[FFmpeg-user] Problem extracting subclips of Avid DNxHD 36 to-the-frame

[Bouke / Videotoolshed]

> On 13 Apr 2023, at 00:49, Media Mouth <communque at gmail.com> wrote:
> 
> I've got a 90 minute source DNxHD36-encoded source clip (Exported from Avid)
> Frame rate is 23.976
> 
> Intended subclip would be from 0 to 778.041666 seconds.
> Testing the following
> ffmpeg -i {Source} -ss 0 -t '778.041666' -c copy {Dest}
> ffmpeg -i {Source} -ss 0 -to '778.041666' -c copy {Dest}
> 
> The resulting clips, when re-imported to Avid come up 18 frames short.
> Should be TRT 12:58:01 but imported clip's TRT is 12:57:07

TRT as in Timecode, or as in TRT? I suspect you’re looking at TC and you did not clock it with a stopwatch.

> I'm thinking it has to do with the frame rate?

Yes, your 12:57:07 is probably timecode. 00:12:57:07 converted to frames is 18655
A frame in 23.976 = 1000 / 23.976 = 0.0417083 seconds
That times 18655 makes ‘about’ the intended length.

Keep in mind that 23,976 timecode is always Non drop, and since it runs slightly slower than the time on the wall, your outcome is as expected.
The total running time will be correct. 

You need to read into the 1000 / 1001 math for 23.976 and 29.97.

Hth,
Bouke