[Ardour-Dev] Ardour Timecode

[Robin Gareus]

On 10/30/2012 03:11 PM, John Emmas wrote:
> On 29 Oct 2012, at 21:34, Reuben Martin wrote:
> 
>> Anything from Apple (using quicktime at least) usually screws up
>> the framerate for NTSC compatible content. Most of the time you
>> will see stuff like 2997/1000 which is wrong. NTSC should always
>> use 1001 for the fame-base. (Those tiny little factional
>> differences add up pretty quickly)
>> 
> 
> Are we talking about dropframe timecode here?  If so, 30000/1001 is
> the wrong fraction surely?  The correct fraction is 30 x 999 /1000
> (or 29.97 precisely). 

[citation needed] :)

Are you saying that 29.97 NDF == 30000/1001 but 29.97 DF == 29.97 ?

> If OMF and AAF are anything to go by, Avid
> represents this internally as 2997/100.

interesting.

> I don't live in a part of the world that uses dropframe timecode but
> that representation looks right to me.

Luckily, I'm on the integer side of the world, too. but 999/1000 is most
certainly is wrong.  29.97000DF = 30.0000DF - 0.1% pull-down.
There are uses for that, too. But..

NTSC is precisely 30000/1001 fps[1,2]. 29.97 is only an approximation
and if you use 29.97 for drop-frame timecode you need to "jam-sync" -
adjust the "clock about 75 milliseconds per day or approximately 1⁄2
second per week" [3].

Yet [4] hints that 29.97DF timecode is indeed counted at 29.97. What a mess.

Does anyone here have access to the SMPTE 12M-* specs?  Looks like it's
gonna cost $175 to find out for sure:
http://store.smpte.org/category-s/1.htm

2c,
robin


[1] http://www.lurkertech.com/lg/timecode/
[2] https://en.wikipedia.org/wiki/SMPTE_timecode
[3] http://www.evertz.com/resources/The-Right-Time.pdf
[4] http://www.poynton.com/notes/video/Timecode/