[Ardour-Users] "Extract LTC from audio and align video" menu option missing from "Transcode/Import Video file" popup

Chris Caudle chris at chriscaudle.org
Mon Apr 9 16:05:25 PDT 2018

> 1) I put a preamp (Sound Devices usbpre2) with nice meters in between the
> F8 and the Irig DUO interface that goes to the phone. I tweaked the levels
> until the LTC and scratch track peaked at -20DB on its outputs.
> Unfortunately I still see the DISCONTINUITY message from ltcdump. And it
> didn't help in Ardour.

I checked some of the different options in ltcdump, and it seems the
discontinuity messages are from ltcdump incorrectly estimating the frame
rate.  The default behavior is described as "autodetect framerate from
LTC" but there is an option to specify the frame rate (-f).  When I used
-f 30/1.001 then there was a single discontinuity message at the very
beginning of the dump, then no more for the remainder of the audio (i.e.
the audio which I had previously extracted from VID_20180405_060536.mp4).

The offset of the video file (around 1 min. if I remember correctly) seems
larger than can be explained by the difference between interpreting as
30fps vs. 29.97fps.  I think that would only account for about 21 seconds

I did notice however that the multi-channel wav file from the Zoom has in
the BWF header
Description      : SPEED=29.970D

Assuming that means 29.97 drop frame, it appears that the timecode
actually recorded with the video is non-drop frame.
I don't work with timecode on a daily basis, but according to the very
readable explanation found here:
the drop-frame timecode is distinguished by "every minute except each
tenth minute, two timecode numbers are dropped from the timecode count.
This drop frame mode of 30 fps timecode remains accurate compared to the
actual time passed, with a strange side effect that two numbers each
minute vanish from the count."

According to
it is (always? customarily?) frame numbers 0 and 1 which are skipped for
drop-frame time code.

I do not see any numbers dropped from the timecode decoded by ltcdump, so
if I understand the explanation from the Final Cut Pro manual correctly,
the timecode is actually non-drop frame, even though the header describes
it as drop-frame.  Again that should only account for 20-something
seconds, so I don't see how that explains the video being imported a
minute later on the timeline.

Is the timecode frame rate on the Zoom set to 29.97D or 29.97ND?

Just more clues so far, nothing that definitively fixes the problem of
offset video import. That discrepancy between the BWF header calling out
29.97D, but not actually having any dropped frame numbers seems wrong. 
Could be a bug in the Zoom firmware, or could just be my incorrect
understanding of how the timecode is decoded by ltcdump.  Bug in the Zoom
firmware feels more likely as my first guess.

Chris Caudle

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