Re: MTC and Time Code Types
Re: MTC and Time Code Types
- Subject: Re: MTC and Time Code Types
- From: Doug Wyatt <email@hidden>
- Date: Sun, 6 Feb 2005 03:43:52 -0800
From:
http://www.phatnav.com/wiki/wiki.phtml?title=SMPTE_time_code
<< To correct this, drop frame SMPTE timecode drops frame numbers 0 and
1 of the first second of every minute, and includes them when the
number of minutes is divisible by ten. This almost perfectly
compensates for the difference in rate, leaving a residual timing error
of roughly 86.4 microseconds per day, an error of only 0.001 ppm. Note:
only timecode frame numbers are dropped. Video frames continue in
sequence. >>
On Feb 5, 2005, at 22:07, Cianflone, Chris wrote:
That's what I was hoping and that is a nice explanation. I think I
am getting more tripped up on what various sources are calling the
various "frame rates" and "counting methods". For example, in my
original e-mail I mentioned the MIDI Spec referring to the following:
> 0 = 24 fps
> 1 = 25 fps
> 2 = 30 fps (Drop Frame)
> 3 = 30 fps (Non-Drop Frame)
So if I am understanding this all correctly, shouldn't this simply be
listed as
0 = 24 fps
1 = 25 fps
2 = 29.97 fps
3 = 30 fps
without any regard to drop/non-drop?
No, 2 is drop frame as described above.
And then I tried a quick experiment in Digital Performer which lists
the following SMPTE formats:
24 fps
25 fps
29.97 fps Drop
29.97 fps Non-Drop
30 fps
All they've added here is 29.97 non-drop which has the exact same
sequence of frame numbers as 30 fps, it's just running at a slightly
different rate -- in the MIDI messages below, notice that there's no
difference between the two (except that presumably the quarter frame
messages are sent at different rates).
However, if I snoop on what MIDI data is getting sent in the above
cases, it looks like this when shuttling to time 0:
24 fps : F0 7F 7F 01 01 00 00 00 00 F7, so
00 = "0 = 24 fps"
25 fps : F0 7F 7F 01 01 20 00 00 00 F7, so
20 = "1 = 25 fps"
29.97 fps Drop : F0 7F 7F 01 01 40 00 00 00 F7, so 40 = "2
= 30 fps (Drop Frame)"
29.97 fps Non-Drop : F0 7F 7F 01 01 60 00 00 00 F7, so 60 = "3 = 30
fps (Non-Drop Frame)"
30 fps : F0 7F 7F 01 01 60 00 00 00 F7, so
60 = "3 = 30 fps (Non-Drop Frame)"
Doug
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