When exporting audio as BWF and you want to ensure metadata integrity, you need to finalize the process with a BWF editor such as Sound Device’s free Wave Agent, available for both Windows and OS X and should be part of everyone’s toolset. You can find more info and download here.
The example shown is a quick multitrack sequence simulating a 5.1 sequence as mono tracks to be exported as a poly file for archive or other purposes. Substitute your own naming convention and timecode per your own needs. Note that is also applies to exporting BWF source clips directly from the bin as well as poly or mono.
As you can see from the above image, I have a basic 6 track audio in a 1080p/23.976 project/timeline starting at 01:00:00:00 and I have renamed my tracks. I export using direct out as seen in these settings:
Opening the exported file in Wave Agent shows the following (click to enlarge)
As indicated, the timecode now shows 01:00:03:18 which is not correct. The Project and and Track info are blank and this is long time request to have that metadata taken from the project and track names automatically.
In order to correct and embed the proper timecode metadata back into the file, uncheck “Preserve Start TC:
Then from the Frame Rate menu, select the frame rate that matches the project from which it was exported:
This will update the timecode by properly embedding the frame rate/sample from midnight into the file.
At this point, I also add the track info back onto the tracks and optionally add a project name that is most likely the same as the file name, but is now embedded in case that gets changed. Be sure to click the “Save” button before leaving the application.
Now this information is part of the BWF file to be re-purposed as needed - even when re-importing back into Media Composer. It seems that the frame rate value is not being defined in the bEXT chunk of the BWF when exported. This is from the Sound Devices webpage on timecode:
TC frame rate: This is the frames per second rate. It is also used to convert the HH:MM:SS:FF time code value to a ‘Samples Since Midnight’ value and visa versa. It is stored in the bEXT chunk as the ‘SPEED’ parameter and in iXML as the ‘TIMECODE_RATE’ parameter.
As seen here in this Sound Devices Tech Note.