPTE ST 2067-2:2020, 5.4), Caption Description Packet (CDP) Track File (SMPTE ST 2067-2:2020, 5.6), Timed Text Virtual Tracks (Caption Description Packet (CDP) Track File, 6.3). See the specification for more technical information related to these features.

See W3C's Making Audio and Video Media Accessible for more general information about accessible sound and moving image media.

SMPTE 2067-2 states that assets can be encrypted with or without authentication required and that "such mechanisms are outside the scope of this specification."

In addition, "when the Signature element of the IMF Packing List is present, digital certificates in the signer’s certificate chain shall conform to the provisions of SMPTE ST 430-2."

A note about the term "master". In 2020, the SMPTE Committee on Diversity, Equity, and Inclusion (DEI) published a statement (Engineering Documents from SMPTE Will Replace Offensive Terminology with Alternative Language) that the "SMPTE Standards Community has recently updated one of its Administrative Guidelines (AG 16) which governs the style of standards and engineering documents. The guidelines identify four offensive terms and provides a multitude of substitutes to use in place of each." Master is included in this list of offensive terms with alternative terms offered as substitute including "primary". This document will use the term "master" when referring to the current published title of the SMPTE standard and in direct quotations. Otherwise, the term "primary" will be used.

A note about naming entries in this resource. Typically, entries are named after format extensions which in this case would be "IMF". However, there is a preexisting entry for the Internet Message Format (IMF) so the short name for Interoperable Master Format is "IMF_Package".