ARRI’s DNxHD Alexa update, Sorenson Squeeze Pro and OP this, OP that
Understanding the different DNxHD MXF formats and getting that into Media Composer
By Scott Simmons | March 15, 2012
ARRI has begun to ship its DNxHD update for their Alexa camera. This announcement that the Alexa would get DNxHD as an onboard recording option (besides ProRes) was welcome news to post-production people like myself who use Avid Media Composer. While ProRes is supported in Avid both by AMA linking and direct MXF creation (in the Media Composer version 6 on Mac that is) DNxHD is Avid's own in-house codec and is a perfect option for compressed hi-def media. But ARRI's onboard DNxHD recording uses an MXF format that's a bit different than Avid's. Read on for a discussion of why that is (and lots of OP).
When the final details came out about this multi-thousand dollar upgrade for the Alexa camera it left a number of editors scratching their heads. There's full DNxHD support (including the mention of DNxHD 444 to come online for free at a later date for those installing the DNxHD option) but these files aren't the exact type Avid Media Composer uses.
This explanation will now devolve into a bit of technical mumbo jumbo that it took me a while to wrap my head around. I prefer an "it just works" type of scenario but often in post-production we have to get much more technical than we would like to ensure that a particular workflow will â¦ work.
Op1a vs OP-Atom
Avid Media Composer uses .MXF files as its native HD file format. That means any media that IS NOT Avid Media Access linked media, as in media imported, loaded, transcoded, consolidated or rendered via any method that isn't an AMA link ends up in an Avid Media Files folder on an Avid media drive.
An Avid MediaFiles folder is found on every Avid media drive and is key to Avid's non-Avid Media Access media management.
That's why Avid media management is so good. The Avid is able to track each file with a unique identifier so it never goes offline as long as the drive is there. Media can be passed among workstations without ever needing a "reconnection dance." Normally these files would be created by Media Composer itself but with the Alexa DNxHD support it can create DNxHD MXF files as well ... only it doesn't create them in the exact format Avid uses.
The Alexa MXF files are what is known as Op1a. Avid MXF files are OP-Atom. As in OP stands for "Operational Pattern." If you want to get really technical then give a read to these Library of Congress Digital Preservation articles that have technical details on these MXF formats: OP1a and the one Media Composer uses, OP-Atom.
The biggest difference I see without getting too much of a headache (there may be more than just this) is that Op1a files include audio and video in the same file while OP-Atom splits them into separate files. So an OP-Atom video clip with stereo audio will be 3 files: 1 for video and 2 audio for each stereo channel.
The 3 files circled above make up one clip (video and stereo audio) in Media Composer.
Because Avid uses the OP-Atom file type, the MXF files that Alexa creates need to be first loaded via Avid Media Access with ARRI's AMA plugin. They can then be consolidated into Media Composer for "proper" Avid media management (FYI I haven't done this myself as of yet but from the research I conducted this looks to be how it's done). This is basically a file copy as Media Composer rewraps the Alexa Op1a MXF files into OP-Atom MXF files so no recompression occurs. You could easily work via AMA link with the Alexa Op1a media but it would be worth the extra consolidation step to get the files into Avid's proper media management. It is a small extra step but in ARRI's MXF DNxHD with Alexa white paper, which can be found at this link, they list the reason they went with OP1a files with a number of bullet points:
â¢ The OP1a MXF format packages picture, sound, and metadata in a single file. This is ideal for both camera acquisition and archiving since audio and video is always kept together and no data is lost if recording is interrupted for any reason.
â¢ OP1a is a streaming format, which means that even incomplete files can be played. This does not only ensure highest data safety but also allows playing clips that are still being transferred (eg. via FTP).
â¢ The OP-Atom MXF format places picture and sound into individual MXF files. Audio is written after video. Using this format for recording in a camera would cause entire takes to be corrupted if the recording is interrupted abnormally, e.g. due to a power loss.
â¢ When ALEXA clips are brought into Avid Media Composer, Newscutter, or Symphony, the OP1a file structure enables that camera metadata is automatically made available in the editor's bins.
â¢ Since ALEXA MXF files use Avid's native DNxHD codec, the OP1a files can be quickly repackaged to the OP-Atom format. This is essentially a copying process since no transcoding is involved. The original image quality is fully retained. Note that ALEXA uses high quality intraframe codecs, which result in bigger files than material encoded using a lower quality interframe long-GOP codec. Therefore, copying times will increase approximately by the ratio of the bit rates.
At first I was disappointed the Alexa was shooting different a type of MXF file but after reading the white paper it makes good sense. Since you'd most likely copy Alexa source media files to your highspeed media drive for editing anyway this different file type isn't too big of a burden.
There are applications that can make OP-Atom MXF files
To take this native, OP-Atom MXF idea one step further we can look to software applications that will transcode to DNxHD MXF files. A good example of this is RED's REDCINE-X Pro which has long been able to transcode to OP-ATOM MXF files. When you choose an Avid workflow you're presented with several options for transcode.
Redcine-X's OP-Atom DNxHD support means it's easy to create Avid DnxHD 36 files for offline editing and then conform a final online from the R3Ds in other online / color grading applications.
When transcoding is complete Redcine-X Pro supplies you with properly formatted OP-Atom files as well as AAF and ALE files for importing the media once it is located on your media drive and Avid is aware that it exists. More on that in a minute.
Sorenson Squeeze Pro and it's new Avid friendly option
Sorenson Media very recently released an update to Squeeze 8, a "pro" version called ... Squeeze 8 Pro. This is a big deal for Media Composer users as it allows for the transcoding of media (including things like Canon DSLR .mov files) into OP-Atom DNxHD MXF files without the need to tie up an actual install of Media Composer to do so. The only other encoding application that I know of that has been able to do that is Telestream Episode. Editors who use Media Composer on a regular basis have long looked for a good transcoding utility that can create proper Avid MXF files from DSLR cameras and since Squeeze has been a Media Composer companion for a long time it's a natural addition. I have always hoped something like Magic Bullet Grinder would add this capability as Squeeze 8 Pro is $1000 but it's still a nice addition to an encoding application I use all the time anyway. You can add DNxHD or ProRes encoding to Squeeze 8 for $99 each.
Page 2: How to use Sorenson Squeeze 8 Pro to transcode Avid OP-Atom MXF media.
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