[opendtv] Re: A bit OT: 24 Frames Drop Frame Timecode
- From: "John Willkie" <johnwillkie@xxxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 30 Nov 2007 14:51:01 -0800
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.
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:
- Using the UNSUBSCRIBE command in your user configuration settings at
FreeLists.org
- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word
unsubscribe in the subject line.
Other related posts:
