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

robertlazarski robertlazarski at gmail.com
Mon Apr 9 20:18:36 PDT 2018

Thanks again Chris!

On Mon, Apr 9, 2018 at 8:21 PM, Chris Caudle <chris at chriscaudle.org> wrote:

> 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 confirmed that too, in the process I learned what "drop frames" are and
what they do.

I decided to try some other fps settings, 29.97D is the default so I tried
29.97ND too. And I tried 25 fps on the F8 and also on the phone camera. I
tried importing all that into Ardour and I am still seeing the late audio
sync problems and no video when importing using the file timestamp.

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.
Yes exactly, I eye balled the time the best I could. I can't figure out an
NTP or GPS time sync option on the F8. I noticed there is an F8 firmware
upgrade available and I'll try that tomorrow to see if it helps at all.

> 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.

I haven't looked at the source yet but that looks like the next step. Any
help here would be much appreciated.

> 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.

Yes, I am using the OpenCamera app and it allows a wide range of variable
bit rates and fps options. I could try any combo, just let me know.

> --
> Chris Caudle
Kind regards,
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