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We've run into an issue where Telestream Flip (or Flip64 or IPTV Flip) unexpectedly changes the timebase of the video from 23.98p NDF to 23.98p DF during transcoding - which effectively changes its duration by a small amount. The transcoding is a mere codec or compression change (e.g. from MPEG to QT or from ProRes HQ to standard ProRes) with no frame rate or other changes. In other words we expect the video duration not to change after transcoding.

The duration however does change - increases by about 4 seconds for every hour of the video - and this in turn results in a captions drift when the captions are supplied as sidecar files as opposed to embedded ones. The captions are in sync with the source file - but slowly drift out of sync with the video transcoded by Telestream Flip - because the duration is different.

Has anyone run into this and if so, what was the workaround?

Thanks!

P.S. More details:

Premiere Pro (2020 14.9.0. Build 52) export does not have the issue. Have not yet tried other encoders or transcodes (e.g. FFMPEG).

We've escalated the issue to Telestream (case 00824478) and their response was:

"the transcode is functioning properly by design" and "You will see these artifacts, or minor differences in duration, when going from an uncompressed source file to a compressed format."

(The funny part is that the issue has nothing to do with compressed or uncompressed - the unexpected duration change occurs on any 23.98p NDF sources regardless of codecs or compression.)

kindzmarauli
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1 Answers1

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Reproducing the issue

We've confirmed the issue in our testing as follows:

  1. We used a 6-second 144-frame 23.98p NDF (non-drop-frame) ProRes HQ video consisting of black frames. mediainfo shows its duration as 6s 6ms (00:00:06:00 NDF TC)

  2. We fed it to a basic Telestream Vantage Flip (v.8.0) transcode that does nothing except transcodes to the same output codec (ProRes HQ). It has no framerate or any other changes. The transcoded video's duration is now 6s 48ms (00:00:06;00 DF TC) per mediainfo.

  3. We then observed that:

    • the output is now DF (not NDF as the source), per mediainfo analysis
    • the runtime (playtime) is different (slightly longer, at a rate of about 4 seconds per hour of playtime) vs. the source, confirmed via mediainfo duration values, as well as in players such as QT Player.

Why this is an issue:

  • as stated in the question: the sidecar captions file that are in sync with the source, are out of sync with the output of Telestream Vantage transcoding - which is the result of an unexpected and undocumented behavior of Telestream Vantage
  • this in turn causes accessibility issues for the consumers of the video who are unable to use captions that drift out of sync by about 8 seconds towards the end of a 2-hour movie

Vendor's response:

Telestream's response was that "it works as designed", i.e. they do not acknowledge the issue and thus, see no need to fix it despite adverse effects on consumers that need correct captions.

Workaround:

The workaround is to re-encode sidecar captions to conform to the altered duration - which requires both a significant effort, and to re-process our library of content that exhibits the issue.

P.S. I am hopeful that this is useful to someone running into the same issue.

kindzmarauli
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