Latest post Fri, May 10 2019 10:46 PM by Rich Laws. 1 replies.
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  • Thu, May 9 2019 12:58 PM

    Help with AMT2 - adding compressed audio to audio only clips


    I am attempting to use AMT2 to add proxy audio tracks to existing clips with PCM (24 bit WAB) MXF files.

    The problem that I am encountering is that in the created proxy audio files - the start offset for the source mob is incorrect for all rates that are NOT 29.97 or 59.94 drop frame.   These two rates work correctly.  If I attempt NDF (with properties flag for DF set to FALSE and timecode string in NDF format) - the created audio MXF file is STILL marked as drop frame and the start tc is offset incorrectly.   Same for 23.976, 25, and 50 rates.  There is a bias that grows with the timecode relative to 30 fps but DF is false for non-30/60 fps.  It's almost as if audio only proxy created is hard coded to be DF and 30 fps.  This works correctly if there is a video track in ALL cases.

    Has ANYONE used AMT2 to add a proxy audio to audio only clips?  Would you mind providing the uncompressed and compressed properties XMLs that you are using?  Maybe I am missing something?  Are there known issues with this?

    Resulting media doesn't play at the correct offset in the timeline.  Again - works for 30fps DF - no other rate or NDF.

    Thanks in advance!


  • Fri, May 10 2019 10:46 PM In reply to

    Re: Help with AMT2 - adding compressed audio to audio only clips

    So... I was able to reverse engineer the issue with the AMT2 making proxy (mpeg1/layer 2) audio for audio only clips.

    As a recap - I was seeing the file start offset in the AMT2 generated file (file mob) being offset from the actual clip master mob start offset (values in audio samples).  All rates were not correct EXCEPT for 29.97 DF and 59.94 DF.  I was able to determine that AMT2 seems to be "stuck in DF mode" for ALL rates, including those that don't support DF.

    Using the compressed properties XML passed for these clips - I used  the actual timecode and did "reverse drop frame" math to create a timecode for the string in the XML that represented what the timecode would have been if the rate was a DF rate.   (Even for 23.976 and 25 fps - I applied DF math - the 2 frame offset every 10 minutes).   

    Passing this recalculated timecode to AMT2 in the compressed properties to add a proxy format to an existing clip, I am now able to create a proxy MXF file that is fully online in the Avid editor.  Checking the metadata - I see that the start offset for  the new proxy audio MXF matches the source PCM start offset.

    I am hoping that an AMT2 developer will take  a challenge to "look at the code" to confirm that audio only clips having a proxy added indeed has an issue as described. My empirical research seems to indicate this.  Again - NOT LOOKING for new AMT2 builds - just confirmation that this is the actual issue.     We have added proxy internally now to over 500 audio only clips (23.976, 25, 29.97 DF/NDF, 50 and 59.94 DF/NDF).  This timecode correction seems to fix the offset and media offline issue.

    Please let me know if this can help any other AMT2 users and I'll explain whatI did.

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