[Ardour-Dev] Ardour Timecode

Robin Gareus robin at gareus.org
Thu Nov 1 13:59:01 PDT 2012

On 11/01/2012 08:17 PM, John Emmas wrote:
> 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)
>             or
> b)  108000 NTSC color frames == 108108 NTSC B&W frames (in terms of real time)

B. because 1 sec_NTSC = 1.001 sec_REAL

> 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. 

This part is explicit in the SMPTE-12M-1999 specs.

 But that brings up another interesting question....  Which of these is
> 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.

A. most likely.

AFAIK the 30fps DF is solely for MTC compatibility or similar legacy A/V
sync systems. Drop-frame timecode is only allowed at 29.97fps
(30/1.001); but I can't find a canonical reference for this statement.

> 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.!

the 3.6ms error / hour is for NTSC's 29.97fps (30/1.001) and not for 30fps.

> Or to put it another way....  dropframe timecode actually drops 108
> frames per 108108 frames - not 108 frames per 108000 frames, as
> everybody thinks.

According to spec, it drops 108 frames per hour _timecode_. ie omit
frame-numbers 00,01 every timecode minute except the minutes which are
integer multiples of 10 (incl. 0).

I can offer:
c) there are 17982 timecode-frames in 10 min drop-frame timecode.

> Of course, as you say, none of this helps in filling out your table.!

Does Avid even have a feature to dial a specific audio-sample number?
FCP does not seem to have that (and if it has, it is hidden deep inside
it somewhere). FCP navigation is by timecode only.


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