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The changing technology of television production
and digital services to viewers means the ways for
moving content - programme video and audio - in
studios is changing too. Not only is there far greater
use of computers and IT-related products, such as
servers, but also reliance on automation and the
re-use of material have expanded. Besides the need
to carry metadata, file transfers are needed to fit
with computer operations and streamed for real-time
operations.
The development of the Material Exchange Format
(MXF) is a remarkable achievement of collaboration
between manufacturers and between major
organisations such as Pro-MPEG, the EBU and the
AAF Association. It establishes interoperability
of content between various applications used in
the television production chain. This leads to
operational efficiency and creative freedom through
a unified networked environment.
MXF is a file transfer format, which is openly
available to all interested parties. It is not
compression-scheme specific and simplifies the
integration of systems using MPEG and DV as
well as future, as yet unspecified, compression
strategies. This means the transportation of these
different files will be independent of content,
not dictating the use of specific manufacturers' equipment. Any required processing can simply be
achieved by automatically invoking the appropriate
hardware or software codec. However, MXF is
designed for operational use and so all handling
processes are seamless to the user. It just works!
Besides offering better interoperability - working
with video and audio between different equipment
and different applications - the other major
contribution is the transport of metadata. By
developing MXF from the beginning as a new file
format, considerable thought has gone into the
implementation and use of metadata. Not only is
this important for the proper functioning of MXF
files, it will also enable powerful new tools for
media management.
MXF is a file format for the exchange of programme
material between servers, tape streamers and to
digital archives. Its contents may be a complete
programme as well as complete packages or
sequences. There are basic facilities available for cuts
between sequences and audio crossfades. This way
the sequences can be assembled into programmes.
MXF is self-contained, holding complete content
without need of external material.
MXF bundles together video, audio, and programme
data, such as text - together termed essence - along
with metadata and places them into a wrapper.
It's body is stream based and carries the essence
and some of the metadata. It holds a sequence of
video frames, each complete with associated audio,
and data essence, plus frame-based metadata. The
latter typically comprises timecode and file format
information for each of the video frames. This
arrangement is also known as an interleaved media
file.
The body can be based on several different types
of material (essence) including MPEG, DV and
uncompressed video and audio, it also uses the
SMPTE KLV data coding system, which has the
advantage of being a recognised standard.
In 1999, work started within Pro-MPEG Forum to
establish a universal file format for the interchange
of programme material between file servers and
workstations. Although networking and file transfers
are already commonplace in broadcasting they are
mostly based on proprietary formats, which may
have restricted functionality and are not fully
interoperable. Typically, although the video and
audio are preserved, metadata is for the most part
not carried, ignored or lost.
For users, the aims for the new format were that it
should be:
- Easy to understand and apply
- Open and, when ready, standardised
- Compression-independent
- Useable on major platform operating systems
and networks
MXF is targeted at professional video and broadcast
applications, which excludes consumer applications
at one end and complex editing and authoring
at the other. It is designed to carry continuous programme material and metadata (see "What is
MXF?"). Typical applications are carrying newly shot
material and finished programmes. This targeting
keeps MXF relatively simple and efficient as it
avoids the complexity and overheads needed for
editing and authoring. However, it is designed to
interoperate with those areas. So Pro-MPEG and the
AAF (Advanced Authoring Format) have and are
continuing to work together to ensure their formats
are compatible (see MXF and AAF).
Traditionally broadcast television has been built
around streaming video and audio. This is logical as
the original scene action and the viewer expectation
is of continuous real-time video and audio - supported by continuous streaming. PAL and NTSC
analogue composite video, and digital SDI (serial
digital interface) and SDTI (serial digital transport
(data) interface) all stream. But computer systems
exchange data by file transfers.
Streaming media:
- Is viewable during transfer - before all the
data is delivered
- Offers minimum delay for live action
- Is point-to-point with no bottlenecks: offers
reliable, continuous operation
Networked media:
- Uses low-priced, standard IT components
- May be stored on a wide variety of devices
including disks and tape
- Offers flexible data exchange, sharing and
distribution
Both real-time streaming and file transfer have their
advantages and both will continue in use. Therefore,
it is essential that both have some degree of
compatibility so they can co-exist and allow material
exchange between them. With this in mind, the
design of MXF makes it a file format that can
stream - creating a seamless bridge between the
two transfer types. Operationally, there is no effort
involved beyond requesting the transfer. So, for
example, advantage can be taken of the flexibility
of AAF in postproduction and, via 'invisible', simple
file conversion, MXF for the finished playout to
tape streamer or server storage. Note that the file
conversion is loss-less for video and audio, if the
compression scheme is not changed.
In a similar way, operational and creative staff want
to concentrate on their tasks and not bother with
compression issues. But is it also true that no single
compression format will suit all applications and
various schemes will continue to be used. Therefore
MXF is compression-independent, offering the same
service regardless of the compression in use. This
allows manufacturers to provide equipment with
multiple compression codecs, which could lead to
seamless working between, for example, MPEG and
DV-based systems.
MXF is an open solution and so has been submitted
to SMPTE for standardisation. Together, the Pro-
MPEG Forum and AAF Association have support from
a substantial cross-section of the industry. In addition,
close collaboration with user groups, such as the
EBU, ensure that users' needs are incorporated. At
the same time, many manufacturers and suppliers of
software and hardware are keen to implement MXF
as soon as possible.
As mentioned, in a move towards early
standardisation, MXF already adheres to SMPTE KLV
guidelines (Key, Length, Value - a method for
wrapping data for transport over networks).
Achieving interoperability is the prime objective of
Pro-MPEG and MXF. This has been implemented in
three areas.
Cross-platform. It will work across different network
protocols and across operating systems including
Windows, Mac, OS, Unix and Linux.
Compression independent. It does not convert
between compression formats; it does make
managing more than one in a single environment
easier. It can handle uncompressed video.
Streaming / transfer bridging. MXF interoperates
seamlessly with streaming media - especially SDTI
where fully transparent interchange is achieved. This
performance is bi-directional: achieved going from
MXF to streaming and vice versa., and means SDTI
fits easily into a file-based environment. This is true
convergence.
A major aim of MXF is the seamless passage of
programme content and its associated metadata.
Also referred to as 'data about data' it exists
in any system today. For example, timecode is a
form of metadata. The problem is that, due to
incompatibilities, this information is currently lost as
the content moves between systems. MXF-enabled
systems will communicate using metadata, video and
audio. MXF metadata may carry information about:
- The file structure
- The body contents - e.g. MPEG or DV, 525 or
625, etc.
- Key words or titles
- Subtitles
- Reference numbers
- Editing notes
- Location, time, date and version number
- .....
The list may be endless. In extreme cases files may
contain more metadata than video or audio content!
However, the efficient implementation of metadata
is seen as the key to material management. The
metadata may be filtered to hold only what is
relevant to the particular operational environment,
thereby cutting possible meta-mountains.
It has no need to carry the editing 'EDL' information.
If the content has come from an edit session, the
editing data will have been used to conform the
material and so has no further use here. Should full
edit archiving be required it should be performed as
a part of the editing operation, before outputting to
MXF. (See "What is MXF?")
The speed of progress of MXF is a tribute to the many
dedicated engineers across the industry working
together towards a common objective. Manufacturers
and users both recognise that there is a window
of opportunity to establish an industry file format
standard. The use of video will increase into an
expanding array of broadcast and allied applications,
as will the use of IT and servers. Any delay in
establishing standards will make the task more
difficult as proprietary solutions will spread into the
format vacuum.
Advanced Authoring Format (AAF) is an industrydriven
open standard for multimedia authoring
and postproduction. It enables content creators to
easily exchange digital media and metadata across
platforms and between applications. It simplifies
project management, saves time and preserves
valuable metadata that was often lost in the past
during media transfers.
MXF is derived from the AAF data model and is
a simple interchange format, primarily to facilitate
the transfer of finished content, whole programmes
or completed sections, between servers and to tape
streamers. MXF also helps with the migration of
playout operations and simpler production systems
into standard networked environments.
The two formats are especially complementary.
Whereas AAF integrates closely with, and
complements, existing media file formats, MXF offers
the same for existing stream formats as well as AAF
files. Both formats can stand on their own and each
has a functionality and design optimised for their
particular spheres of application. At the same time,
one does not depend on the other. For example, a
whole broadcast system may use only MXF and a
postproduction house, just AAF, but a broadcaster
with a post facility may well use both.
While the MXF and AAF are complementary, there
are many differences. One is that AAF may carry
references to outside material held in other places,
to be used in an edit whereas MXF is always
complete and self-contained: not requiring any access
to outside material. In addition AAF includes basic
video transition processing whereas MXF, carrying
completed programme material, has no need of that.
MXF is driven by user needs and so has a strong commercial base. Even normally competitive
manufacturers are working together at high speed to provide an open industry solution to file
interchange. Working together, targets have been met; MXF is now submitted to SMPTE and is well on
its way to helping media flow in the television/IT convergent world. End users will be able to better
manage their media and to concentrate on their prime tasks of productivity and creativity.
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