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

Chris Caudle chris at chriscaudle.org
Wed Apr 11 06:21:48 PDT 2018

On Mon, April 9, 2018 10:18 pm, robertlazarski wrote:
> I confirmed that too, in the process I learned what "drop frames" are and
> what they do.

Yeah, I realized I only partially understood how drop frame timecode
works, so I had to spend some time going through the details.
The ATSC standard used for digital TV broadcast in the US and some Asian
countries calls out 30fps and 29.97fps, I was hoping that everyone would
just switch to straight 30fps and drop frame would go away, but I guess
during the transition period from analog to digital broadcasts they went
with 29.97002997002997... for compatibility, and then just never switched.
 We're stuck with it forever probably.

Anyway, I didn't get a chance to try stepping through the video import
last night, but I did do a sanity check on video file import, and using a
file I generated with ltcgen and ffmpeg I verified that I could import a
video file and it would line up on the timeline exactly where I expected.

I used a bitmap editor to generate a 720x480 bitmap, it was just a single
color background with "LTC Test" text added so that I had a starting frame
for the video.  Named test_image.bmp for reference below.

I created an audio file with timecode starting at 6:05:29 to match your
earlier file (length was picked as some semi-random value a little over 5 
min long):
ltcgen -f 30/1.001df -l 00:05:32:15 -t 06:05:29:14 time_code_test_060529.wav

Created video with command:
ffmpeg -loop 1 -i test_image.bmp -i time_code_test_060529.wav -c:v libx264
-tune stillimage -c:a aac -b:a 192k -pix_fmt yuv420p -r 30000/1001
-shortest timecode_test_video.mp4

When I opened timecode_test_video.mp4 in Ardour it lined up right at 6:05:29.
So that seems to point at something in your specific video file having
some feature that confuses the ardour import.
You should be able to duplicate using those command lines I pasted in
above, you just need to create a single bmp file in a graphics editor to
start with.

> 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.

And the app is set at 29.97fps? I'm still confused about why ffmpeg is
flagging the video file as 29.89fps. Maybe just a rounding error since
that original file is only 42 seconds long.  Have you tried with a file
from that phone which is a few minutes long?

Chris Caudle

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