[Ardour-Dev] Ardour Timecode
reuben.m at gmail.com
Tue Oct 30 22:04:06 PDT 2012
No that's correct. Remember that NTSC is an analog format. Back when they
changed over from black and white (plain 30 fps) to color, they wanted to
maintain compatibility with old devices, so the color signal has the time base
slightly shifted to 1 + 1/1000 second. Just enough so that both signal types
could by broadcast over top of each other without interfearing.
Dropframe is just a convention for the timecode, not for the actual frame
rate. The time code drifts from the actual frame count and then drops a
timecode frame every 10 min to re-sync.
How the best way should be to represent that in the digital domain has been
the source of many arguments. :-D Most editors lock to the DF timing. But then
after the content is rendered and played out for broadcast, it's played out at
the 30/1.001 rate. But that leaves audio (especially digital audio) up in the
air because that tiny difference is not so insignificant for audio that is
sampled at 48k+. You can fudge the video timestamps at 30fps without really
causing problems for video, but doing that with the audio has potentual to
cause cracking sounds.
When analog and digital worlds collide, it's a PITA.
Perhaps some of that was addressed in the new ATSC spec...
On Tuesday, October 30, 2012 05:39:29 PM John Emmas wrote:
> On 30 Oct 2012, at 16:56, Robin Gareus wrote:
> > Are you saying that 29.97 NDF == 30000/1001 but 29.97 DF == 29.97 ?
> No, I'm saying that the intention of dropframe timecode is to drop
> (precisely) 108 frames per hour from a source that hitherto ran at 30fps.
> 30 x 1000 / 1001 is not the right formula for achieving 108 dropped frames
> per hour.
> 30 x 999 / 1000 is the correct formula.
> I don't use 29.97 NDF but as far as I can tell, 30 x 999 x 1000 would still
> be the correct pulldown formula.
> I've never encountered 30000/1001 in my own timecode workings but as I say,
> I'm not in a region that uses dropframe or 29.97.
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