MPEG-21 is a paradigm shift in multimedia standards.
Published by the Motion Picture Experts Group (MPEG) and intended as a framework to identify, prescribe, manage, and protect content, MPEG-21 reshapes the notion of audio and video streams by providing an open, multimedia framework oriented around users and objects. Instead of a traditional focus on content, MPEG-21 is geared more toward the relationship between users and content. The latter is treated as individual or sets of objects, and rules are applied to these objects as they pertain to users.
MPEG-21's primary objective is to define technology needed to support users in exchanging, accessing, consuming, trading or manipulating Digital Items. In this sense, a "user" is anyone who interacts with Digital Items inside the MPEG-21 Multimedia Framework.
The original MPEG-21 specification (ISO/IEC 21000:2003) contained 9 parts. The latest spec (ISO/IEC 21000:2019) contains 22 parts.
MPEG-21 refers to these multimedia objects as Digital Items and to users as simply, users. The primary objective of MPEG-21 is to support users in exchanging, accessing, consuming, trading, and manipulating Digital Items (audio/visual content, defined as objects). Being object-based, MPEG-21 does not differentiate its handling of audio, video, pictures, or other multimedia content, as was the case with prior standards.
How We Got Here from There
MPEG-21's history is a bit confusing at times. MPEG has historically been codec and format centric. MPEG-21 marks a departure from this philosophy.
Designed specifically to facilitate the servicing of different user preferences from a single content source, MPEG-21 does this by shifting the delivery system from a content provider focus (e.g. maintaining multiple slightly different iterations of the same content) to a content delivery focus. The MPEG-21 framework allows flexibility of the delivery mechanism, facilitating differentiated content delivery to the end user. Its primary purpose is to streamline and improve the process of content delivery, rather than acting as a static method of delivery and/or a multimedia storage medium (as are MPEG-4 and MPEG-2, respectively).
Beginning as a working standard embodied in ISO/IEC 18034, as it began to gain a foothold it was eventually reborn as ISO/IEC 21000. A departure from what one traditionally thinks of as an MPEG standard, MPEG-21 is both an Object-Oriented format and a Digital Rights Management (DRM) standard. Unlike MPEG-2, MPEG-4, and MPEG-D - functionally groups of multimedia codecs - MPEG-21 is a framework that describes multimedia content as objects, while the MPEG-2/4/D stanards are meant to guide the packaging and delivery of content. You may think of MPEG-21's relationship to MPEG-2 and MPEG-4, as Atmos and audio objects are to Dolby multi-channel surround sound audio formats such as TrueHD. Atmos enhances TrueHD, but it does not replace it. In fact, to the contrary, it relies upon it. Likewise, MPEG-21's framework cannot exist without leaning on pre-existing codecs such as MPEG-4. It doesn't focus on codecs, formatting, and players as MPEG has historically. Rather, MPEG-21 is designed to be agnostic toward the content, focusing instead on users and delivery mechanisms.
A Paradigm Shift in Content Management
MPEG-21 is targetted squarely at content distributors. In spite of its purported focus on user experience, the fact it is really about user content control, and placing that control in the hands of broadcasters. Introducing the concept of so-called Digital Items, MPEG-21 is designed as a framework where users interact with one another, and the object of their interaction is a so-called Digital Item. To facilitate this interaction, ISO 21000 defines a "Rights Expression Language." This standard is a set of rules that manage restrictions for digital content usage. This "language" is XML based.
MPEG-21 is designed to embrace a world where the consumption and distribution of multimedia content varies by user, form factor, and content provider. MPEG-2 was created specifically to accommodate DVD media. As that industry evolved into higher definition (e.g. Blu-ray) formats, MPEG-2 evolved with it. MPEG-4 addressed a shift in content consumption from physical formats (such as DVDs) to streaming, which addressed not only the evolution of the Internet's ability to transmit vast quantities of data quickly, but also a trend toward stand-alone multimedia devices capable of more visual and sonic details (e.g. "Smart" TVs). MPEG-21 has been designed to support the latest wave in media consumption trends, where not only are media presentation devices ubiquitous, but their capabilities vary considerably.
MPEG-21 allows a content publisher to create rules around who gets content delivered, how it is delivered, and what content is delivered. It's akin to providing pointers to your end users (such as a website URL), where the onus of determining what content the end user actually receives is shifted from the user's device to the distribution device. Think of how MPEG-2 or MPEG-4 movies work. If a standard definition DVD (MPEG-2) or Blu-ray DVD (MPEG-4) is presented as a file to the end user, the user's device must differentiate between the audio and video streams it wants to present to the user. The user's device receives ALL possible streams (including different languages). Their device bears the weight of differentiation and choice of the presentation layer to the user. While obviously shaped by their preferences, the user's experience is dependent upon the set of available content derived by their equipment. It is not simply a factor of what content exists, but also of the ability of the equipment to present such content in a form the user desires.
Imagine the content and device are capable of presenting multiple streams that result in slightly different presentations to the user (e.g. different languages), and multiple user preferences are in play for the same content delivery. Now, imgaine shifting the burden of correct content presentation away from the playback device, and shift it further up the distribution chain to (for example) the content broadcaster. This is how MPEG-21 operates. The result is each end user device receives only ONE stream, customized to the end user's desires. Whereas under the traditional pardaigm, the user's device actually receives multiple streams even though it only presents one. The burden is on the playback device. MPEG-21 has the effect of moving this point of delineation upstream in the content delivery process. The user no longer receives the entire gamut of possible content.
This process creates several noteworthy impacts to both the end user and the content distributor:
- Allows the distributor to have more control over the content distribution.
- Reduces bandwidth requirements between the distributor and end user.
- Less sophisticated devices may be utilized to receive the content, as now the end user's device is no longer responsible for processing the differentiation of content at the presentation layer.
The final result is greater efficiency and speed across the entire process. Only the data needed is sent. Extraneous information that would be ignored by the end user's device is never exchanged.
Digital Rights Management
I won't get into the weeds on this, but wanted to mention that through its architecture, MPEG-21 is also designed to implicitly act as a Digital Rights Management (DRM) tool. Obviously, if the broadcaster controls the content flow at a more granular level, this helps to protect the content from illicit copying, as no single user is ever receiving the entire spectrum of the work of art. But, there is another layer as well. The standard's XML-based "Rights Expression Language" (REL) is perfectly positioned to incorporate DRM functions as well, during the content delivery process to the end user's device.
MPEG-21 Multimedia Files (.m21)
Just like MPEG-2 and MPEG-4, the MPEG-21 standard defines a specific media type and file format ISO/IEC 21000-9:2008, based heavily on the MPEG-4 file type defined under ISO Base Media File Format ISO/IEC 14496-12.
The portion of the MPEG-21 standard we are concerned with here is Part 9: File Format in the MPEG-21 ISO standard (officially, ISO/IEC 21000-9:2005/Amd 1:2008 Information technology — Multimedia framework (MPEG-21) — Part 9: File Format AMENDMENT 1: MIME type registration). Ratified in 2008, its official file extensions are .m21 and .mp21.
While the original concept was announced publicly in 2003 and began to be adopted in 2008, the notion of MPEG-21 file types was not ratified until 2010. To this day, they remain rare to encounter in the wild. There is no native support for it in any of the common operating systems (iOS, Android, Linux, Windows, Mac). It is poorly supported from an end-user perspective, and generally offers little to nothing of particular interest to consumers.