Re: "Extract LTC from audio and align video" menu option missing from "Transcode/Import Video file" popup

On Mon, April 9, 2018 6:05 pm, Chris Caudle wrote:
> 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.

Actually, I think I was mis-remembering an earlier set of files.  With the
most recent files (the ones which start at timecode 06:05:29), Ardour
actually imports the audio file at 06:05:29 if the session frame rate is
set to 29.97 drop.

I'm looking more closely at the time code in the audio and video files,
and something seems wrong in the video timecode.
The BWF header in the file from the Zoom says origination time is 6:05:29.
The origination time is 0x03ebc8b00, or decimal 1,052,543,744 (sample
count since midnight for beginning sample).  That comes out to 6.0911
hours, 6:05:28. Maybe I missed a rounding, but pretty close  to the
6:05:29 in the origination time field.
Anyway, before I lose the point, I can set the secondary clock display in
Ardour to sample count, and when I zoom way in the audio file is imported
exactly at sample 1,052,543,744.
That is at 6:05:06:02 on the frame count when the session rate is 29.97,
or 6:05:28 when set to 29.97 drop, so I think the audio file really is
using drop frame.

I double checked the audio from the video, using ltcdump -f 30/1.001df,
and I see the skipped timecode values at the top of the minute:
00000000   06:05:59.29 |  1211682  1213283
00000000   06:06:00.02 |  1213284  1214884

Note frame numbers 00 and 01 are skipped, as expected for drop frame time
code.

I think you said you set the RTC on the Zoom by hand, right?  So that
might explain the slight offset of the timecode starting at 6:05:34 and
your phone displaying 6:05:37, it probably takes about 3 seconds to type
in the time and hit the button to start the clock running again, so I'm
not putting too much into that.

The only message I see in the Ardour log is that the video is being
aligned to 1,055,787,966 samples, nothing additional in the terminal
window where I started ardour to indicate how that was determined.
At this point I'm stumped at how to debug any more externally, I may have
to bug Robin about where the LTC decoding happens in Ardour and see if I
can figure out how to put a breakpoint in the debugger and step through
the calculations that happen to determine where the video file should be
imported.

I need to do some research on ffprobe and ffmpeg to see how the frame
rates get reported, whether that is stored in metadata for the file or
whether it is calculated somehow.

Just a thought, does your phone have any video quality settings you can
tweak?  I'm thinking specifically about something like variable bit rate. 
I would not think that would leak through into the frame rate
calculations, but I'm grasping at straws at this point.
I don't have any other good examples of video files with time code
embedded, I may have to try to generate some artificially by using ltcgen
to create a time code audio track that I can mux with an existing video
file.

-- 
Chris Caudle


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