Project Transfer – OMF: DIALOGUE EDITING

OMF file! – created by Avid in 1992 –, is, in my experience, quick and efficient (although I have never worked with an EDL, even knowing I should with dialogue editing…).  At school, I’ve done this many times between Digital Performer and Cubase and vice-versa, and always worked pretty well.

So, Open Media Framework Interchange. Great for transferring a variety of media (actually audio, video, graphics and still images) between heterogeneous platforms. However, most audio applications do not support video files storage [4]. It exports the media contained in the timeline, although it doesn’t include slates and outtakes, so if working with alternative takes they must be turned into normal audio tracks.

OMF Interchange is specifically designed to permit different applications to create, edit, enhance, modify, playback, and then transmit the same compositions and media in object form without loss of information. It uses a numeric representation that can easily be converted to a platform’s native format.

Let us be just a bit more technical:

This format includes rules for identifying the original sources of the digital media data.

(extremely simplified) STRUCTURE

Within OMF different kinds of media data structures are called “objects”. An object can be simple such as a Filler object that describes a blank segment of media, or an object that includes a complex structure of other objects (which is called “Mob”). OMF objects describe how the data is stored, which enables applications to convert data to their native form when necessary.

OMF Interchange provides structures for three distinct working-bases:

  • digital media data
  • media sources
  • compositions

Digital media data is what we play: audio, video, and graphics. An OMF Interchange file includes information about the format of the data (sample rate and compression technique) in addition to the data itself. OMF Interchange files allow applications to store the digital media data in separate files. When digital media data is in a separate file, the source information includes hints for locating the file that contains the data.

Sources describe the digital media data and the original, physical sources of the data.

composition is a description of all the information required to play a media presentation (dealing with various types of media from a variety of sources), comprising the logical organization of a time-based media presentation. Compositions manage time in a way that allows a nonlinear approach to editing, unlimited access to physical source material, and synchronized playback of physical sources that do not share the same native sample rate. It does not actually contain the digital media data such as the video, audio, graphics, or animation. Instead, it points to sections of source data. The used application is actually playing a composition, following its references to the digital data.

HOW DOES IT WORK

The audio tracks might contain volume and fader information, and the video track might be a combination of multiple video sources, including transition effects at the boundaries between sources (such as wipes, dissolves, or fades).

Sources and compositions work separately in order to reduce memory usage when, for example, more than one composition contains the same (or sections of the) source.

It identifies the data by storing the file source mob ID and describing the starting position and the length of the data. Each file has a file Header object that provides efficient access to top-level objects such as mobs and Media Data objects. It also contains properties that specify the OMF Interchange Specification version and when the file was last modified.

Types of media data Required formats
video, graphic, and still image RGBA Component Image (RGBA)Color Difference Component Image (CDCI)TIFF
Animation Stored as video
Audio Audio Interchange File Format (AIFC) – uncompressed~RIFF Waveform Audio File Format (WAVE)

Source Clips to Represent Media

Compositions use Source Clips to represent the media for a Segment. A Source Clip in a Composition Mob represents a section of digital media data in a file. Source Clips do not contain either the media data itself, they reference the media data.

Media Descriptors

A Source Mob includes a Media Descriptor that describes the kind of source and describes the format of the media data. For example, a Media Descriptor for a film source might contain the film aspect ratio.

An AIFC Audio Descriptor describes the characteristics of audio samples in the AIFC format. A WAVE Audio Descriptor describes the characteristics of audio data in the WAVE format.

The information in these types of media descriptors is necessary for interpreting the sample data for editing or playback.

Media descriptors for file Source Mobs can also include information about the location where the digital media data is stored.

Mob and Header Classes [1]

Composition Mob (CMOB)—describes the editing information and media that constitute a media production.

Master Mob (MMOB)—provides an indirect way for a Composition Mob to reference a Source Mob and provides a way to synchronize data in multiple Source Mobs.

Source Mob (SMOB)—describes media data; can be digital media data that is accessible to OMF (in which case it is a file Source Mob) or other media data stored in videotape, audio tape, film, or some other storage format.

Header (HEAD)—specifies OMF Interchange file wide information; there is exactly one Header object in each OMFI file.

OMF-1, OMF-2

OMF-1 and OMF-2 (released in 1996) are not compatible formats.

Usually, most DAW’s work with both, although one cannot convert an OMF-2 into an OMF-1.

So, it was easy as to  click “Import” the OMF and all the audio files appeared clear and identified at their right time location in the timeline. In most DAW’s, when importing an OMF there are some determinant options available:

  • Import all media files, which includes the media files not referenced by events.
  • Import volume automation and envelopes of the Volume Automation Track of each track.
  • Import at Timecode Position which will insert the media at their original timecode positions (as it was last saved in the OMF file). If, for example, the timecode position of a sound clip starts at 00:00:00:00 and meanwhile the project’s timecode offset is altered to 00:00:30:00, the media contained in this interval will run out of the timelime, so the media is attached to its original position.
  • Import at Absolute Time will insert the elements contained in the OMF file starting at the timecode position defined on the DAW’s project and keeping the relative distances between the elements.
GENERAL RECOMMENDATIONS

Stereo files  –  at the export, if the OMF contains stereo files, make sure that the targeted application for import is able to read OMF files with included stereo files.
Muted files – are not transfered with the OMF.
Media data – should be embedded, especially if you are switching the operating systems from Windows to MAC OS because file paths could be interpreted wrongly [4].
IMPORT / CUBASE
If importing a OMF file with a different timecode position then the project’s to which one is  importing, the project will conform the timecode positions to those of the OMF. For example, if the opened project starts at 00:04:00:00 and the OMF imported contains media starting at 00:00:00:00, the Cubase project will change its own timecode positions to start at 00:00:00:00.

Speaking from personal experience, I never had issues when importing OMF files to Cubase. However, when importing to ProTools from other DAW, everything fails. It is extremely frustrating and I don’t know where does the glitch come from. What happens is that ProTools, apparently picks a random track in the OMF, calls it “A1” and that’s the only thing I ended up seeing on the editor window. A couple of years at school, where we had Digital Performer, OMF between it and Cubase worked flawlessly… Just so you know and hopefully save you hours of work.


References:

[1] http://www.linuxmedialabs.com/Downloads/LSI/omfspec21.pdf, 04-11-2012

[2] Purcell, John (2007), Dialogue Editing For Motion Pictures, Focal Press

[3] Cubase Operation Manual

[4] https://www.steinberg.net/en/support/knowledgebase_new/show_details/kb_show/project-exchange-formats-supported-by-nuendo-and-cubase/kb_back/2020.html?tx_p77sbknowledgebase_pi1%5Bproduct%5D=23, 16-04-2012

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