[opendtv] Re: A bit OT: 24 Frames Drop Frame Timecode
- From: "Russ Wood" <russ.wood@xxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 3 Dec 2007 08:54:54 -0000
Thanks for all your answers so far.
It isn't me who wants 23.976 Drop, it is our customers who are demanding
it. In this instance our customers are the captioning houses (this is
for the Swift Captioning workstation), they claim their customers want
23.976 Drop.
Russ Wood
-----Original Message-----
From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx]
On Behalf Of John Willkie
Sent: 30 November 2007 22:51
To: opendtv@xxxxxxxxxxxxx
Subject: [opendtv] Re: A bit OT: 24 Frames Drop Frame Timecode
John & Russ;
I appreciate the feedback, and I'll leave the math to you. I have only
the merest experience in writing to time code.
I'd be interested in knowing why Russ feels he needs 23.976, since it
apparently isn't a valid frame rate for SMPTE time code.
However, here's my proposed solution. Use the scheme I proposed for
29.97 (taken from the MPEG-2 video spec), and, for every 5 frames at
29.97 fps, count 4 frames at 23.976.
80% of 30 is 24, so this is probably how all those devices that convert
time code do the conversion.
And, thankfully, nothing in this requires the use of fractions or
microsecond calculations.
Let me know if the math work out. I have no problem with fractions, but
I tend to only use integers in my coding.
I'll leave for others which is the easiest to implement in practice and
code.
John Willkie
www.EtherGuideSystems.com
-----Mensaje original-----
De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx]
En nombre de John Shutt Enviado el: Friday, November 30, 2007 1:07 PM
Para: opendtv@xxxxxxxxxxxxx
Asunto: [opendtv] Re: A bit OT: 24 Frames Drop Frame Timecode
John,
You described drop frame time code only for 29.97 fps.
With 29.97 drop frame time code you need to drop 18 frames every ten
minutes. So you drop two frames every minute except for every tenth
minute.
However, 29.97 is really 30/1.001, so you really needed to drop 17.982
frames every ten minutes, and in a 24 hour period you're still off of
real time by 2.59 frames. Acceptable for editing, and you jam sync once
per day if you have to maintain real time over several days.
With 24/1.001 fps, you have to drop 14.3856144 frames every ten minutes
in order to stay current with real time. That's a much harder thing to
do.
I came up with "Drop one frame every odd minute, drop two frames every
even minute, except drop only one frame every odd ten minutes, except
drop only one frame every odd hour."
That keeps you aligned with real time for the first 2 1/2 hours, but you
are
off by one frame at 3 hours 20 minutes. In a 24 hour period you are
away from real time by 4.472 frames.
Russ' solution is pretty much the same thing with the one frame every
odd minute and 2 frames every even minute, but he add in additional
times to make up for the gradual drift in "my" simplified solution.
I think that my simplified solution would be easier for a human to
remember and calculate for editing purposes, but Russ' solution stays
with real time much more closely.
Sounds like a great paper to write for SMPTE, because I don't think
there are any standards for drop frame time code in 23.976 fps. The
Sony HDCam manual says you must use non-drop frame when shooting 23.976.
John S.
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