[Ardour-Dev] Ardour Timecode
johne53 at tiscali.co.uk
Thu Nov 1 12:17:32 PDT 2012
On 1 Nov 2012, at 17:55, Robin Gareus wrote:
> mmh. but not for NTSC :(
> I managed to take a peek into the SMPTE 12M-1999 spec.
> long story short, on page two it defines:
> 1 sec_NTSC = 1.001 sec_REAL
> It furthermore states that the total error accumulated due to drop-frame
> timecode is -3.6ms per hour or about -86ms over a 24-hour period.
> The spec describes how to count in drop-frame timecode but does not
> explicitly mention at which rate it is counted.
Hi Robin. It's easy to see why people are confused about this. It raises a couple of very interesting questions. Firstly, which of these is true?
a) 107892 NTSC color frames == 108000 NTSC B&W frames (in terms of real time)
b) 108000 NTSC color frames == 108108 NTSC B&W frames (in terms of real time)
It seems that I (and Avid) have always assumed the former. But if a 3.6mS error is part of the spec, than it must be the latter. But that brings up another interesting question.... Which of these is true?
a) At 30fps the intention of dropframe timecode is to drop 108 frames per realtime hour.
b) At 30fps the intention of dropframe timecode is to drop 108 frames per 1.001 realtime hours.
I'd bet that the entire world believes a) is true. Whereas if a 3.6mS error is correct according to the spec, b) must actually be true.!
Or to put it another way.... dropframe timecode actually drops 108 frames per 108108 frames - not 108 frames per 108000 frames, as everybody thinks.
Of course, as you say, none of this helps in filling out your table.!
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